diff --git a/Makefile b/Makefile
index 4a1c807..2957229 100644
--- a/Makefile
+++ b/Makefile
@@ -7,7 +7,9 @@ AGENT_BIN      := $(BIN_DIR)/restic-manager-agent
 VERSION        ?= $(shell git describe --tags --always --dirty 2>/dev/null || echo dev)
 COMMIT         ?= $(shell git rev-parse HEAD 2>/dev/null || echo none)
 DATE           ?= $(shell date -u +%Y-%m-%dT%H:%M:%SZ)
-LDFLAGS        := -s -w -X main.version=$(VERSION) -X main.commit=$(COMMIT) -X main.date=$(DATE)
+VERSION_PKG    := gitea.dcglab.co.uk/steve/restic-manager/internal/version
+LDFLAGS        := -s -w -X main.version=$(VERSION) -X main.commit=$(COMMIT) -X main.date=$(DATE) \
+                  -X $(VERSION_PKG).Version=$(VERSION) -X $(VERSION_PKG).Commit=$(COMMIT)
 GOFLAGS        := -trimpath
 DOCKER_IMAGE   ?= gitea.dcglab.co.uk/steve/restic-manager
 DOCKER_TAG     ?= dev
diff --git a/cmd/agent/main.go b/cmd/agent/main.go
index d1b5041..e04dd2d 100644
--- a/cmd/agent/main.go
+++ b/cmd/agent/main.go
@@ -148,6 +148,7 @@ func run() error {
 		resticBin:                 resticBin,
 		resticVer:                 snap.ResticVersion,
 		resticSupportsNoOwnership: resticSupportsNoOwnership,
+		serverURL:                 cfg.ServerURL,
 		secrets:                   sec,
 		scheduler:                 scheduler.New(),
 	}
@@ -214,6 +215,7 @@ type dispatcher struct {
 	resticBin                 string
 	resticVer                 string // e.g. "0.17.1"; empty if restic isn't installed yet
 	resticSupportsNoOwnership bool   // captured at startup from `restic restore --help`
+	serverURL                 string // base URL of the server (used by the self-update fetch)
 	secrets                   *secrets.Store
 	scheduler                 *scheduler.Scheduler
 
@@ -395,10 +397,12 @@ func (d *dispatcher) handle(ctx context.Context, env api.Envelope, tx wsclient.S
 				"up_kbps", up, "down_kbps", down)
 		}
 
-	case api.MsgAgentUpdateAvail:
-		var p api.AgentUpdateAvailablePayload
-		_ = env.UnmarshalPayload(&p)
-		slog.Info("ws agent: update available", "version", p.LatestVersion, "url", p.PackageURL)
+	case api.MsgCommandUpdate:
+		var p api.CommandUpdatePayload
+		if err := env.UnmarshalPayload(&p); err != nil {
+			return fmt.Errorf("command.update: %w", err)
+		}
+		go d.runUpdate(ctx, p, tx)
 
 	default:
 		slog.Debug("ws agent: ignored message", "type", env.Type)
diff --git a/cmd/agent/update_dispatch.go b/cmd/agent/update_dispatch.go
new file mode 100644
index 0000000..50bdddf
--- /dev/null
+++ b/cmd/agent/update_dispatch.go
@@ -0,0 +1,65 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"time"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/agent/updater"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/agent/wsclient"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+)
+
+// runUpdate handles a server-dispatched command.update. It logs progress
+// via log.stream so the live job page captures pre-restart state, then
+// calls the platform updater. On Linux the updater calls os.Exit; on
+// Windows it spawns a detached helper and returns, with the agent then
+// exiting.
+//
+// The terminal job state is set by the server, not the agent: success
+// is "agent re-hellos with matching version" rather than anything the
+// agent itself can assert. The only `job.finished` we send from here is
+// on the failure path, before any restart attempt.
+func (d *dispatcher) runUpdate(ctx context.Context, p api.CommandUpdatePayload, tx wsclient.Sender) {
+	logf := func(format string, args ...any) {
+		line := fmt.Sprintf(format, args...)
+		slog.Info("ws agent: update: " + line)
+		env, err := api.Marshal(api.MsgLogStream, "", api.LogStreamLine{
+			JobID:   p.JobID,
+			TS:      time.Now().UTC(),
+			Stream:  api.LogStdout,
+			Payload: line,
+		})
+		if err == nil {
+			_ = tx.Send(env)
+		}
+	}
+
+	startedEnv, err := api.Marshal(api.MsgJobStarted, "", api.JobStartedPayload{
+		JobID:     p.JobID,
+		Kind:      api.JobUpdate,
+		StartedAt: time.Now().UTC(),
+	})
+	if err == nil {
+		_ = tx.Send(startedEnv)
+	}
+
+	logf("fetching new binary from %s", d.serverURL)
+	if err := updater.Update(ctx, d.serverURL); err != nil {
+		logf("update failed: %v", err)
+		finishedEnv, mErr := api.Marshal(api.MsgJobFinished, "", api.JobFinishedPayload{
+			JobID:      p.JobID,
+			Status:     api.JobFailed,
+			FinishedAt: time.Now().UTC(),
+			Error:      err.Error(),
+		})
+		if mErr == nil {
+			_ = tx.Send(finishedEnv)
+		}
+		return
+	}
+	// Unreachable on Linux (Update calls os.Exit). On Windows control
+	// returns here while the detached helper does the swap-and-restart;
+	// the agent then exits cleanly so SCM hands off.
+}
diff --git a/cmd/server/main.go b/cmd/server/main.go
index 8d52bb8..dcd0d38 100644
--- a/cmd/server/main.go
+++ b/cmd/server/main.go
@@ -17,6 +17,7 @@ import (
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/crypto"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/notification"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/config"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/fleetupdate"
 	rmhttp "gitea.dcglab.co.uk/steve/restic-manager/internal/server/http"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/maintenance"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/oidc"
@@ -91,6 +92,7 @@ func run() error {
 
 	notifHub := notification.NewHub(st, aead, cfg.BaseURL)
 	alertEngine := alert.NewEngine(st, notifHub)
+	updateWatcher := ws.NewUpdateWatcher(st, alertEngine)
 
 	renderer, err := ui.New()
 	if err != nil {
@@ -116,6 +118,7 @@ func run() error {
 		JobHub:          jobHub,
 		AlertEngine:     alertEngine,
 		NotificationHub: notifHub,
+		UpdateWatcher:   updateWatcher,
 		UI:              renderer,
 		Version:         version,
 		OIDC:            oidcClient,
@@ -147,10 +150,17 @@ func run() error {
 
 	srv := rmhttp.New(deps)
 
+	// Fleet-update worker — built after the HTTP server because the
+	// dispatcher delegates back into srv.DispatchHostUpdate.
+	fleetWorker := fleetupdate.NewWorker(st, hub,
+		&serverDispatcher{srv: srv}, alertEngine)
+	srv.SetFleetWorker(fleetWorker)
+
 	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
 	go alertEngine.Run(ctx)
+	go updateWatcher.Run(ctx)
 
 	errCh := make(chan error, 1)
 	go func() {
@@ -243,3 +253,12 @@ func run() error {
 	}
 	return nil
 }
+
+// serverDispatcher adapts the http.Server's DispatchHostUpdate method
+// to the fleetupdate.Dispatcher interface. Lives in main so the
+// http and fleetupdate packages don't need to know about each other.
+type serverDispatcher struct{ srv *rmhttp.Server }
+
+func (d *serverDispatcher) DispatchUpdate(ctx context.Context, hostID, actorUserID string) (string, string, error) {
+	return d.srv.DispatchHostUpdate(ctx, hostID, actorUserID)
+}
diff --git a/deploy/install/restic-manager-agent.service b/deploy/install/restic-manager-agent.service
index d6ad407..1e3bcc8 100644
--- a/deploy/install/restic-manager-agent.service
+++ b/deploy/install/restic-manager-agent.service
@@ -52,7 +52,12 @@ ProtectSystem=full
 # whenever a new SecretsKey is minted, so we need a targeted
 # write-exemption for that dir. No exemption for the rest of /etc:
 # the agent has no business editing /etc/passwd, /etc/sudoers, etc.
-ReadWritePaths=/etc/restic-manager
+#
+# /usr/local/bin is writable so the self-update flow (P6-01) can
+# atomic-rename a fresh binary over the running one. Permitting the
+# whole directory (rather than just the binary path) is required
+# because os.Rename takes a write lock on the parent dir.
+ReadWritePaths=/etc/restic-manager /usr/local/bin
 ProtectHostname=true
 ProtectKernelTunables=true
 ProtectKernelModules=true
diff --git a/internal/agent/updater/updater.go b/internal/agent/updater/updater.go
new file mode 100644
index 0000000..90b4f96
--- /dev/null
+++ b/internal/agent/updater/updater.go
@@ -0,0 +1,100 @@
+// Package updater carries the agent's self-update logic.
+//
+// The flow is operator-driven: the server dispatches a command.update
+// WS envelope, the agent fetches a fresh binary from the server's
+// /agent/binary endpoint, atomic-renames it over the running binary
+// (Linux) or hands off to a detached helper script (Windows), and
+// exits cleanly so the service manager restarts under the new
+// binary. See docs/superpowers/specs/2026-05-06-p6-01-02-...
+//
+// Platform-specific code is build-tagged into updater_unix.go /
+// updater_windows.go. This file holds the shared HTTP fetch + path
+// helpers + the test seam.
+package updater
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"path/filepath"
+	"runtime"
+	"time"
+)
+
+// fetch downloads the new binary into <binaryPath>.new, fsyncs, chmods.
+// Returns the path of the staged file (always binaryPath + ".new").
+func fetch(ctx context.Context, serverURL, binaryPath string) (string, error) {
+	url := fmt.Sprintf("%s/agent/binary?os=%s&arch=%s", serverURL, runtime.GOOS, runtime.GOARCH)
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	if err != nil {
+		return "", err
+	}
+	c := &http.Client{Timeout: 5 * time.Minute}
+	res, err := c.Do(req)
+	if err != nil {
+		return "", err
+	}
+	defer func() { _ = res.Body.Close() }()
+	if res.StatusCode != http.StatusOK {
+		return "", fmt.Errorf("agent binary fetch: %s", res.Status)
+	}
+
+	stagePath := binaryPath + ".new"
+	f, err := os.OpenFile(stagePath, os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o755)
+	if err != nil {
+		return "", err
+	}
+	if _, copyErr := io.Copy(f, res.Body); copyErr != nil {
+		_ = f.Close()
+		_ = os.Remove(stagePath)
+		return "", copyErr
+	}
+	if syncErr := f.Sync(); syncErr != nil {
+		_ = f.Close()
+		_ = os.Remove(stagePath)
+		return "", syncErr
+	}
+	if closeErr := f.Close(); closeErr != nil {
+		_ = os.Remove(stagePath)
+		return "", closeErr
+	}
+	if err := os.Chmod(stagePath, 0o755); err != nil {
+		_ = os.Remove(stagePath)
+		return "", err
+	}
+	return stagePath, nil
+}
+
+// resolveOwnBinary returns the absolute path of the running binary.
+// Refuses /proc/self/exe — that's what os.Executable returns on some
+// systems but the path can't be renamed across.
+func resolveOwnBinary() (string, error) {
+	p, err := os.Executable()
+	if err != nil {
+		return "", err
+	}
+	abs, err := filepath.Abs(p)
+	if err != nil {
+		return "", err
+	}
+	if abs == "/proc/self/exe" {
+		return "", fmt.Errorf("cannot resolve own binary path (/proc/self/exe)")
+	}
+	return abs, nil
+}
+
+// UpdateForTest is the platform-neutral test seam. In production the
+// platform-specific Update fetches, swaps, then exits the process.
+// UpdateForTest stops short of the exit so unit tests can assert on
+// file state.
+func UpdateForTest(serverURL, binaryPath string) error {
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
+	defer cancel()
+	stage, err := fetch(ctx, serverURL, binaryPath)
+	if err != nil {
+		return err
+	}
+	return swap(stage, binaryPath)
+}
diff --git a/internal/agent/updater/updater_test.go b/internal/agent/updater/updater_test.go
new file mode 100644
index 0000000..435ef21
--- /dev/null
+++ b/internal/agent/updater/updater_test.go
@@ -0,0 +1,87 @@
+//go:build !windows
+
+package updater
+
+import (
+	"bytes"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"runtime"
+	"testing"
+)
+
+// TestUpdate_LinuxAtomicSwap stages a fake "running binary" file, runs
+// UpdateForTest against a fake /agent/binary server, and asserts that
+// the binary was swapped, .old preserves the previous bytes, and .new
+// was renamed away.
+func TestUpdate_LinuxAtomicSwap(t *testing.T) {
+	tmp := t.TempDir()
+	binPath := filepath.Join(tmp, "agent")
+	if err := os.WriteFile(binPath, []byte("OLD"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	newBytes := []byte("NEW BINARY CONTENTS")
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/agent/binary" {
+			http.NotFound(w, r)
+			return
+		}
+		gotOS, gotArch := r.URL.Query().Get("os"), r.URL.Query().Get("arch")
+		if gotOS != runtime.GOOS || gotArch != runtime.GOARCH {
+			t.Errorf("query mismatch: got os=%s arch=%s want %s/%s",
+				gotOS, gotArch, runtime.GOOS, runtime.GOARCH)
+		}
+		_, _ = io.Copy(w, bytes.NewReader(newBytes))
+	}))
+	defer srv.Close()
+
+	if err := UpdateForTest(srv.URL, binPath); err != nil {
+		t.Fatalf("update: %v", err)
+	}
+
+	got, err := os.ReadFile(binPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != string(newBytes) {
+		t.Fatalf("binary contents: got %q want %q", got, newBytes)
+	}
+	old, err := os.ReadFile(binPath + ".old")
+	if err != nil {
+		t.Fatalf("agent.old missing: %v", err)
+	}
+	if string(old) != "OLD" {
+		t.Fatalf("agent.old contents: got %q want %q", old, "OLD")
+	}
+	if _, err := os.Stat(binPath + ".new"); !os.IsNotExist(err) {
+		t.Fatalf("agent.new should be absent after swap, got err=%v", err)
+	}
+}
+
+// TestUpdate_FetchHTTPError surfaces the server's status when the
+// binary is not published for this os/arch.
+func TestUpdate_FetchHTTPError(t *testing.T) {
+	tmp := t.TempDir()
+	binPath := filepath.Join(tmp, "agent")
+	if err := os.WriteFile(binPath, []byte("OLD"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, `{"error":"binary_not_published"}`, http.StatusNotFound)
+	}))
+	defer srv.Close()
+
+	err := UpdateForTest(srv.URL, binPath)
+	if err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	got, _ := os.ReadFile(binPath)
+	if string(got) != "OLD" {
+		t.Fatalf("binary should not have changed, got %q", got)
+	}
+}
diff --git a/internal/agent/updater/updater_unix.go b/internal/agent/updater/updater_unix.go
new file mode 100644
index 0000000..81ebf50
--- /dev/null
+++ b/internal/agent/updater/updater_unix.go
@@ -0,0 +1,73 @@
+//go:build !windows
+
+package updater
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"time"
+)
+
+// Update fetches the new binary, swaps it in, then exits so systemd
+// restarts the process under the new binary. The caller should close
+// the WS connection cleanly (so the server transitions the host to
+// disconnected immediately rather than waiting for the heartbeat
+// sweep) before invoking.
+//
+// Service-user assumption: the agent runs as root under the
+// systemd-shipped unit, which can write the binary path directly.
+// If the agent ever moves to a non-root service user, this breaks —
+// would need a setuid helper or an out-of-process update service.
+func Update(ctx context.Context, serverURL string) error {
+	binPath, err := resolveOwnBinary()
+	if err != nil {
+		return err
+	}
+	stage, err := fetch(ctx, serverURL, binPath)
+	if err != nil {
+		return err
+	}
+	if err := swap(stage, binPath); err != nil {
+		return err
+	}
+	slog.Info("agent self-update: binary swapped, exiting for systemd restart",
+		"binary", binPath)
+	// Give logger / WS close-frame a moment to flush, then exit.
+	time.Sleep(200 * time.Millisecond)
+	os.Exit(0)
+	return nil // unreachable
+}
+
+// swap copies the running binary to <bin>.old (M1 — keep one revision
+// back for hand-rolled rollback), then atomic-renames the staged
+// binary into place. Linux supports rename-while-open so this works
+// even though the running process holds the source open.
+func swap(stagePath, binPath string) error {
+	src, err := os.Open(binPath)
+	if err != nil {
+		return fmt.Errorf("open running binary: %w", err)
+	}
+	defer func() { _ = src.Close() }()
+	dst, err := os.OpenFile(binPath+".old", os.O_CREATE|os.O_WRONLY|os.O_TRUNC, 0o755)
+	if err != nil {
+		return fmt.Errorf("open .old: %w", err)
+	}
+	if _, err := io.Copy(dst, src); err != nil {
+		_ = dst.Close()
+		return fmt.Errorf("copy to .old: %w", err)
+	}
+	if err := dst.Sync(); err != nil {
+		_ = dst.Close()
+		return err
+	}
+	if err := dst.Close(); err != nil {
+		return err
+	}
+	if err := os.Rename(stagePath, binPath); err != nil {
+		return fmt.Errorf("rename .new over running binary: %w", err)
+	}
+	return nil
+}
diff --git a/internal/agent/updater/updater_windows.go b/internal/agent/updater/updater_windows.go
new file mode 100644
index 0000000..0806472
--- /dev/null
+++ b/internal/agent/updater/updater_windows.go
@@ -0,0 +1,73 @@
+//go:build windows
+
+package updater
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"syscall"
+	"time"
+)
+
+// helperScript is rendered with fmt.Sprintf, args order:
+//
+//	%[1]s — running binary path (source for the .old copy)
+//	%[2]s — .old path
+//	%[3]s — staged .new path
+//	%[4]s — running binary path (rename target)
+const helperScript = `@echo off
+timeout /t 3 /nobreak >nul
+copy /Y "%[1]s" "%[2]s"
+sc stop restic-manager-agent
+:wait
+sc query restic-manager-agent | find "STOPPED" >nul
+if errorlevel 1 (timeout /t 1 /nobreak >nul & goto wait)
+move /Y "%[3]s" "%[4]s"
+sc start restic-manager-agent
+del "%%~f0"
+`
+
+// Update on Windows can't overwrite the running .exe in-process
+// (exclusive file lock), so we stage the new binary, write a small
+// detached helper script that waits, stops the service, swaps the
+// binary, and starts the service, then exit cleanly. SCM treats
+// clean exits after sc stop as intentional and does not auto-restart;
+// the helper's final sc start handles that.
+func Update(ctx context.Context, serverURL string) error {
+	binPath, err := resolveOwnBinary()
+	if err != nil {
+		return err
+	}
+	stage, err := fetch(ctx, serverURL, binPath)
+	if err != nil {
+		return err
+	}
+	helperPath := filepath.Join(filepath.Dir(binPath), "agent-update.cmd")
+	body := fmt.Sprintf(helperScript, binPath, binPath+".old", stage, binPath)
+	if err := os.WriteFile(helperPath, []byte(body), 0o755); err != nil {
+		return err
+	}
+	cmd := exec.Command("cmd.exe", "/c", helperPath)
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		HideWindow:    true,
+		CreationFlags: 0x00000008 | 0x08000000, // DETACHED_PROCESS | CREATE_NO_WINDOW
+	}
+	if err := cmd.Start(); err != nil {
+		return err
+	}
+	slog.Info("agent self-update: helper spawned, exiting cleanly",
+		"binary", binPath, "helper", helperPath)
+	time.Sleep(200 * time.Millisecond)
+	os.Exit(0)
+	return nil // unreachable
+}
+
+// swap is unused on Windows — the helper script does the swap.
+// Defined to satisfy the build (UpdateForTest references it).
+func swap(_, _ string) error {
+	return fmt.Errorf("updater.swap not implemented on Windows; use the helper script via Update")
+}
diff --git a/internal/alert/update_alerts.go b/internal/alert/update_alerts.go
new file mode 100644
index 0000000..9a7da6e
--- /dev/null
+++ b/internal/alert/update_alerts.go
@@ -0,0 +1,63 @@
+package alert
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"time"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/notification"
+)
+
+// Alert-kind constants for P6 self-update flows.
+const (
+	// KindUpdateFailed is raised when an agent fails to come back with
+	// the expected version after a command.update dispatch (timeout or
+	// version-mismatch). Resolved by a subsequent matching hello.
+	KindUpdateFailed = "update_failed"
+
+	// KindFleetUpdateHalted is raised when the fleet-update worker
+	// stops mid-run because a host failed to update or went offline.
+	// Host-less alert (system-scoped). Manually resolved by an admin.
+	KindFleetUpdateHalted = "fleet_update_halted"
+)
+
+// RaiseUpdateFailed records a per-host update failure. dedupKey is the
+// hostID so a re-dispatch on the same host touches the existing alert
+// rather than spawning a duplicate.
+func (e *Engine) RaiseUpdateFailed(ctx context.Context, hostID, jobID, reason string, when time.Time) {
+	msg := fmt.Sprintf("Agent update failed (job %s): %s", jobID, reason)
+	e.raiseAndNotify(ctx, hostID, KindUpdateFailed, hostID, "warning", msg, when)
+}
+
+// ResolveUpdateFailed clears any open update_failed alert for hostID.
+// Called from the WS hello path when the agent reconnects with the
+// target version.
+func (e *Engine) ResolveUpdateFailed(ctx context.Context, hostID string, when time.Time) {
+	e.resolveAndNotify(ctx, hostID, KindUpdateFailed, hostID, when)
+}
+
+// RaiseFleetUpdateHalted is host-less — the fleet update is a
+// system-level concept. We persist it via the dedicated host-less
+// alert path so the alerts table's host_id column carries NULL.
+func (e *Engine) RaiseFleetUpdateHalted(ctx context.Context, fleetUpdateID, reason string, when time.Time) {
+	msg := fmt.Sprintf("Fleet update %s halted: %s", fleetUpdateID, reason)
+	id, didRaise, err := e.store.RaiseOrTouchSystem(ctx, KindFleetUpdateHalted, fleetUpdateID, "warning", msg, when)
+	if err != nil {
+		slog.Warn("alert: raise fleet_update_halted", "fu_id", fleetUpdateID, "err", err)
+		return
+	}
+	if !didRaise {
+		return
+	}
+	go e.hub.Dispatch(ctx, notification.Payload{
+		Event:    notification.EventRaised,
+		AlertID:  id,
+		Severity: "warning",
+		Kind:     KindFleetUpdateHalted,
+		HostID:   "",
+		HostName: "",
+		Message:  msg,
+		RaisedAt: when,
+	})
+}
diff --git a/internal/api/messages.go b/internal/api/messages.go
index 8ea18f2..b9d8a2e 100644
--- a/internal/api/messages.go
+++ b/internal/api/messages.go
@@ -63,6 +63,7 @@ const (
 	JobUnlock  JobKind = "unlock"
 	JobRestore JobKind = "restore"
 	JobDiff    JobKind = "diff"
+	JobUpdate  JobKind = "update"
 )
 
 // JobStatus is the lifecycle state of a job.
@@ -361,13 +362,14 @@ type ConfigUpdatePayload struct {
 	BandwidthDownKBps *int `json:"bandwidth_down_kbps,omitempty"`
 }
 
-// AgentUpdateAvailablePayload — informational only; the agent does
-// NOT self-update. See spec.md §4.2 for the package-manager-based
-// update model.
-type AgentUpdateAvailablePayload struct {
-	LatestVersion string `json:"latest_version"`
-	PackageURL    string `json:"package_url"` // apt repo / choco source
-	Changelog     string `json:"changelog,omitempty"`
+// CommandUpdatePayload carries no operational data — the agent
+// already knows its own os/arch and fetches from its configured
+// server URL via /agent/binary. JobID is the server-issued id of
+// the update job; the agent echoes it on log.stream lines so the
+// live job log captures pre-restart progress, then either exits
+// (Linux) or hands off to a detached helper script (Windows).
+type CommandUpdatePayload struct {
+	JobID string `json:"job_id"`
 }
 
 // TreeListRequestPayload is the body of a tree.list RPC. Used by the
diff --git a/internal/api/wire.go b/internal/api/wire.go
index 005827f..4573738 100644
--- a/internal/api/wire.go
+++ b/internal/api/wire.go
@@ -29,12 +29,12 @@ const (
 
 // Server → agent message types.
 const (
-	MsgCommandRun       MessageType = "command.run"
-	MsgCommandCancel    MessageType = "command.cancel"
-	MsgScheduleSet      MessageType = "schedule.set"
-	MsgConfigUpdate     MessageType = "config.update"
-	MsgAgentUpdateAvail MessageType = "agent.update.available"
-	MsgTreeList         MessageType = "tree.list" // sync RPC: list a snapshot's children
+	MsgCommandRun    MessageType = "command.run"
+	MsgCommandCancel MessageType = "command.cancel"
+	MsgScheduleSet   MessageType = "schedule.set"
+	MsgConfigUpdate  MessageType = "config.update"
+	MsgCommandUpdate MessageType = "command.update"
+	MsgTreeList      MessageType = "tree.list" // sync RPC: list a snapshot's children
 )
 
 // Envelope is the framing for every WS message in either direction.
diff --git a/internal/server/fleetupdate/worker.go b/internal/server/fleetupdate/worker.go
new file mode 100644
index 0000000..1442832
--- /dev/null
+++ b/internal/server/fleetupdate/worker.go
@@ -0,0 +1,221 @@
+// Package fleetupdate drives a rolling, sequential agent self-update
+// over a list of hosts. One worker goroutine per Start() call (gated
+// at the store layer to at-most-one-running-fleet-update).
+package fleetupdate
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log/slog"
+	"time"
+
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+)
+
+// Hub is the slim "is this host connected?" surface.
+type Hub interface {
+	Connected(hostID string) bool
+}
+
+// Dispatcher sends one command.update envelope. The implementer also
+// creates the jobs row, writes audit, and registers with the update
+// watcher. Pre-checks are the dispatcher's responsibility — the worker
+// passes through whatever error it returns.
+type Dispatcher interface {
+	DispatchUpdate(ctx context.Context, hostID string, actorUserID string) (jobID string, code string, err error)
+}
+
+// AlertRaiser is the slim view of the alert engine's host-less raise
+// path. Used to emit fleet_update_halted on first failure.
+type AlertRaiser interface {
+	RaiseFleetUpdateHalted(ctx context.Context, fleetUpdateID, reason string, when time.Time)
+}
+
+// Worker is the long-lived fleet-update orchestrator. There is at most
+// one *running* fleet update at a time (enforced by the store).
+type Worker struct {
+	store  *store.Store
+	hub    Hub
+	disp   Dispatcher
+	alerts AlertRaiser
+
+	// targetVersion is the version every dispatched agent is expected
+	// to come back with. Captured at Start time to avoid drift.
+	targetVersion string
+
+	// pollPeriod controls the cadence at which the worker re-reads the
+	// host row to check for the version transition. Exposed for tests.
+	pollPeriod time.Duration
+	// hostTimeout bounds how long the worker waits for one host to
+	// reach the target version before halting.
+	hostTimeout time.Duration
+}
+
+// NewWorker builds an unstarted worker. targetVersion is set on each
+// Start call; the values here are defaults.
+func NewWorker(st *store.Store, hub Hub, disp Dispatcher, alerts AlertRaiser) *Worker {
+	return &Worker{
+		store:       st,
+		hub:         hub,
+		disp:        disp,
+		alerts:      alerts,
+		pollPeriod:  1 * time.Second,
+		hostTimeout: 95 * time.Second,
+	}
+}
+
+// Start creates the parent + child rows, then spawns the per-host
+// worker goroutine. Returns the new fleet_update_id on success.
+// store.ErrFleetUpdateRunning bubbles up unchanged.
+func (w *Worker) Start(ctx context.Context, userID, targetVersion string, hostIDs []string) (string, error) {
+	if userID == "" || targetVersion == "" {
+		return "", errors.New("fleetupdate: userID and targetVersion required")
+	}
+	if len(hostIDs) == 0 {
+		return "", errors.New("fleetupdate: at least one host required")
+	}
+	fuID := ulid.Make().String()
+	now := time.Now().UTC()
+	if err := w.store.CreateFleetUpdate(ctx, store.FleetUpdate{
+		ID:              fuID,
+		StartedAt:       now,
+		StartedByUserID: userID,
+		TargetVersion:   targetVersion,
+		Status:          "running",
+	}, hostIDs); err != nil {
+		return "", err
+	}
+
+	// The goroutine outlives the request that started it; carry a
+	// detached context so an HTTP-handler ctx cancel doesn't abort
+	// the long roll.
+	bg := context.WithoutCancel(ctx)
+	go w.run(bg, fuID, userID, targetVersion)
+	return fuID, nil
+}
+
+// Cancel marks the fleet update cancelled. The running goroutine
+// observes the new status on its next pre-check and exits without
+// dispatching further hosts. The currently-dispatched job is left to
+// finish on its own — cancelling agent-side is out of scope for v1.
+func (w *Worker) Cancel(ctx context.Context, fuID string) error {
+	return w.store.CancelFleetUpdate(ctx, fuID, time.Now().UTC())
+}
+
+// run is the per-host loop. Halts on first failure; emits one alert
+// on transition.
+func (w *Worker) run(ctx context.Context, fuID, userID, targetVersion string) {
+	w.targetVersion = targetVersion
+
+	for {
+		// Check the parent row's status — picks up Cancel.
+		fu, err := w.store.ActiveFleetUpdate(ctx)
+		if err != nil {
+			slog.Warn("fleetupdate: read active", "fu_id", fuID, "err", err)
+			return
+		}
+		if fu == nil || fu.ID != fuID {
+			// Cancelled, halted, or completed externally. Done.
+			return
+		}
+
+		pending, err := w.store.ListPendingFleetUpdateHosts(ctx, fuID)
+		if err != nil {
+			slog.Warn("fleetupdate: list pending", "fu_id", fuID, "err", err)
+			return
+		}
+		if len(pending) == 0 {
+			now := time.Now().UTC()
+			if err := w.store.CompleteFleetUpdate(ctx, fuID, now); err != nil {
+				slog.Warn("fleetupdate: complete", "fu_id", fuID, "err", err)
+			}
+			return
+		}
+
+		next := pending[0]
+		w.processHost(ctx, fuID, userID, next)
+	}
+}
+
+// processHost handles one host slot. Marks it skipped, succeeded, or
+// failed (and halts the fleet on failure).
+func (w *Worker) processHost(ctx context.Context, fuID, userID string, slot store.FleetUpdateHost) {
+	hostID := slot.HostID
+	_ = w.store.SetFleetUpdateCurrentHost(ctx, fuID, hostID)
+
+	// Pre-flight: re-read the host. The dispatch path repeats most of
+	// these checks but doing them up-front lets us emit the right
+	// per-host status (skipped vs failed) without consuming a job row.
+	host, err := w.store.GetHost(ctx, hostID)
+	if err != nil || host == nil {
+		_ = w.store.SetFleetUpdateHostStatus(ctx, fuID, hostID, "skipped", "host not found", "")
+		return
+	}
+	if host.AgentVersion != "" && host.AgentVersion == w.targetVersion {
+		_ = w.store.SetFleetUpdateHostStatus(ctx, fuID, hostID, "skipped", "already at target version", "")
+		return
+	}
+	if !w.hub.Connected(hostID) {
+		reason := fmt.Sprintf("host went offline: %s", hostID)
+		_ = w.store.SetFleetUpdateHostStatus(ctx, fuID, hostID, "failed", reason, "")
+		w.halt(ctx, fuID, reason)
+		return
+	}
+
+	// Dispatch.
+	_ = w.store.SetFleetUpdateHostStatus(ctx, fuID, hostID, "running", "", "")
+	jobID, code, err := w.disp.DispatchUpdate(ctx, hostID, userID)
+	if err != nil || code != "" {
+		reason := dispatchErrorReason(code, err)
+		_ = w.store.SetFleetUpdateHostStatus(ctx, fuID, hostID, "failed", reason, jobID)
+		w.halt(ctx, fuID, reason)
+		return
+	}
+
+	// Poll until the host's recorded agent_version matches target, or
+	// timeout.
+	deadline := time.Now().Add(w.hostTimeout)
+	for time.Now().Before(deadline) {
+		// Honour cancellation between polls.
+		fu, err := w.store.ActiveFleetUpdate(ctx)
+		if err == nil && (fu == nil || fu.ID != fuID) {
+			// Cancelled mid-host; leave the slot in 'running' for the
+			// admin to inspect. No further dispatches.
+			return
+		}
+		time.Sleep(w.pollPeriod)
+		h, err := w.store.GetHost(ctx, hostID)
+		if err == nil && h != nil && h.AgentVersion == w.targetVersion {
+			if err := w.store.SetFleetUpdateHostStatus(ctx, fuID, hostID, "succeeded", "", jobID); err != nil {
+				slog.Warn("fleetupdate: set succeeded", "fu_id", fuID, "host_id", hostID, "err", err)
+			}
+			return
+		}
+	}
+	reason := fmt.Sprintf("timeout waiting for %s to reach %s", hostID, w.targetVersion)
+	_ = w.store.SetFleetUpdateHostStatus(ctx, fuID, hostID, "failed", reason, jobID)
+	w.halt(ctx, fuID, reason)
+}
+
+func (w *Worker) halt(ctx context.Context, fuID, reason string) {
+	now := time.Now().UTC()
+	if err := w.store.HaltFleetUpdate(ctx, fuID, reason, now); err != nil {
+		slog.Warn("fleetupdate: halt", "fu_id", fuID, "err", err)
+	}
+	if w.alerts != nil {
+		w.alerts.RaiseFleetUpdateHalted(ctx, fuID, reason, now)
+	}
+}
+
+func dispatchErrorReason(code string, err error) string {
+	if code != "" {
+		return "dispatch failed: " + code
+	}
+	if err != nil {
+		return err.Error()
+	}
+	return "dispatch failed"
+}
diff --git a/internal/server/fleetupdate/worker_test.go b/internal/server/fleetupdate/worker_test.go
new file mode 100644
index 0000000..c1cdac1
--- /dev/null
+++ b/internal/server/fleetupdate/worker_test.go
@@ -0,0 +1,344 @@
+package fleetupdate
+
+import (
+	"context"
+	"errors"
+	"path/filepath"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+)
+
+type fakeHub struct {
+	mu     sync.Mutex
+	online map[string]bool
+}
+
+func (f *fakeHub) Connected(hostID string) bool {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return f.online[hostID]
+}
+
+type fakeDispatcher struct {
+	mu    sync.Mutex
+	calls []string // host IDs
+	// after dispatch, set the host's agent_version to this on the
+	// store so the worker observes the version transition.
+	st         *store.Store
+	target     string
+	delayMS    int
+	failOnHost map[string]string // host → error code
+}
+
+func (f *fakeDispatcher) DispatchUpdate(ctx context.Context, hostID, _ string) (string, string, error) {
+	f.mu.Lock()
+	f.calls = append(f.calls, hostID)
+	if code, ok := f.failOnHost[hostID]; ok {
+		f.mu.Unlock()
+		return "", code, nil
+	}
+	st := f.st
+	target := f.target
+	delay := f.delayMS
+	f.mu.Unlock()
+
+	jobID := ulid.Make().String()
+	if st != nil {
+		_ = st.CreateJob(context.Background(), store.Job{
+			ID: jobID, HostID: hostID, Kind: "update",
+			ActorKind: "user", CreatedAt: time.Now().UTC(),
+		})
+	}
+	if st != nil && target != "" {
+		go func() {
+			if delay > 0 {
+				time.Sleep(time.Duration(delay) * time.Millisecond)
+			}
+			_ = st.MarkHostHello(context.Background(), hostID, target, "0.17", api.CurrentProtocolVersion, time.Now().UTC())
+		}()
+	}
+	return jobID, "", nil
+}
+
+type recAlert struct {
+	mu      sync.Mutex
+	reasons []string
+}
+
+func (r *recAlert) RaiseFleetUpdateHalted(_ context.Context, _ string, reason string, _ time.Time) {
+	r.mu.Lock()
+	r.reasons = append(r.reasons, reason)
+	r.mu.Unlock()
+}
+
+func openStore(t *testing.T) *store.Store {
+	t.Helper()
+	dir := t.TempDir()
+	st, err := store.Open(context.Background(), filepath.Join(dir, "rm.db"))
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	t.Cleanup(func() { _ = st.Close() })
+	return st
+}
+
+func mustCreateAdmin(t *testing.T, st *store.Store) string {
+	t.Helper()
+	uid := ulid.Make().String()
+	if err := st.CreateUser(context.Background(), store.User{
+		ID: uid, Username: "u-" + uid[:6],
+		PasswordHash: "x", Role: store.RoleAdmin, CreatedAt: time.Now().UTC(),
+	}); err != nil {
+		t.Fatalf("user: %v", err)
+	}
+	return uid
+}
+
+func mustCreateHost(t *testing.T, st *store.Store, name, version string) string {
+	t.Helper()
+	hostID := ulid.Make().String()
+	if err := st.CreateHost(context.Background(), store.Host{
+		ID: hostID, Name: name, OS: "linux", Arch: "amd64",
+		EnrolledAt: time.Now().UTC(),
+	}, "deadbeef-"+hostID, ""); err != nil {
+		t.Fatalf("host: %v", err)
+	}
+	if version != "" {
+		if err := st.MarkHostHello(context.Background(), hostID, version, "0.17", api.CurrentProtocolVersion, time.Now().UTC()); err != nil {
+			t.Fatalf("hello: %v", err)
+		}
+	}
+	return hostID
+}
+
+func waitForStatus(t *testing.T, st *store.Store, fuID, want string, timeout time.Duration) *store.FleetUpdate {
+	t.Helper()
+	deadline := time.Now().Add(timeout)
+	for time.Now().Before(deadline) {
+		fu, _, err := st.GetFleetUpdate(context.Background(), fuID)
+		if err == nil && fu != nil && fu.Status == want {
+			return fu
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+	t.Fatalf("status never reached %q", want)
+	return nil
+}
+
+func TestWorkerTwoHostsBothSucceed(t *testing.T) {
+	st := openStore(t)
+	uid := mustCreateAdmin(t, st)
+	h1 := mustCreateHost(t, st, "h1", "v0")
+	h2 := mustCreateHost(t, st, "h2", "v0")
+
+	hub := &fakeHub{online: map[string]bool{h1: true, h2: true}}
+	disp := &fakeDispatcher{st: st, target: "v2", delayMS: 30}
+	alerts := &recAlert{}
+	w := NewWorker(st, hub, disp, alerts)
+	w.pollPeriod = 20 * time.Millisecond
+	w.hostTimeout = 2 * time.Second
+
+	fuID, err := w.Start(context.Background(), uid, "v2", []string{h1, h2})
+	if err != nil {
+		t.Fatalf("start: %v", err)
+	}
+	waitForStatus(t, st, fuID, "completed", 5*time.Second)
+	_, hosts, _ := st.GetFleetUpdate(context.Background(), fuID)
+	for _, h := range hosts {
+		if h.Status != "succeeded" {
+			t.Errorf("host %s status %q want succeeded", h.HostID, h.Status)
+		}
+	}
+	if n := len(alerts.reasons); n != 0 {
+		t.Errorf("unexpected halt alert: %v", alerts.reasons)
+	}
+}
+
+func TestWorkerSecondHostTimesOutHalts(t *testing.T) {
+	st := openStore(t)
+	uid := mustCreateAdmin(t, st)
+	h1 := mustCreateHost(t, st, "h1", "v0")
+	h2 := mustCreateHost(t, st, "h2", "v0")
+	h3 := mustCreateHost(t, st, "h3", "v0")
+
+	hub := &fakeHub{online: map[string]bool{h1: true, h2: true, h3: true}}
+	// h1 dispatches normally (transitions to v2). h2 dispatch returns
+	// success but never transitions.
+	disp := &fakeDispatcher{st: st, target: "v2", delayMS: 20, failOnHost: map[string]string{
+		h2: "", // not a code-failure; simulate by clearing target on this disp run
+	}}
+	// Actually: drop h2 from the auto-transition by faking with a
+	// per-host store setter. Easiest: subclass via a wrapper.
+	_ = disp
+	customDisp := &perHostDispatcher{base: disp, st: st, target: "v2", noTransition: map[string]bool{h2: true}}
+
+	alerts := &recAlert{}
+	w := NewWorker(st, hub, customDisp, alerts)
+	w.pollPeriod = 20 * time.Millisecond
+	w.hostTimeout = 200 * time.Millisecond
+
+	fuID, err := w.Start(context.Background(), uid, "v2", []string{h1, h2, h3})
+	if err != nil {
+		t.Fatalf("start: %v", err)
+	}
+	waitForStatus(t, st, fuID, "halted", 3*time.Second)
+	_, hosts, _ := st.GetFleetUpdate(context.Background(), fuID)
+	gotStatus := map[string]string{}
+	for _, h := range hosts {
+		gotStatus[h.HostID] = h.Status
+	}
+	if gotStatus[h1] != "succeeded" {
+		t.Errorf("h1: %q", gotStatus[h1])
+	}
+	if gotStatus[h2] != "failed" {
+		t.Errorf("h2: %q", gotStatus[h2])
+	}
+	if gotStatus[h3] != "pending" {
+		t.Errorf("h3: %q", gotStatus[h3])
+	}
+	alerts.mu.Lock()
+	defer alerts.mu.Unlock()
+	if len(alerts.reasons) != 1 {
+		t.Errorf("alert reasons: %v", alerts.reasons)
+	}
+}
+
+// perHostDispatcher lets a test omit the auto-transition for selected
+// hosts so we can simulate timeout.
+type perHostDispatcher struct {
+	mu           sync.Mutex
+	base         *fakeDispatcher
+	st           *store.Store
+	target       string
+	noTransition map[string]bool
+}
+
+func (p *perHostDispatcher) DispatchUpdate(_ context.Context, hostID, _ string) (string, string, error) {
+	p.mu.Lock()
+	skip := p.noTransition[hostID]
+	p.mu.Unlock()
+	jobID := ulid.Make().String()
+	_ = p.st.CreateJob(context.Background(), store.Job{
+		ID: jobID, HostID: hostID, Kind: "update",
+		ActorKind: "user", CreatedAt: time.Now().UTC(),
+	})
+	if !skip {
+		go func() {
+			time.Sleep(20 * time.Millisecond)
+			_ = p.st.MarkHostHello(context.Background(), hostID, p.target, "0.17", api.CurrentProtocolVersion, time.Now().UTC())
+		}()
+	}
+	return jobID, "", nil
+}
+
+func TestWorkerHostOfflineHalts(t *testing.T) {
+	st := openStore(t)
+	uid := mustCreateAdmin(t, st)
+	h1 := mustCreateHost(t, st, "h1", "v0")
+	h2 := mustCreateHost(t, st, "h2", "v0")
+	hub := &fakeHub{online: map[string]bool{h1: false, h2: true}}
+	disp := &fakeDispatcher{st: st, target: "v2"}
+	alerts := &recAlert{}
+	w := NewWorker(st, hub, disp, alerts)
+	w.pollPeriod = 20 * time.Millisecond
+	w.hostTimeout = 500 * time.Millisecond
+
+	fuID, err := w.Start(context.Background(), uid, "v2", []string{h1, h2})
+	if err != nil {
+		t.Fatalf("start: %v", err)
+	}
+	waitForStatus(t, st, fuID, "halted", 2*time.Second)
+	_, hosts, _ := st.GetFleetUpdate(context.Background(), fuID)
+	if hosts[0].Status != "failed" {
+		t.Errorf("h1 status: %q", hosts[0].Status)
+	}
+	if hosts[1].Status != "pending" {
+		t.Errorf("h2 status: %q", hosts[1].Status)
+	}
+}
+
+func TestWorkerAlreadyAtTargetSkipped(t *testing.T) {
+	st := openStore(t)
+	uid := mustCreateAdmin(t, st)
+	h1 := mustCreateHost(t, st, "h1", "v2")
+	h2 := mustCreateHost(t, st, "h2", "v0")
+	hub := &fakeHub{online: map[string]bool{h1: true, h2: true}}
+	disp := &fakeDispatcher{st: st, target: "v2", delayMS: 20}
+	alerts := &recAlert{}
+	w := NewWorker(st, hub, disp, alerts)
+	w.pollPeriod = 20 * time.Millisecond
+	w.hostTimeout = 2 * time.Second
+
+	fuID, err := w.Start(context.Background(), uid, "v2", []string{h1, h2})
+	if err != nil {
+		t.Fatalf("start: %v", err)
+	}
+	waitForStatus(t, st, fuID, "completed", 4*time.Second)
+	_, hosts, _ := st.GetFleetUpdate(context.Background(), fuID)
+	want := map[string]string{h1: "skipped", h2: "succeeded"}
+	for _, h := range hosts {
+		if h.Status != want[h.HostID] {
+			t.Errorf("host %s: got %q want %q", h.HostID, h.Status, want[h.HostID])
+		}
+	}
+}
+
+func TestWorkerCancelMidRun(t *testing.T) {
+	st := openStore(t)
+	uid := mustCreateAdmin(t, st)
+	h1 := mustCreateHost(t, st, "h1", "v0")
+	h2 := mustCreateHost(t, st, "h2", "v0")
+	hub := &fakeHub{online: map[string]bool{h1: true, h2: true}}
+	// h1's transition is delayed long enough that we can cancel
+	// before it lands; h2 should never be touched.
+	disp := &fakeDispatcher{st: st, target: "v2", delayMS: 500}
+	alerts := &recAlert{}
+	w := NewWorker(st, hub, disp, alerts)
+	w.pollPeriod = 50 * time.Millisecond
+	w.hostTimeout = 5 * time.Second
+
+	fuID, err := w.Start(context.Background(), uid, "v2", []string{h1, h2})
+	if err != nil {
+		t.Fatalf("start: %v", err)
+	}
+	// Give the worker a moment to dispatch h1.
+	time.Sleep(100 * time.Millisecond)
+	if err := w.Cancel(context.Background(), fuID); err != nil {
+		t.Fatalf("cancel: %v", err)
+	}
+	waitForStatus(t, st, fuID, "cancelled", 2*time.Second)
+
+	// h2 should never be dispatched.
+	disp.mu.Lock()
+	defer disp.mu.Unlock()
+	for _, c := range disp.calls {
+		if c == h2 {
+			t.Errorf("h2 dispatched after cancel")
+		}
+	}
+}
+
+func TestWorkerStartWhileActiveErrors(t *testing.T) {
+	st := openStore(t)
+	uid := mustCreateAdmin(t, st)
+	h1 := mustCreateHost(t, st, "h1", "v0")
+	h2 := mustCreateHost(t, st, "h2", "v0")
+	hub := &fakeHub{online: map[string]bool{h1: true, h2: true}}
+	disp := &fakeDispatcher{st: st, target: "v2", delayMS: 5_000}
+	w := NewWorker(st, hub, disp, &recAlert{})
+	w.pollPeriod = 50 * time.Millisecond
+	w.hostTimeout = 2 * time.Second
+	if _, err := w.Start(context.Background(), uid, "v2", []string{h1}); err != nil {
+		t.Fatalf("first start: %v", err)
+	}
+	_, err := w.Start(context.Background(), uid, "v2", []string{h2})
+	if !errors.Is(err, store.ErrFleetUpdateRunning) {
+		t.Fatalf("err: %v want ErrFleetUpdateRunning", err)
+	}
+}
diff --git a/internal/server/http/dashboard_filter_test.go b/internal/server/http/dashboard_filter_test.go
index 61b2b47..bc58b85 100644
--- a/internal/server/http/dashboard_filter_test.go
+++ b/internal/server/http/dashboard_filter_test.go
@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
 )
 
 func makeFilterHosts() []store.Host {
@@ -98,6 +99,23 @@ func TestSortDashboardHostsColumns(t *testing.T) {
 	}
 }
 
+// TestFilterAndSortDashboardUpdatesBehind: ?updates=behind narrows
+// to hosts whose agent_version is non-empty AND != server's version.
+func TestFilterAndSortDashboardUpdatesBehind(t *testing.T) {
+	t.Parallel()
+	hosts := []store.Host{
+		{ID: "01a", Name: "alpha", AgentVersion: "v0.0.1", Status: "online"},
+		{ID: "01b", Name: "bravo", AgentVersion: version.Version, Status: "online"},
+		{ID: "01c", Name: "charlie", AgentVersion: "", Status: "online"}, // never seen
+		{ID: "01d", Name: "delta", AgentVersion: "v0.0.1", Status: "offline"},
+	}
+	got := filterAndSortDashboardHosts(hosts, dashboardFilter{Updates: "behind", Sort: "name", Dir: "asc"})
+	// alpha + delta both behind; bravo (current) and charlie (empty) excluded.
+	if len(got) != 2 || got[0].Name != "alpha" || got[1].Name != "delta" {
+		t.Errorf("updates=behind: got %v", namesOf(got))
+	}
+}
+
 // TestParseDashboardFilterDefaults: empty query gives sort=name asc.
 func TestParseDashboardFilterDefaults(t *testing.T) {
 	t.Parallel()
diff --git a/internal/server/http/fleet_update.go b/internal/server/http/fleet_update.go
new file mode 100644
index 0000000..42c67f8
--- /dev/null
+++ b/internal/server/http/fleet_update.go
@@ -0,0 +1,379 @@
+// fleet_update.go — admin-only fleet rolling-update endpoints + page.
+//
+// Surface:
+//   - POST /api/fleet/update          → starts a fleet update (JSON)
+//   - POST /api/fleet-updates/{id}/cancel
+//   - GET  /api/fleet-updates/{id}    → JSON parent + per-host array
+//   - GET  /settings/fleet-update     → admin UI page
+//   - GET  /settings/fleet-update/partial → htmx polling fragment
+//
+// All routes are mounted in the admin band (see routes()).
+package http
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"log/slog"
+	stdhttp "net/http"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
+)
+
+// fleetUpdateStartReq is the JSON body for POST /api/fleet/update.
+// Both fields are optional: empty target_version defaults to the
+// server's current version, empty host_ids derives the out-of-date
+// online subset.
+type fleetUpdateStartReq struct {
+	TargetVersion string   `json:"target_version,omitempty"`
+	HostIDs       []string `json:"host_ids,omitempty"`
+}
+
+// fleetUpdateHostView is one row in the JSON response for GET
+// /api/fleet-updates/{id}. Hostname is hydrated from the store so
+// callers don't need a second round-trip per host.
+type fleetUpdateHostView struct {
+	HostID       string `json:"host_id"`
+	HostName     string `json:"host_name,omitempty"`
+	Position     int    `json:"position"`
+	Status       string `json:"status"`
+	JobID        string `json:"job_id,omitempty"`
+	FailedReason string `json:"failed_reason,omitempty"`
+}
+
+// fleetUpdateView is the JSON projection of the parent + children.
+type fleetUpdateView struct {
+	ID              string                `json:"id"`
+	StartedAt       string                `json:"started_at"`
+	StartedByUserID string                `json:"started_by_user_id"`
+	TargetVersion   string                `json:"target_version"`
+	Status          string                `json:"status"`
+	CurrentHostID   string                `json:"current_host_id,omitempty"`
+	HaltedReason    string                `json:"halted_reason,omitempty"`
+	CompletedAt     *string               `json:"completed_at,omitempty"`
+	Hosts           []fleetUpdateHostView `json:"hosts"`
+}
+
+// fleetUpdatePage backs both the full /settings/fleet-update page
+// and the partial polled fragment. Idle / Active are mutually
+// exclusive: if Active is non-nil, render the progress view.
+type fleetUpdatePage struct {
+	// Idle-state fields.
+	OutOfDateHosts []store.Host // online hosts whose version != target
+	TargetVersion  string
+
+	// Active-state fields. Nil when no fleet update has ever run.
+	Active     *store.FleetUpdate
+	ActiveRows []fleetUpdateHostView
+
+	// Common.
+	HostNames map[string]string
+	// PollURL is the partial endpoint htmx polls every few seconds.
+	PollURL string
+}
+
+// handleAPIFleetUpdateStart is POST /api/fleet/update.
+func (s *Server) handleAPIFleetUpdateStart(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	user, ok := s.requireUser(r)
+	if !ok {
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
+		return
+	}
+	if s.deps.FleetWorker == nil {
+		writeJSONError(w, stdhttp.StatusServiceUnavailable, "fleet_worker_unavailable", "")
+		return
+	}
+	var body fleetUpdateStartReq
+	// Empty body is fine — both fields are optional.
+	if r.ContentLength != 0 {
+		if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+			writeJSONError(w, stdhttp.StatusBadRequest, "invalid_json", err.Error())
+			return
+		}
+	}
+	target := body.TargetVersion
+	if target == "" {
+		target = version.Version
+	}
+	hostIDs := body.HostIDs
+	if len(hostIDs) == 0 {
+		derived, err := s.deriveOutOfDateOnlineHostIDs(r.Context(), target)
+		if err != nil {
+			writeJSONError(w, stdhttp.StatusInternalServerError, "internal", err.Error())
+			return
+		}
+		hostIDs = derived
+	}
+	if len(hostIDs) == 0 {
+		writeJSONError(w, stdhttp.StatusConflict, "no_hosts_eligible",
+			"no online hosts are out of date")
+		return
+	}
+
+	fuID, err := s.deps.FleetWorker.Start(r.Context(), user.ID, target, hostIDs)
+	if err != nil {
+		if errors.Is(err, store.ErrFleetUpdateRunning) {
+			writeJSONError(w, stdhttp.StatusConflict, "fleet_update_in_progress", err.Error())
+			return
+		}
+		writeJSONError(w, stdhttp.StatusInternalServerError, "internal", err.Error())
+		return
+	}
+
+	auditPayload, _ := json.Marshal(map[string]any{
+		"fleet_update_id": fuID,
+		"target_version":  target,
+		"host_count":      len(hostIDs),
+	})
+	_ = s.deps.Store.AppendAudit(r.Context(), store.AuditEntry{
+		ID: ulid.Make().String(), UserID: &user.ID, Actor: "user",
+		Action:     "fleet.update_started",
+		TargetKind: ptr("fleet_update"), TargetID: &fuID,
+		TS:      time.Now().UTC(),
+		Payload: auditPayload,
+	})
+
+	writeJSON(w, stdhttp.StatusAccepted, map[string]string{"fleet_update_id": fuID})
+}
+
+// handleAPIFleetUpdateCancel is POST /api/fleet-updates/{id}/cancel.
+func (s *Server) handleAPIFleetUpdateCancel(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	user, ok := s.requireUser(r)
+	if !ok {
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
+		return
+	}
+	if s.deps.FleetWorker == nil {
+		writeJSONError(w, stdhttp.StatusServiceUnavailable, "fleet_worker_unavailable", "")
+		return
+	}
+	fuID := chi.URLParam(r, "id")
+	if fuID == "" {
+		writeJSONError(w, stdhttp.StatusBadRequest, "missing_id", "")
+		return
+	}
+	fu, _, err := s.deps.Store.GetFleetUpdate(r.Context(), fuID)
+	if err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			writeJSONError(w, stdhttp.StatusNotFound, "fleet_update_not_found", "")
+			return
+		}
+		writeJSONError(w, stdhttp.StatusInternalServerError, "internal", err.Error())
+		return
+	}
+	if fu.Status != "running" {
+		writeJSONError(w, stdhttp.StatusConflict, "fleet_update_not_running",
+			"fleet update is not in the running state")
+		return
+	}
+	if err := s.deps.FleetWorker.Cancel(r.Context(), fuID); err != nil {
+		writeJSONError(w, stdhttp.StatusInternalServerError, "internal", err.Error())
+		return
+	}
+	_ = s.deps.Store.AppendAudit(r.Context(), store.AuditEntry{
+		ID: ulid.Make().String(), UserID: &user.ID, Actor: "user",
+		Action:     "fleet.update_cancelled",
+		TargetKind: ptr("fleet_update"), TargetID: &fuID,
+		TS: time.Now().UTC(),
+	})
+	w.WriteHeader(stdhttp.StatusNoContent)
+}
+
+// handleAPIFleetUpdateGet is GET /api/fleet-updates/{id}.
+func (s *Server) handleAPIFleetUpdateGet(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	if _, ok := s.requireUser(r); !ok {
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
+		return
+	}
+	fuID := chi.URLParam(r, "id")
+	fu, hosts, err := s.deps.Store.GetFleetUpdate(r.Context(), fuID)
+	if err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			writeJSONError(w, stdhttp.StatusNotFound, "fleet_update_not_found", "")
+			return
+		}
+		writeJSONError(w, stdhttp.StatusInternalServerError, "internal", err.Error())
+		return
+	}
+	names := s.hostNameMap(r)
+	view := fleetUpdateView{
+		ID:              fu.ID,
+		StartedAt:       fu.StartedAt.UTC().Format(time.RFC3339Nano),
+		StartedByUserID: fu.StartedByUserID,
+		TargetVersion:   fu.TargetVersion,
+		Status:          fu.Status,
+		CurrentHostID:   fu.CurrentHostID,
+		HaltedReason:    fu.HaltedReason,
+		Hosts:           make([]fleetUpdateHostView, 0, len(hosts)),
+	}
+	if fu.CompletedAt != nil {
+		s := fu.CompletedAt.UTC().Format(time.RFC3339Nano)
+		view.CompletedAt = &s
+	}
+	for _, h := range hosts {
+		view.Hosts = append(view.Hosts, fleetUpdateHostView{
+			HostID:       h.HostID,
+			HostName:     names[h.HostID],
+			Position:     h.Position,
+			Status:       h.Status,
+			JobID:        h.JobID,
+			FailedReason: h.FailedReason,
+		})
+	}
+	writeJSON(w, stdhttp.StatusOK, view)
+}
+
+// handleUIFleetUpdate renders /settings/fleet-update.
+func (s *Server) handleUIFleetUpdate(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	u := s.requireUIUser(w, r)
+	if u == nil {
+		return
+	}
+	page, err := s.buildFleetUpdatePage(r)
+	if err != nil {
+		slog.Error("ui fleet update: build page", "err", err)
+		stdhttp.Error(w, "internal", stdhttp.StatusInternalServerError)
+		return
+	}
+	view := s.baseView(r, u)
+	view.Title = "Fleet update · restic-manager"
+	view.Active = "settings"
+	view.Page = page
+	if err := s.deps.UI.Render(w, "fleet_update", view); err != nil {
+		slog.Error("ui fleet update: render", "err", err)
+	}
+}
+
+// handleUIFleetUpdatePartial renders just the inner panel for htmx
+// auto-refresh polling — same data, no chrome.
+func (s *Server) handleUIFleetUpdatePartial(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	u := s.requireUIUser(w, r)
+	if u == nil {
+		return
+	}
+	page, err := s.buildFleetUpdatePage(r)
+	if err != nil {
+		slog.Error("ui fleet update partial: build page", "err", err)
+		stdhttp.Error(w, "internal", stdhttp.StatusInternalServerError)
+		return
+	}
+	view := s.baseView(r, u)
+	view.Page = page
+	if err := s.deps.UI.RenderPartial(w, "fleet_update_inner", view); err != nil {
+		slog.Error("ui fleet update partial: render", "err", err)
+	}
+}
+
+// buildFleetUpdatePage assembles the data both /settings/fleet-update
+// and its partial render against. Resolves the most-recent fleet
+// update (active OR completed/cancelled/halted) so the page can show
+// the last roll's result instead of disappearing into "idle" the
+// instant a roll finishes.
+func (s *Server) buildFleetUpdatePage(r *stdhttp.Request) (fleetUpdatePage, error) {
+	page := fleetUpdatePage{
+		TargetVersion: version.Version,
+		HostNames:     map[string]string{},
+		PollURL:       "/settings/fleet-update/partial",
+	}
+	hosts, err := s.deps.Store.ListHosts(r.Context())
+	if err != nil {
+		return page, err
+	}
+	for _, h := range hosts {
+		page.HostNames[h.ID] = h.Name
+	}
+
+	active, err := s.deps.Store.ActiveFleetUpdate(r.Context())
+	if err != nil {
+		return page, err
+	}
+	mostRecent := active
+	if mostRecent == nil {
+		// Fall back to the most recent terminal row so the page can
+		// show "completed" / "halted" / "cancelled" once the worker
+		// finishes. One small bespoke query — keeps the page from
+		// flashing back to "idle" the instant a roll wraps up.
+		var id string
+		err := s.deps.Store.DB().QueryRowContext(r.Context(),
+			`SELECT id FROM fleet_updates ORDER BY started_at DESC LIMIT 1`).
+			Scan(&id)
+		if err == nil {
+			fu, _, gerr := s.deps.Store.GetFleetUpdate(r.Context(), id)
+			if gerr == nil {
+				mostRecent = fu
+			}
+		}
+	}
+
+	if mostRecent != nil {
+		_, rows, gerr := s.deps.Store.GetFleetUpdate(r.Context(), mostRecent.ID)
+		if gerr == nil {
+			page.Active = mostRecent
+			page.ActiveRows = make([]fleetUpdateHostView, 0, len(rows))
+			for _, hr := range rows {
+				page.ActiveRows = append(page.ActiveRows, fleetUpdateHostView{
+					HostID:       hr.HostID,
+					HostName:     page.HostNames[hr.HostID],
+					Position:     hr.Position,
+					Status:       hr.Status,
+					JobID:        hr.JobID,
+					FailedReason: hr.FailedReason,
+				})
+			}
+		}
+	}
+
+	// Idle list (or "still out of date" reference even when an active
+	// roll is running — cheap to compute, harmless to attach).
+	for _, h := range hosts {
+		if h.Status != "online" {
+			continue
+		}
+		if h.AgentVersion == "" || h.AgentVersion == page.TargetVersion {
+			continue
+		}
+		page.OutOfDateHosts = append(page.OutOfDateHosts, h)
+	}
+	return page, nil
+}
+
+// deriveOutOfDateOnlineHostIDs returns the list of host IDs that
+// (a) are online (Hub.Connected) and (b) have an agent_version that's
+// non-empty AND != target. Used by the start endpoint when the caller
+// omits host_ids.
+func (s *Server) deriveOutOfDateOnlineHostIDs(ctx context.Context, target string) ([]string, error) {
+	hosts, err := s.deps.Store.ListHosts(ctx)
+	if err != nil {
+		return nil, err
+	}
+	out := []string{}
+	for _, h := range hosts {
+		if h.AgentVersion == "" || h.AgentVersion == target {
+			continue
+		}
+		if !s.deps.Hub.Connected(h.ID) {
+			continue
+		}
+		out = append(out, h.ID)
+	}
+	return out, nil
+}
+
+// hostNameMap returns hostID → name; used to hydrate fleet-update
+// JSON responses.
+func (s *Server) hostNameMap(r *stdhttp.Request) map[string]string {
+	out := map[string]string{}
+	hosts, err := s.deps.Store.ListHosts(r.Context())
+	if err != nil {
+		return out
+	}
+	for _, h := range hosts {
+		out[h.ID] = h.Name
+	}
+	return out
+}
diff --git a/internal/server/http/fleet_update_test.go b/internal/server/http/fleet_update_test.go
new file mode 100644
index 0000000..ca82561
--- /dev/null
+++ b/internal/server/http/fleet_update_test.go
@@ -0,0 +1,334 @@
+// fleet_update_test.go — coverage for the P6-15 fleet-update HTTP
+// surface: start/cancel/get JSON endpoints + RBAC.
+package http
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	stdhttp "net/http"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ws"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
+)
+
+// fakeFleetWorker stands in for *fleetupdate.Worker in HTTP tests.
+// It records what was passed to Start/Cancel and lets tests inject
+// canned errors. Satisfies the FleetWorker interface in
+// host_update.go.
+type fakeFleetWorker struct {
+	mu sync.Mutex
+
+	startCalls []fakeStartCall
+	startID    string
+	startErr   error
+
+	cancelCalls []string
+	cancelErr   error
+}
+
+type fakeStartCall struct {
+	UserID  string
+	Target  string
+	HostIDs []string
+}
+
+func (f *fakeFleetWorker) Start(_ context.Context, userID, target string, hostIDs []string) (string, error) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.startCalls = append(f.startCalls, fakeStartCall{userID, target, append([]string(nil), hostIDs...)})
+	if f.startErr != nil {
+		return "", f.startErr
+	}
+	return f.startID, nil
+}
+
+func (f *fakeFleetWorker) Cancel(_ context.Context, id string) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.cancelCalls = append(f.cancelCalls, id)
+	return f.cancelErr
+}
+
+// helloOnlineHost is the smallest setup that lets the dispatch /
+// derivation logic see a host as "online + version mismatch".
+// Returns the host id.
+func helloOnlineHost(t *testing.T, srv *Server, st *store.Store, name, agentVer string) string {
+	t.Helper()
+	id := makeHost(t, st, name)
+	if err := st.MarkHostHello(context.Background(), id, agentVer, "0.17", api.CurrentProtocolVersion, time.Now().UTC()); err != nil {
+		t.Fatalf("mark hello: %v", err)
+	}
+	// Mark connected on the hub so deriveOutOfDateOnlineHostIDs
+	// considers it online without needing a real WS handshake. The
+	// Conn has a nil websocket pointer — tests never call Send on it.
+	srv.deps.Hub.Register(id, ws.NewConn(id, nil))
+	return id
+}
+
+func TestFleetUpdateStartHappyPath(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	worker := &fakeFleetWorker{startID: ulid.Make().String()}
+	srv.deps.FleetWorker = worker
+
+	cookie, uid := loginAsAdminWithID(t, st)
+	hostID := helloOnlineHost(t, srv, st, "fu-host", "v0")
+
+	body := map[string]any{"host_ids": []string{hostID}}
+	raw, _ := json.Marshal(body)
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/fleet/update", bytes.NewReader(raw))
+	req.AddCookie(cookie)
+	req.Header.Set("Content-Type", "application/json")
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusAccepted {
+		t.Fatalf("status: got %d, want 202", res.StatusCode)
+	}
+	var out struct {
+		FleetUpdateID string `json:"fleet_update_id"`
+	}
+	if err := json.NewDecoder(res.Body).Decode(&out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if out.FleetUpdateID != worker.startID {
+		t.Fatalf("fleet_update_id: got %q, want %q", out.FleetUpdateID, worker.startID)
+	}
+	worker.mu.Lock()
+	if len(worker.startCalls) != 1 || worker.startCalls[0].UserID != uid {
+		t.Fatalf("start calls: %+v", worker.startCalls)
+	}
+	if got := worker.startCalls[0].HostIDs; len(got) != 1 || got[0] != hostID {
+		t.Fatalf("host_ids: %v", got)
+	}
+	worker.mu.Unlock()
+
+	// Audit row.
+	var n int
+	if err := st.DB().QueryRow(
+		`SELECT COUNT(*) FROM audit_log WHERE action = 'fleet.update_started' AND target_id = ?`,
+		out.FleetUpdateID).Scan(&n); err != nil {
+		t.Fatalf("audit count: %v", err)
+	}
+	if n != 1 {
+		t.Fatalf("audit rows: got %d, want 1", n)
+	}
+}
+
+func TestFleetUpdateStartConflictWhenAlreadyRunning(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	worker := &fakeFleetWorker{startErr: store.ErrFleetUpdateRunning}
+	srv.deps.FleetWorker = worker
+	cookie := loginAsAdmin(t, st)
+	_ = helloOnlineHost(t, srv, st, "fu-host", "v0")
+
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/fleet/update", bytes.NewReader([]byte(`{}`)))
+	req.AddCookie(cookie)
+	req.Header.Set("Content-Type", "application/json")
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusConflict {
+		t.Fatalf("status: got %d, want 409", res.StatusCode)
+	}
+	body := readJSONError(t, res.Body)
+	if body.Code != "fleet_update_in_progress" {
+		t.Fatalf("code: %q", body.Code)
+	}
+}
+
+func TestFleetUpdateStartDerivesHostIDsWhenEmpty(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	worker := &fakeFleetWorker{startID: ulid.Make().String()}
+	srv.deps.FleetWorker = worker
+	cookie := loginAsAdmin(t, st)
+
+	// Two online + out-of-date, one online + at-target, one offline.
+	a := helloOnlineHost(t, srv, st, "behind-a", "v0")
+	b := helloOnlineHost(t, srv, st, "behind-b", "v0")
+	_ = helloOnlineHost(t, srv, st, "uptodate", version.Version)
+	offlineID := makeHost(t, st, "offline-host")
+	if err := st.MarkHostHello(context.Background(), offlineID, "v0", "0.17", api.CurrentProtocolVersion, time.Now().UTC()); err != nil {
+		t.Fatalf("mark hello: %v", err)
+	}
+	// Don't MarkOnline → derivation should skip.
+
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/fleet/update", bytes.NewReader([]byte(`{}`)))
+	req.AddCookie(cookie)
+	req.Header.Set("Content-Type", "application/json")
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusAccepted {
+		t.Fatalf("status: got %d, want 202", res.StatusCode)
+	}
+	worker.mu.Lock()
+	defer worker.mu.Unlock()
+	if len(worker.startCalls) != 1 {
+		t.Fatalf("start calls: %d", len(worker.startCalls))
+	}
+	got := worker.startCalls[0].HostIDs
+	want := map[string]bool{a: true, b: true}
+	if len(got) != 2 || !want[got[0]] || !want[got[1]] {
+		t.Fatalf("derived host_ids: got %v, want both of %v", got, []string{a, b})
+	}
+}
+
+func TestFleetUpdateCancelHappyPath(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	worker := &fakeFleetWorker{}
+	srv.deps.FleetWorker = worker
+	cookie := loginAsAdmin(t, st)
+
+	// Seed a running fleet update directly.
+	fuID := ulid.Make().String()
+	uid := ulid.Make().String()
+	if err := st.CreateUser(context.Background(), store.User{
+		ID: uid, Username: "starter", PasswordHash: "x",
+		Role: store.RoleAdmin, CreatedAt: time.Now().UTC(),
+	}); err != nil {
+		t.Fatalf("seed user: %v", err)
+	}
+	hostID := makeHost(t, st, "fu-cancel-host")
+	if err := st.CreateFleetUpdate(context.Background(),
+		store.FleetUpdate{ID: fuID, StartedByUserID: uid, TargetVersion: "v1"},
+		[]string{hostID}); err != nil {
+		t.Fatalf("seed fleet update: %v", err)
+	}
+
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/fleet-updates/"+fuID+"/cancel", nil)
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusNoContent {
+		t.Fatalf("status: got %d, want 204", res.StatusCode)
+	}
+	worker.mu.Lock()
+	if len(worker.cancelCalls) != 1 || worker.cancelCalls[0] != fuID {
+		t.Fatalf("cancel calls: %v", worker.cancelCalls)
+	}
+	worker.mu.Unlock()
+}
+
+func TestFleetUpdateCancelNotRunning(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	srv.deps.FleetWorker = &fakeFleetWorker{}
+	cookie := loginAsAdmin(t, st)
+
+	// Seed + complete one so it's no longer running.
+	fuID := ulid.Make().String()
+	uid := ulid.Make().String()
+	_ = st.CreateUser(context.Background(), store.User{
+		ID: uid, Username: "starter2", PasswordHash: "x",
+		Role: store.RoleAdmin, CreatedAt: time.Now().UTC(),
+	})
+	hostID := makeHost(t, st, "fu-done-host")
+	_ = st.CreateFleetUpdate(context.Background(),
+		store.FleetUpdate{ID: fuID, StartedByUserID: uid, TargetVersion: "v1"},
+		[]string{hostID})
+	if err := st.CompleteFleetUpdate(context.Background(), fuID, time.Now().UTC()); err != nil {
+		t.Fatalf("complete: %v", err)
+	}
+
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/fleet-updates/"+fuID+"/cancel", nil)
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusConflict {
+		t.Fatalf("status: got %d, want 409", res.StatusCode)
+	}
+	body := readJSONError(t, res.Body)
+	if body.Code != "fleet_update_not_running" {
+		t.Fatalf("code: %q", body.Code)
+	}
+}
+
+func TestFleetUpdateGetHydrates(t *testing.T) {
+	t.Parallel()
+	_, ts, st := rawTestServer(t)
+	cookie := loginAsAdmin(t, st)
+
+	uid := ulid.Make().String()
+	_ = st.CreateUser(context.Background(), store.User{
+		ID: uid, Username: "starter3", PasswordHash: "x",
+		Role: store.RoleAdmin, CreatedAt: time.Now().UTC(),
+	})
+	hostID := makeHost(t, st, "fu-get-host")
+	fuID := ulid.Make().String()
+	if err := st.CreateFleetUpdate(context.Background(),
+		store.FleetUpdate{ID: fuID, StartedByUserID: uid, TargetVersion: "v1.2.3"},
+		[]string{hostID}); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+
+	req, _ := stdhttp.NewRequest("GET", ts.URL+"/api/fleet-updates/"+fuID, nil)
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusOK {
+		t.Fatalf("status: got %d, want 200", res.StatusCode)
+	}
+	var got fleetUpdateView
+	if err := json.NewDecoder(res.Body).Decode(&got); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if got.ID != fuID || got.TargetVersion != "v1.2.3" || got.Status != "running" {
+		t.Fatalf("parent: %+v", got)
+	}
+	if len(got.Hosts) != 1 || got.Hosts[0].HostID != hostID || got.Hosts[0].HostName != "fu-get-host" {
+		t.Fatalf("hosts: %+v", got.Hosts)
+	}
+}
+
+func TestFleetUpdateRBAC(t *testing.T) {
+	t.Parallel()
+	_, ts, st := rawTestServer(t)
+
+	for _, role := range []store.Role{store.RoleViewer, store.RoleOperator} {
+		role := role
+		t.Run(string(role), func(t *testing.T) {
+			cookie := loginAsRole(t, st, role)
+			req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/fleet/update", bytes.NewReader([]byte(`{}`)))
+			req.AddCookie(cookie)
+			req.Header.Set("Content-Type", "application/json")
+			res, err := stdhttp.DefaultClient.Do(req)
+			if err != nil {
+				t.Fatalf("do: %v", err)
+			}
+			defer res.Body.Close()
+			if res.StatusCode != stdhttp.StatusForbidden {
+				t.Fatalf("status: got %d, want 403", res.StatusCode)
+			}
+		})
+	}
+}
+
+// Sanity check that fakeFleetWorker satisfies the FleetWorker iface.
+var _ FleetWorker = (*fakeFleetWorker)(nil)
diff --git a/internal/server/http/host_update.go b/internal/server/http/host_update.go
new file mode 100644
index 0000000..b1a2033
--- /dev/null
+++ b/internal/server/http/host_update.go
@@ -0,0 +1,217 @@
+package http
+
+import (
+	"context"
+	"encoding/json"
+	stdhttp "net/http"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
+)
+
+// UpdateWatcher is the slim view of the ws.updateWatcher this package
+// uses for tracking in-flight update dispatches. Defined as an
+// interface so a test can inject a stub.
+type UpdateWatcher interface {
+	Track(jobID, hostID string)
+}
+
+// FleetWorker is the slim view of the fleetupdate.Worker this package
+// uses. Kept here for forward compatibility with P6-15 — the host
+// update endpoint itself does not use it.
+type FleetWorker interface {
+	Start(ctx context.Context, userID, targetVersion string, hostIDs []string) (string, error)
+	Cancel(ctx context.Context, fleetUpdateID string) error
+}
+
+// dispatchHostUpdateResult communicates structured outcomes from the
+// shared dispatch path so both the HTTP handler and the fleet worker
+// can format errors in their own idiom.
+type dispatchHostUpdateResult struct {
+	JobID  string
+	Code   string // "" on success
+	Status int    // HTTP status the JSON handler should use on error
+	Msg    string // human-readable detail (optional)
+}
+
+// dispatchHostUpdate is the shared "send command.update to one host"
+// path. It performs every pre-check (host exists, online, version
+// mismatch, no in-flight update) and on success creates the jobs row,
+// audits, dispatches the WS envelope, and tracks the watcher entry.
+//
+// Pre-checks are returned as structured codes rather than HTTP errors
+// so the fleet worker can map them onto its own per-host status enum
+// without parsing strings.
+func (s *Server) dispatchHostUpdate(ctx context.Context, hostID string, actorKind string, actorID *string) dispatchHostUpdateResult {
+	host, err := s.deps.Store.GetHost(ctx, hostID)
+	if err != nil || host == nil {
+		return dispatchHostUpdateResult{Code: "host_not_found", Status: stdhttp.StatusNotFound}
+	}
+	if !s.deps.Hub.Connected(host.ID) {
+		return dispatchHostUpdateResult{
+			Code: "host_offline", Status: stdhttp.StatusConflict,
+			Msg: "agent is not currently connected",
+		}
+	}
+	if host.AgentVersion != "" && host.AgentVersion == version.Version {
+		return dispatchHostUpdateResult{
+			Code: "already_up_to_date", Status: stdhttp.StatusConflict,
+			Msg: "agent already running version " + version.Version,
+		}
+	}
+	existing, err := s.deps.Store.RunningUpdateJobForHost(ctx, hostID)
+	if err != nil {
+		return dispatchHostUpdateResult{Code: "internal", Status: stdhttp.StatusInternalServerError, Msg: err.Error()}
+	}
+	if existing != "" {
+		return dispatchHostUpdateResult{
+			Code: "update_in_progress", Status: stdhttp.StatusConflict,
+			Msg:   "an update job is already in flight for this host",
+			JobID: existing,
+		}
+	}
+
+	jobID := ulid.Make().String()
+	now := time.Now().UTC()
+	if err := s.deps.Store.CreateJob(ctx, store.Job{
+		ID: jobID, HostID: hostID, Kind: "update",
+		ActorKind: actorKind, ActorID: actorID,
+		CreatedAt: now,
+	}); err != nil {
+		return dispatchHostUpdateResult{Code: "internal", Status: stdhttp.StatusInternalServerError, Msg: err.Error()}
+	}
+	env, err := api.Marshal(api.MsgCommandUpdate, ulid.Make().String(), api.CommandUpdatePayload{
+		JobID: jobID,
+	})
+	if err != nil {
+		return dispatchHostUpdateResult{Code: "internal", Status: stdhttp.StatusInternalServerError, Msg: err.Error()}
+	}
+	if err := s.deps.Hub.Send(ctx, hostID, env); err != nil {
+		// Roll the job to failed so we don't leak a queued row.
+		_ = s.deps.Store.MarkJobFinished(ctx, jobID, "failed", -1, nil, err.Error(), time.Now().UTC())
+		return dispatchHostUpdateResult{
+			Code: "host_offline", Status: stdhttp.StatusConflict, Msg: err.Error(),
+		}
+	}
+	if s.deps.UpdateWatcher != nil {
+		s.deps.UpdateWatcher.Track(jobID, hostID)
+	}
+
+	auditPayload, _ := json.Marshal(map[string]string{
+		"job_id":         jobID,
+		"target_version": version.Version,
+	})
+	_ = s.deps.Store.AppendAudit(ctx, store.AuditEntry{
+		ID:         ulid.Make().String(),
+		UserID:     actorID,
+		Actor:      actorKind,
+		Action:     "host.update_dispatched",
+		TargetKind: ptr("host"),
+		TargetID:   &hostID,
+		TS:         now,
+		Payload:    auditPayload,
+	})
+
+	return dispatchHostUpdateResult{JobID: jobID}
+}
+
+// handleHostUpdate is POST /api/hosts/{id}/update — JSON, admin-only.
+func (s *Server) handleHostUpdate(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	user, ok := s.requireUser(r)
+	if !ok {
+		writeJSONError(w, stdhttp.StatusUnauthorized, "unauthorised", "")
+		return
+	}
+	hostID := chi.URLParam(r, "id")
+	if hostID == "" {
+		writeJSONError(w, stdhttp.StatusBadRequest, "missing_host_id", "")
+		return
+	}
+	actor := "user"
+	var actorID *string
+	if user != nil {
+		actorID = &user.ID
+	}
+	res := s.dispatchHostUpdate(r.Context(), hostID, actor, actorID)
+	if res.Code != "" {
+		writeJSONError(w, res.Status, res.Code, res.Msg)
+		return
+	}
+	writeJSON(w, stdhttp.StatusAccepted, map[string]string{"job_id": res.JobID})
+}
+
+// handleHostUpdateForm is the HTMX-friendly POST /hosts/{id}/update
+// variant. On success it sets HX-Redirect to the job detail page; on
+// pre-check failures it renders an inline error banner.
+func (s *Server) handleHostUpdateForm(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	user, ok := s.requireUser(r)
+	if !ok {
+		stdhttp.Error(w, "unauthorised", stdhttp.StatusUnauthorized)
+		return
+	}
+	hostID := chi.URLParam(r, "id")
+	if hostID == "" {
+		stdhttp.Error(w, "missing host_id", stdhttp.StatusBadRequest)
+		return
+	}
+	actor := "user"
+	var actorID *string
+	if user != nil {
+		actorID = &user.ID
+	}
+	res := s.dispatchHostUpdate(r.Context(), hostID, actor, actorID)
+	if res.Code != "" {
+		// Inline banner for HTMX swaps. Mirrors what host_credentials
+		// returns on validation errors — small text/html fragment.
+		w.Header().Set("Content-Type", "text/html; charset=utf-8")
+		w.WriteHeader(res.Status)
+		msg := hostUpdateErrorMessage(res.Code, res.Msg)
+		_, _ = w.Write([]byte(`<div class="banner banner-error" role="alert">` + htmlEscape(msg) + `</div>`))
+		return
+	}
+	w.Header().Set("HX-Redirect", "/jobs/"+res.JobID)
+	w.WriteHeader(stdhttp.StatusOK)
+}
+
+func hostUpdateErrorMessage(code, msg string) string {
+	switch code {
+	case "host_not_found":
+		return "Host not found."
+	case "host_offline":
+		return "Agent is offline; can't deliver the update command."
+	case "already_up_to_date":
+		return "Agent is already running the current version."
+	case "update_in_progress":
+		return "An update is already in progress for this host."
+	}
+	if msg != "" {
+		return msg
+	}
+	return "Update dispatch failed."
+}
+
+// htmlEscape is a minimal HTML-attr-safe escaper. Avoids pulling html/template
+// for a one-shot inline banner.
+func htmlEscape(s string) string {
+	out := make([]byte, 0, len(s))
+	for i := 0; i < len(s); i++ {
+		switch s[i] {
+		case '&':
+			out = append(out, []byte("&amp;")...)
+		case '<':
+			out = append(out, []byte("&lt;")...)
+		case '>':
+			out = append(out, []byte("&gt;")...)
+		case '"':
+			out = append(out, []byte("&quot;")...)
+		default:
+			out = append(out, s[i])
+		}
+	}
+	return string(out)
+}
diff --git a/internal/server/http/host_update_test.go b/internal/server/http/host_update_test.go
new file mode 100644
index 0000000..30cc0ce
--- /dev/null
+++ b/internal/server/http/host_update_test.go
@@ -0,0 +1,270 @@
+// host_update_test.go — covers POST /api/hosts/{id}/update.
+package http
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	stdhttp "net/http"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/coder/websocket"
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
+)
+
+// stubWatcher records Track calls so tests can assert the watcher was
+// notified.
+type stubWatcher struct {
+	mu      sync.Mutex
+	tracked []string // hostIDs
+}
+
+func (s *stubWatcher) Track(_, hostID string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.tracked = append(s.tracked, hostID)
+}
+
+func TestHostUpdateHappyPath(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	watcher := &stubWatcher{}
+	srv.deps.UpdateWatcher = watcher
+	hostID, token := enrolHostForWS(t, srv, st, "upd-host")
+	c := agentDial(t, srv, ts, hostID, token)
+	sendHello(t, c, "upd-host")
+	_ = drainUntil(t, c, api.MsgScheduleSet)
+
+	// Force a version mismatch so the dispatch isn't short-circuited.
+	if err := st.MarkHostHello(context.Background(), hostID, "v0", "0.17", api.CurrentProtocolVersion, time.Now().UTC()); err != nil {
+		t.Fatalf("mark hello: %v", err)
+	}
+
+	cookie := loginAsAdmin(t, st)
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/hosts/"+hostID+"/update", nil)
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusAccepted {
+		t.Fatalf("status: got %d, want 202", res.StatusCode)
+	}
+	var out struct {
+		JobID string `json:"job_id"`
+	}
+	if err := json.NewDecoder(res.Body).Decode(&out); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if out.JobID == "" {
+		t.Fatal("missing job_id in response")
+	}
+
+	// command.update envelope arrives.
+	deadline := time.Now().Add(2 * time.Second)
+	var got api.Envelope
+	for time.Now().Before(deadline) {
+		ctx, cancel := context.WithTimeout(context.Background(), 500*time.Millisecond)
+		mt, raw, rerr := c.Read(ctx)
+		cancel()
+		if rerr != nil {
+			break
+		}
+		if mt != websocket.MessageText {
+			continue
+		}
+		if !strings.Contains(string(raw), `"command.update"`) {
+			continue
+		}
+		_ = json.Unmarshal(raw, &got)
+		break
+	}
+	if got.Type != api.MsgCommandUpdate {
+		t.Fatal("never received command.update envelope")
+	}
+	var cp api.CommandUpdatePayload
+	if err := got.UnmarshalPayload(&cp); err != nil {
+		t.Fatalf("payload: %v", err)
+	}
+	if cp.JobID != out.JobID {
+		t.Fatalf("payload job_id: got %q want %q", cp.JobID, out.JobID)
+	}
+
+	// Watcher tracked.
+	watcher.mu.Lock()
+	defer watcher.mu.Unlock()
+	if len(watcher.tracked) != 1 || watcher.tracked[0] != hostID {
+		t.Fatalf("watcher tracked: %v", watcher.tracked)
+	}
+
+	// Audit row exists.
+	var n int
+	if err := st.DB().QueryRow(
+		`SELECT COUNT(*) FROM audit_log WHERE action = 'host.update_dispatched' AND target_id = ?`,
+		hostID).Scan(&n); err != nil {
+		t.Fatalf("audit count: %v", err)
+	}
+	if n != 1 {
+		t.Fatalf("audit rows: got %d, want 1", n)
+	}
+}
+
+func TestHostUpdateNotFound(t *testing.T) {
+	t.Parallel()
+	_, ts, st := rawTestServer(t)
+	cookie := loginAsAdmin(t, st)
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/hosts/no-such/update", nil)
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusNotFound {
+		t.Fatalf("status: got %d want 404", res.StatusCode)
+	}
+}
+
+func TestHostUpdateOffline(t *testing.T) {
+	t.Parallel()
+	_, ts, st := rawTestServer(t)
+	hostID := ulid.Make().String()
+	if err := st.CreateHost(context.Background(), store.Host{
+		ID: hostID, Name: "off", OS: "linux", Arch: "amd64",
+		EnrolledAt: time.Now().UTC(),
+	}, "deadbeef", ""); err != nil {
+		t.Fatalf("create: %v", err)
+	}
+	cookie := loginAsAdmin(t, st)
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/hosts/"+hostID+"/update", nil)
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusConflict {
+		t.Fatalf("status: got %d want 409", res.StatusCode)
+	}
+	body := readJSONError(t, res.Body)
+	if body.Code != "host_offline" {
+		t.Fatalf("code: %q", body.Code)
+	}
+}
+
+func TestHostUpdateAlreadyUpToDate(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	hostID, token := enrolHostForWS(t, srv, st, "uptodate-host")
+	c := agentDial(t, srv, ts, hostID, token)
+	sendHello(t, c, "uptodate-host")
+	_ = drainUntil(t, c, api.MsgScheduleSet)
+
+	// Force agent_version == version.Version.
+	if err := st.MarkHostHello(context.Background(), hostID, version.Version, "0.17", api.CurrentProtocolVersion, time.Now().UTC()); err != nil {
+		t.Fatalf("mark hello: %v", err)
+	}
+
+	cookie := loginAsAdmin(t, st)
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/hosts/"+hostID+"/update", nil)
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusConflict {
+		t.Fatalf("status: got %d want 409", res.StatusCode)
+	}
+	body := readJSONError(t, res.Body)
+	if body.Code != "already_up_to_date" {
+		t.Fatalf("code: %q", body.Code)
+	}
+}
+
+func TestHostUpdateInProgress(t *testing.T) {
+	t.Parallel()
+	srv, ts, st := rawTestServer(t)
+	hostID, token := enrolHostForWS(t, srv, st, "inprog-host")
+	c := agentDial(t, srv, ts, hostID, token)
+	sendHello(t, c, "inprog-host")
+	_ = drainUntil(t, c, api.MsgScheduleSet)
+	if err := st.MarkHostHello(context.Background(), hostID, "v0", "0.17", api.CurrentProtocolVersion, time.Now().UTC()); err != nil {
+		t.Fatalf("mark hello: %v", err)
+	}
+
+	// Pre-seed an in-flight update job.
+	jobID := ulid.Make().String()
+	if err := st.CreateJob(context.Background(), store.Job{
+		ID: jobID, HostID: hostID, Kind: "update",
+		ActorKind: "user", CreatedAt: time.Now().UTC(),
+	}); err != nil {
+		t.Fatalf("seed job: %v", err)
+	}
+
+	cookie := loginAsAdmin(t, st)
+	req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/hosts/"+hostID+"/update", nil)
+	req.AddCookie(cookie)
+	res, err := stdhttp.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("do: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusConflict {
+		t.Fatalf("status: got %d want 409", res.StatusCode)
+	}
+	body := readJSONError(t, res.Body)
+	if body.Code != "update_in_progress" {
+		t.Fatalf("code: %q", body.Code)
+	}
+}
+
+func TestHostUpdateRBAC(t *testing.T) {
+	t.Parallel()
+	_, ts, st := rawTestServer(t)
+	hostID := ulid.Make().String()
+	if err := st.CreateHost(context.Background(), store.Host{
+		ID: hostID, Name: "rbac-host", OS: "linux", Arch: "amd64",
+		EnrolledAt: time.Now().UTC(),
+	}, "deadbeef", ""); err != nil {
+		t.Fatalf("create: %v", err)
+	}
+	for _, role := range []store.Role{store.RoleViewer, store.RoleOperator} {
+		role := role
+		t.Run(string(role), func(t *testing.T) {
+			cookie := loginAsRole(t, st, role)
+			req, _ := stdhttp.NewRequest("POST", ts.URL+"/api/hosts/"+hostID+"/update", nil)
+			req.AddCookie(cookie)
+			res, err := stdhttp.DefaultClient.Do(req)
+			if err != nil {
+				t.Fatalf("do: %v", err)
+			}
+			defer res.Body.Close()
+			if res.StatusCode != stdhttp.StatusForbidden {
+				t.Fatalf("status for %s: got %d want 403", role, res.StatusCode)
+			}
+		})
+	}
+}
+
+type jsonErrBody struct {
+	Code    string `json:"code"`
+	Message string `json:"message,omitempty"`
+}
+
+func readJSONError(t *testing.T, body io.Reader) jsonErrBody {
+	t.Helper()
+	var out jsonErrBody
+	if err := json.NewDecoder(body).Decode(&out); err != nil {
+		t.Fatalf("decode error body: %v", err)
+	}
+	return out
+}
diff --git a/internal/server/http/hosts.go b/internal/server/http/hosts.go
index 59f913b..d355626 100644
--- a/internal/server/http/hosts.go
+++ b/internal/server/http/hosts.go
@@ -4,6 +4,7 @@ import (
 	stdhttp "net/http"
 
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
 )
 
 // hostView is the JSON projection of a Host row. Same shape as the
@@ -27,6 +28,8 @@ type hostView struct {
 	RepoSizeBytes    int64    `json:"repo_size_bytes"`
 	SnapshotCount    int      `json:"snapshot_count"`
 	OpenAlertCount   int      `json:"open_alert_count"`
+	UpdateAvailable  bool     `json:"update_available"`
+	TargetVersion    string   `json:"target_version,omitempty"`
 }
 
 // handleListHosts returns the full fleet as JSON. Authenticated; the
@@ -85,6 +88,8 @@ func hostToView(h store.Host) hostView {
 		RepoSizeBytes:    h.RepoSizeBytes,
 		SnapshotCount:    h.SnapshotCount,
 		OpenAlertCount:   h.OpenAlertCount,
+		TargetVersion:    version.Version,
+		UpdateAvailable:  h.AgentVersion != "" && h.AgentVersion != version.Version,
 	}
 	if v.Tags == nil {
 		v.Tags = []string{}
diff --git a/internal/server/http/server.go b/internal/server/http/server.go
index 5db2438..17ecc7a 100644
--- a/internal/server/http/server.go
+++ b/internal/server/http/server.go
@@ -39,6 +39,13 @@ type Deps struct {
 	// NotificationHub (optional, wired in G1) is used by the test-fire
 	// endpoint to dispatch a single synthetic payload through a channel.
 	NotificationHub *notification.Hub
+	// UpdateWatcher tracks in-flight agent self-update dispatches and
+	// reconciles them against incoming hello envelopes. Optional;
+	// nil = no-op (handlers degrade by skipping the Track call).
+	UpdateWatcher UpdateWatcher
+	// FleetWorker drives the rolling fleet-update worker. Optional;
+	// nil = fleet update endpoints (P6-15) report unavailable.
+	FleetWorker FleetWorker
 	// Version is the binary's build version, surfaced in the chrome.
 	// Empty falls back to "dev".
 	Version string
@@ -123,8 +130,9 @@ func (s *Server) routes(r chi.Router) {
 	r.Post("/api/agents/announce", s.handleAnnounce)
 	r.Get("/agent/binary", s.handleAgentBinary)
 	r.Get("/install/*", s.handleInstallAsset)
+	r.Get("/api/version", s.handleVersion)
 	if s.deps.Hub != nil {
-		r.Mount("/ws/agent", ws.AgentHandler(ws.HandlerDeps{
+		hd := ws.HandlerDeps{
 			Hub:            s.deps.Hub,
 			Store:          s.deps.Store,
 			JobHub:         s.deps.JobHub,
@@ -132,7 +140,11 @@ func (s *Server) routes(r chi.Router) {
 			OnHello:        s.onAgentHello,
 			OnScheduleAck:  s.applyScheduleAck,
 			OnScheduleFire: s.dispatchScheduledJob,
-		}))
+		}
+		if w, ok := s.deps.UpdateWatcher.(*ws.UpdateWatcher); ok && w != nil {
+			hd.UpdateWatcher = w
+		}
+		r.Mount("/ws/agent", ws.AgentHandler(hd))
 	}
 	r.Get("/ws/agent/pending", s.handlePendingWS)
 	r.Mount("/static/", staticHandler())
@@ -270,6 +282,14 @@ func (s *Server) routes(r chi.Router) {
 	r.Group(func(r chi.Router) {
 		r.Use(s.requireRole(store.RoleAdmin))
 
+		r.Post("/api/hosts/{id}/update", s.handleHostUpdate)
+		r.Post("/hosts/{id}/update", s.handleHostUpdateForm)
+
+		// Fleet update (P6-15): rolling update across many hosts.
+		r.Post("/api/fleet/update", s.handleAPIFleetUpdateStart)
+		r.Post("/api/fleet-updates/{id}/cancel", s.handleAPIFleetUpdateCancel)
+		r.Get("/api/fleet-updates/{id}", s.handleAPIFleetUpdateGet)
+
 		r.Get("/api/users", s.handleAPIUsersList)
 		r.Post("/api/users", s.handleAPIUserCreate)
 		r.Get("/api/users/{id}", s.handleAPIUserGet)
@@ -283,6 +303,8 @@ func (s *Server) routes(r chi.Router) {
 		if s.deps.UI != nil {
 			r.Post("/hosts/{id}/delete", s.handleUIHostDelete)
 			r.Get("/settings", s.handleUISettings)
+			r.Get("/settings/fleet-update", s.handleUIFleetUpdate)
+			r.Get("/settings/fleet-update/partial", s.handleUIFleetUpdatePartial)
 			r.Get("/settings/users", s.handleUIUsersList)
 			r.Get("/settings/users/new", s.handleUIUserNewGet)
 			r.Post("/settings/users/new", s.handleUIUserNewPost)
@@ -321,6 +343,27 @@ func (s *Server) Shutdown(ctx context.Context) error {
 	return s.srv.Shutdown(ctx)
 }
 
+// SetFleetWorker installs the fleet-update worker post-construction.
+// Used to break the wiring loop in cmd/server (the worker depends on a
+// dispatcher that delegates back into the server's host-update path).
+func (s *Server) SetFleetWorker(fw FleetWorker) { s.deps.FleetWorker = fw }
+
+// DispatchHostUpdate is the public entry point for callers (the fleet
+// worker) that need to drive the same dispatch path the HTTP handler
+// uses, without going through HTTP. Returns the structured result so
+// the caller can map error codes to its own status enum.
+func (s *Server) DispatchHostUpdate(ctx context.Context, hostID, actorUserID string) (jobID string, code string, err error) {
+	var actorID *string
+	if actorUserID != "" {
+		actorID = &actorUserID
+	}
+	res := s.dispatchHostUpdate(ctx, hostID, "user", actorID)
+	if res.Code != "" {
+		return res.JobID, res.Code, nil
+	}
+	return res.JobID, "", nil
+}
+
 // Addr returns the configured listen address. Useful in tests when
 // the caller passes :0 to get a random port.
 func (s *Server) Addr() string { return s.srv.Addr }
diff --git a/internal/server/http/ui_handlers.go b/internal/server/http/ui_handlers.go
index ffa75d2..c569c27 100644
--- a/internal/server/http/ui_handlers.go
+++ b/internal/server/http/ui_handlers.go
@@ -23,6 +23,7 @@ import (
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ui"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/server/ws"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
 	"gitea.dcglab.co.uk/steve/restic-manager/web"
 )
 
@@ -155,6 +156,10 @@ type dashboardPage struct {
 	// when it's already active). Pre-computed so the template stays
 	// dumb.
 	SortURL map[string]string
+	// UpdatesBehind is the count of online hosts whose agent_version
+	// trails the server. Surfaces as the dashboard "N hosts behind"
+	// hero tile and links to ?updates=behind.
+	UpdatesBehind int
 }
 
 // dashboardFilter holds the parsed query-string filter state.
@@ -165,6 +170,10 @@ type dashboardFilter struct {
 	Tag        string // mirrors ActiveTag for round-trip on links
 	Sort       string // column key (see sortDashboard)
 	Dir        string // "asc" | "desc"
+	// Updates narrows to hosts whose agent is behind the server's
+	// version. Only valid value today is "behind"; empty means no
+	// filter.
+	Updates string
 }
 
 // dashboardHostRow carries a host plus the per-row Run-now decision
@@ -180,6 +189,13 @@ type dashboardHostRow struct {
 	// NextRun is the next-fire time of RunAllScheduleID (when set),
 	// computed server-side from its cron. nil otherwise.
 	NextRun *time.Time
+	// UpdateAvailable is true when the host's agent has connected at
+	// least once AND its agent_version differs from the server's. Used
+	// by the host_row partial to render the update-available chip.
+	UpdateAvailable bool
+	// TargetVersion is the server's build version, surfaced in the
+	// chip's tooltip and label.
+	TargetVersion string
 }
 
 // pickRunAllSchedule returns the ID of the single schedule whose
@@ -255,7 +271,11 @@ func (s *Server) handleUIDashboard(w stdhttp.ResponseWriter, r *stdhttp.Request)
 	// calls per host — fine at fleet sizes we care about.
 	rows := make([]dashboardHostRow, 0, len(hosts))
 	for _, h := range hosts {
-		row := dashboardHostRow{Host: h}
+		row := dashboardHostRow{
+			Host:            h,
+			TargetVersion:   version.Version,
+			UpdateAvailable: h.AgentVersion != "" && h.AgentVersion != version.Version,
+		}
 		groups, gerr := s.deps.Store.ListSourceGroupsByHost(r.Context(), h.ID)
 		if gerr != nil {
 			slog.Warn("ui dashboard: list source groups", "host_id", h.ID, "err", gerr)
@@ -289,6 +309,13 @@ func (s *Server) handleUIDashboard(w stdhttp.ResponseWriter, r *stdhttp.Request)
 		critOpenCount = len(crit)
 	}
 
+	updatesBehind := 0
+	for _, h := range allHosts {
+		if h.Status == "online" && h.AgentVersion != "" && h.AgentVersion != version.Version {
+			updatesBehind++
+		}
+	}
+
 	view := s.baseView(r, u)
 	view.Page = dashboardPage{
 		Hosts:         rows,
@@ -302,6 +329,7 @@ func (s *Server) handleUIDashboard(w stdhttp.ResponseWriter, r *stdhttp.Request)
 		Filter:        filter,
 		RefreshURL:    "/?" + filter.encode(),
 		SortURL:       buildDashboardSortURLs(filter),
+		UpdatesBehind: updatesBehind,
 	}
 	if err := s.deps.UI.Render(w, "dashboard", view); err != nil {
 		slog.Error("ui: render dashboard", "err", err)
@@ -320,6 +348,7 @@ func parseDashboardFilter(q url.Values) dashboardFilter {
 		Tag:        q.Get("tag"),
 		Sort:       q.Get("sort"),
 		Dir:        q.Get("dir"),
+		Updates:    q.Get("updates"),
 	}
 	if f.Sort == "" {
 		f.Sort = "name"
@@ -352,6 +381,9 @@ func (f dashboardFilter) encode() string {
 	if f.Dir != "" && f.Dir != "asc" {
 		v.Set("dir", f.Dir)
 	}
+	if f.Updates != "" {
+		v.Set("updates", f.Updates)
+	}
 	return v.Encode()
 }
 
@@ -402,6 +434,11 @@ func filterAndSortDashboardHosts(hosts []store.Host, f dashboardFilter) []store.
 				continue
 			}
 		}
+		if f.Updates == "behind" {
+			if h.AgentVersion == "" || h.AgentVersion == version.Version {
+				continue
+			}
+		}
 		out = append(out, h)
 	}
 	sortDashboardHosts(out, f.Sort, f.Dir)
@@ -809,6 +846,20 @@ type hostChromeData struct {
 	SourceGroupCount int
 	ScheduleCount    int
 	ScheduleVersion  int64 // host_schedule_version (latest desired)
+	// UpdateAvailable + TargetVersion drive the agent-out-of-date chip
+	// in the host detail header. UpdateAvailable is true iff the host
+	// has connected at least once AND its agent_version != server's.
+	UpdateAvailable bool
+	TargetVersion   string
+	// Online + UpdateInProgress drive the per-host "Update agent"
+	// button on host_detail. Online mirrors hub.Connected; pulled here
+	// so the button can disable when the host is unreachable.
+	Online           bool
+	UpdateInProgress bool
+	// CanAdmin is true when the viewing user has admin role; used to
+	// gate the "Update agent" button. Kept on the chrome struct so any
+	// page reusing host_chrome already has it for free.
+	CanAdmin bool
 	// KnownTags is the union of tags already in use across the fleet,
 	// used for autocomplete on the host-tags edit form. Cheap query.
 	KnownTags []string
@@ -834,6 +885,14 @@ type hostChromeData struct {
 // render the page with stale counts than 500 the whole tab.
 func (s *Server) loadHostChrome(r *stdhttp.Request, host store.Host, subtab, crumb string) hostChromeData {
 	d := hostChromeData{Host: host, SubTab: subtab, Crumb: crumb}
+	d.TargetVersion = version.Version
+	d.UpdateAvailable = host.AgentVersion != "" && host.AgentVersion != version.Version
+	if s.deps.Hub != nil {
+		d.Online = s.deps.Hub.Connected(host.ID)
+	}
+	if existing, _ := s.deps.Store.RunningUpdateJobForHost(r.Context(), host.ID); existing != "" {
+		d.UpdateInProgress = true
+	}
 	if groups, err := s.deps.Store.ListSourceGroupsByHost(r.Context(), host.ID); err == nil {
 		d.SourceGroupCount = len(groups)
 	} else {
@@ -972,8 +1031,10 @@ func (s *Server) handleUIHostDetail(w stdhttp.ResponseWriter, r *stdhttp.Request
 
 	view := s.baseView(r, u)
 	view.Title = host.Name + " · restic-manager"
+	chrome := s.loadHostChrome(r, *host, "snapshots", "snapshots")
+	chrome.CanAdmin = u.Role == string(store.RoleAdmin)
 	view.Page = hostDetailPage{
-		hostChromeData: s.loadHostChrome(r, *host, "snapshots", "snapshots"),
+		hostChromeData: chrome,
 		Snapshots:      shown,
 		SnapshotsShown: len(shown),
 		LegacyRestic:   !restic.Env{Version: host.ResticVersion}.AtLeastVersion(0, 17),
diff --git a/internal/server/http/version.go b/internal/server/http/version.go
new file mode 100644
index 0000000..33cc17f
--- /dev/null
+++ b/internal/server/http/version.go
@@ -0,0 +1,20 @@
+package http
+
+import (
+	"encoding/json"
+	stdhttp "net/http"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
+)
+
+// handleVersion exposes the server's build-time identifying constants
+// (set via -ldflags). Public-band — no secrets surface here, the agent
+// updater compares its own agent_version byte-for-byte against the
+// Version field to drive the "out of date" signal.
+func (s *Server) handleVersion(w stdhttp.ResponseWriter, r *stdhttp.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	_ = json.NewEncoder(w).Encode(map[string]string{
+		"version": version.Version,
+		"commit":  version.Commit,
+	})
+}
diff --git a/internal/server/http/version_test.go b/internal/server/http/version_test.go
new file mode 100644
index 0000000..b012818
--- /dev/null
+++ b/internal/server/http/version_test.go
@@ -0,0 +1,42 @@
+package http
+
+import (
+	"encoding/json"
+	stdhttp "net/http"
+	"testing"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
+)
+
+func TestVersionEndpoint(t *testing.T) {
+	t.Parallel()
+
+	prevV, prevC := version.Version, version.Commit
+	version.Version = "v9.9.9-test"
+	version.Commit = "abc1234"
+	t.Cleanup(func() {
+		version.Version = prevV
+		version.Commit = prevC
+	})
+
+	_, url, _ := newTestServerWithHub(t)
+
+	res, err := stdhttp.Get(url + "/api/version")
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	defer res.Body.Close()
+	if res.StatusCode != stdhttp.StatusOK {
+		t.Fatalf("status: got %d want 200", res.StatusCode)
+	}
+	var body map[string]string
+	if err := json.NewDecoder(res.Body).Decode(&body); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if body["version"] != "v9.9.9-test" {
+		t.Fatalf("version: got %q", body["version"])
+	}
+	if body["commit"] != "abc1234" {
+		t.Fatalf("commit: got %q", body["commit"])
+	}
+}
diff --git a/internal/server/ui/ui.go b/internal/server/ui/ui.go
index 3b3c446..45e5af7 100644
--- a/internal/server/ui/ui.go
+++ b/internal/server/ui/ui.go
@@ -108,6 +108,8 @@ func New() (*Renderer, error) {
 		"templates/partials/tree_node.html",
 		"templates/partials/alert_row.html",
 		"templates/partials/crit_banner.html",
+		"templates/partials/fleet_update_inner.html",
+		"templates/partials/host_update_chip.html",
 	}
 
 	pageEntries, err := fs.Glob(web.FS, "templates/pages/*.html")
diff --git a/internal/server/ws/handler.go b/internal/server/ws/handler.go
index df74332..312f568 100644
--- a/internal/server/ws/handler.go
+++ b/internal/server/ws/handler.go
@@ -16,6 +16,7 @@ import (
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/api"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/auth"
 	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/version"
 )
 
 // HandlerDeps is the set of collaborators the agent WS handler needs.
@@ -26,6 +27,9 @@ type HandlerDeps struct {
 	// AlertEngine receives job-finished and host-online events so the
 	// alert engine can evaluate its rules. Optional; nil = no-op.
 	AlertEngine *alert.Engine
+	// UpdateWatcher reconciles in-flight agent-update dispatches against
+	// hello envelopes. Optional; nil = no-op.
+	UpdateWatcher *UpdateWatcher
 	// OnHello is called once per successful hello, after the host row
 	// has been touched and the conn registered. Used by the HTTP
 	// layer to push host_credentials down as a config.update before
@@ -147,6 +151,9 @@ func runAgentLoop(ctx context.Context, c *Conn, hostID string, deps HandlerDeps)
 	if deps.AlertEngine != nil {
 		deps.AlertEngine.NotifyHostOnline(hostID)
 	}
+	if deps.UpdateWatcher != nil {
+		deps.UpdateWatcher.OnHello(ctx, hostID, helloPayload.AgentVersion, version.Version)
+	}
 
 	deps.Hub.Register(hostID, c)
 	defer deps.Hub.Unregister(hostID, c)
diff --git a/internal/server/ws/update_watch.go b/internal/server/ws/update_watch.go
new file mode 100644
index 0000000..be2fef8
--- /dev/null
+++ b/internal/server/ws/update_watch.go
@@ -0,0 +1,151 @@
+package ws
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"sync"
+	"time"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+)
+
+// updateTimeout bounds how long the watcher waits for an agent to come
+// back with its new version after a command.update dispatch. var (not
+// const) so tests can shrink it.
+var updateTimeout = 90 * time.Second
+
+// AlertRaiser is the slim subset of *alert.Engine the update watcher
+// touches. Defined here (not in the alert package) so the dependency
+// arrow points the right way.
+type AlertRaiser interface {
+	RaiseUpdateFailed(ctx context.Context, hostID, jobID, reason string, when time.Time)
+	ResolveUpdateFailed(ctx context.Context, hostID string, when time.Time)
+}
+
+// UpdateWatcher tracks in-flight agent-update dispatches and reconciles
+// them against incoming hello envelopes. Entries land on Track and
+// resolve via OnHello (success path) or the periodic sweep (timeout).
+type UpdateWatcher struct {
+	store  *store.Store
+	alerts AlertRaiser
+
+	mu      sync.Mutex
+	entries map[string]*updateEntry // hostID → entry
+
+	tickPeriod time.Duration
+}
+
+type updateEntry struct {
+	jobID     string
+	startedAt time.Time
+	// terminated is set once the entry has reached a terminal state so
+	// late OnHellos don't resurrect it.
+	terminated bool
+}
+
+// NewUpdateWatcher builds an unstarted watcher. Call Run in a goroutine
+// to start the periodic sweep.
+func NewUpdateWatcher(st *store.Store, alerts AlertRaiser) *UpdateWatcher {
+	return &UpdateWatcher{
+		store:      st,
+		alerts:     alerts,
+		entries:    make(map[string]*updateEntry),
+		tickPeriod: 5 * time.Second,
+	}
+}
+
+// Track registers a freshly-dispatched update job. A subsequent Track
+// for the same host replaces the prior entry (last-write-wins).
+func (w *UpdateWatcher) Track(jobID, hostID string) {
+	if w == nil {
+		return
+	}
+	w.mu.Lock()
+	w.entries[hostID] = &updateEntry{jobID: jobID, startedAt: time.Now()}
+	w.mu.Unlock()
+}
+
+// OnHello is called by the WS handler after a successful hello has been
+// persisted. If a tracked update for the host matches the targetVersion,
+// the job is marked succeeded and any open update_failed alert is
+// auto-resolved. A non-matching version is a no-op (the watcher keeps
+// waiting until the timeout).
+func (w *UpdateWatcher) OnHello(ctx context.Context, hostID, agentVersion, targetVersion string) {
+	if w == nil {
+		return
+	}
+	w.mu.Lock()
+	e, ok := w.entries[hostID]
+	if !ok || e.terminated {
+		w.mu.Unlock()
+		return
+	}
+	if agentVersion != targetVersion {
+		// Not the version we asked for — keep waiting.
+		w.mu.Unlock()
+		return
+	}
+	e.terminated = true
+	jobID := e.jobID
+	delete(w.entries, hostID)
+	w.mu.Unlock()
+
+	now := time.Now().UTC()
+	if err := w.store.MarkJobFinished(ctx, jobID, "succeeded", 0, nil, "", now); err != nil {
+		slog.Warn("ws update watcher: mark succeeded", "job_id", jobID, "host_id", hostID, "err", err)
+	}
+	if w.alerts != nil {
+		w.alerts.ResolveUpdateFailed(ctx, hostID, now)
+	}
+}
+
+// Run drives the periodic sweep. Returns when ctx is done.
+func (w *UpdateWatcher) Run(ctx context.Context) {
+	if w == nil {
+		return
+	}
+	t := time.NewTicker(w.tickPeriod)
+	defer t.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case now := <-t.C:
+			w.sweep(ctx, now)
+		}
+	}
+}
+
+func (w *UpdateWatcher) sweep(ctx context.Context, now time.Time) {
+	type expired struct {
+		hostID string
+		jobID  string
+		age    time.Duration
+	}
+	var toFail []expired
+	w.mu.Lock()
+	for hostID, e := range w.entries {
+		if e.terminated {
+			continue
+		}
+		if now.Sub(e.startedAt) >= updateTimeout {
+			toFail = append(toFail, expired{hostID: hostID, jobID: e.jobID, age: now.Sub(e.startedAt)})
+			e.terminated = true
+			delete(w.entries, hostID)
+		}
+	}
+	w.mu.Unlock()
+
+	for _, x := range toFail {
+		reason := fmt.Sprintf("timeout: agent did not reconnect within %s", updateTimeout)
+		stamp := now.UTC()
+		errMsg := reason
+		if err := w.store.MarkJobFinished(ctx, x.jobID, "failed", -1, nil, errMsg, stamp); err != nil {
+			slog.Warn("ws update watcher: mark failed", "job_id", x.jobID, "host_id", x.hostID, "err", err)
+		}
+		if w.alerts != nil {
+			w.alerts.RaiseUpdateFailed(ctx, x.hostID, x.jobID, reason, stamp)
+		}
+	}
+}
diff --git a/internal/server/ws/update_watch_test.go b/internal/server/ws/update_watch_test.go
new file mode 100644
index 0000000..4081501
--- /dev/null
+++ b/internal/server/ws/update_watch_test.go
@@ -0,0 +1,161 @@
+package ws
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/oklog/ulid/v2"
+
+	"gitea.dcglab.co.uk/steve/restic-manager/internal/store"
+)
+
+type fakeAlerts struct {
+	mu       sync.Mutex
+	raised   []string // hostIDs
+	resolved []string
+	reasons  []string
+}
+
+func (f *fakeAlerts) RaiseUpdateFailed(_ context.Context, hostID, _ /*jobID*/, reason string, _ time.Time) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.raised = append(f.raised, hostID)
+	f.reasons = append(f.reasons, reason)
+}
+
+func (f *fakeAlerts) ResolveUpdateFailed(_ context.Context, hostID string, _ time.Time) {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	f.resolved = append(f.resolved, hostID)
+}
+
+func seedJob(t *testing.T, st *store.Store, hostID string) string {
+	t.Helper()
+	jobID := ulid.Make().String()
+	if err := st.CreateJob(context.Background(), store.Job{
+		ID: jobID, HostID: hostID, Kind: "update",
+		ActorKind: "user", CreatedAt: time.Now().UTC(),
+	}); err != nil {
+		t.Fatalf("create job: %v", err)
+	}
+	return jobID
+}
+
+func TestUpdateWatcherOnHelloSuccess(t *testing.T) {
+	st := openWSTestStore(t)
+	hostID := ulid.Make().String()
+	seedHostWS(t, st, hostID)
+	jobID := seedJob(t, st, hostID)
+
+	a := &fakeAlerts{}
+	w := NewUpdateWatcher(st, a)
+	w.Track(jobID, hostID)
+
+	w.OnHello(context.Background(), hostID, "v2", "v2")
+
+	job, err := st.GetJob(context.Background(), jobID)
+	if err != nil {
+		t.Fatalf("get job: %v", err)
+	}
+	if job.Status != "succeeded" {
+		t.Fatalf("status: got %q want succeeded", job.Status)
+	}
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if len(a.resolved) != 1 || a.resolved[0] != hostID {
+		t.Fatalf("resolve calls: %v", a.resolved)
+	}
+	if len(a.raised) != 0 {
+		t.Fatalf("unexpected raises: %v", a.raised)
+	}
+}
+
+func TestUpdateWatcherTimeout(t *testing.T) {
+	prev := updateTimeout
+	updateTimeout = 50 * time.Millisecond
+	t.Cleanup(func() { updateTimeout = prev })
+
+	st := openWSTestStore(t)
+	hostID := ulid.Make().String()
+	seedHostWS(t, st, hostID)
+	jobID := seedJob(t, st, hostID)
+
+	a := &fakeAlerts{}
+	w := NewUpdateWatcher(st, a)
+	w.Track(jobID, hostID)
+
+	time.Sleep(80 * time.Millisecond)
+	w.sweep(context.Background(), time.Now())
+
+	job, err := st.GetJob(context.Background(), jobID)
+	if err != nil {
+		t.Fatalf("get job: %v", err)
+	}
+	if job.Status != "failed" {
+		t.Fatalf("status: got %q want failed", job.Status)
+	}
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if len(a.raised) != 1 || a.raised[0] != hostID {
+		t.Fatalf("raise calls: %v", a.raised)
+	}
+	if len(a.reasons) == 0 || a.reasons[0] == "" {
+		t.Fatalf("missing reason")
+	}
+}
+
+func TestUpdateWatcherMismatchedVersionNoOp(t *testing.T) {
+	st := openWSTestStore(t)
+	hostID := ulid.Make().String()
+	seedHostWS(t, st, hostID)
+	jobID := seedJob(t, st, hostID)
+
+	a := &fakeAlerts{}
+	w := NewUpdateWatcher(st, a)
+	w.Track(jobID, hostID)
+
+	w.OnHello(context.Background(), hostID, "v1", "v2")
+
+	job, _ := st.GetJob(context.Background(), jobID)
+	if job.Status == "succeeded" || job.Status == "failed" {
+		t.Fatalf("status flipped on mismatched hello: %q", job.Status)
+	}
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if len(a.raised) != 0 || len(a.resolved) != 0 {
+		t.Fatalf("unexpected alert calls raised=%v resolved=%v", a.raised, a.resolved)
+	}
+}
+
+func TestUpdateWatcherHelloAfterTimeoutIsNoOp(t *testing.T) {
+	prev := updateTimeout
+	updateTimeout = 50 * time.Millisecond
+	t.Cleanup(func() { updateTimeout = prev })
+
+	st := openWSTestStore(t)
+	hostID := ulid.Make().String()
+	seedHostWS(t, st, hostID)
+	jobID := seedJob(t, st, hostID)
+
+	a := &fakeAlerts{}
+	w := NewUpdateWatcher(st, a)
+	w.Track(jobID, hostID)
+
+	time.Sleep(80 * time.Millisecond)
+	w.sweep(context.Background(), time.Now())
+
+	// Hello arrives after sweep — entry already gone, must be no-op.
+	w.OnHello(context.Background(), hostID, "v2", "v2")
+
+	job, _ := st.GetJob(context.Background(), jobID)
+	if job.Status != "failed" {
+		t.Fatalf("status flipped from failed → %q", job.Status)
+	}
+	a.mu.Lock()
+	defer a.mu.Unlock()
+	if len(a.resolved) != 0 {
+		t.Fatalf("late hello triggered ResolveUpdateFailed: %v", a.resolved)
+	}
+}
diff --git a/internal/store/alerts.go b/internal/store/alerts.go
index b12d6fa..f16b9bc 100644
--- a/internal/store/alerts.go
+++ b/internal/store/alerts.go
@@ -77,6 +77,56 @@ func (s *Store) RaiseOrTouch(ctx context.Context, hostID, kind, dedupKey, severi
 	return id, true, nil
 }
 
+// RaiseOrTouchSystem is the host-less variant of RaiseOrTouch — the
+// alert row's host_id is stored as NULL, so the FK to hosts is bypassed.
+// Used by fleet-wide alerts (e.g. fleet_update_halted) where the
+// failure surface isn't pinned to a single host.
+func (s *Store) RaiseOrTouchSystem(ctx context.Context, kind, dedupKey, severity, message string, when time.Time) (id string, didRaise bool, err error) {
+	tx, err := s.db.BeginTx(ctx, nil)
+	if err != nil {
+		return "", false, fmt.Errorf("store: begin: %w", err)
+	}
+	defer func() { _ = tx.Rollback() }()
+
+	row := tx.QueryRowContext(ctx,
+		`SELECT id FROM alerts
+		   WHERE host_id IS NULL AND kind = ? AND dedup_key = ? AND resolved_at IS NULL
+		   LIMIT 1`,
+		kind, dedupKey)
+	var existing string
+	switch err := row.Scan(&existing); {
+	case err == nil:
+		_, uerr := tx.ExecContext(ctx,
+			`UPDATE alerts SET last_seen_at = ?, message = ? WHERE id = ?`,
+			when.UTC().Format(time.RFC3339Nano), message, existing)
+		if uerr != nil {
+			return "", false, fmt.Errorf("store: touch alert: %w", uerr)
+		}
+		if err := tx.Commit(); err != nil {
+			return "", false, err
+		}
+		return existing, false, nil
+	case errors.Is(err, sql.ErrNoRows):
+		// fall through to insert
+	default:
+		return "", false, fmt.Errorf("store: lookup alert: %w", err)
+	}
+
+	id = ulid.Make().String()
+	whenStr := when.UTC().Format(time.RFC3339Nano)
+	_, err = tx.ExecContext(ctx,
+		`INSERT INTO alerts (id, host_id, kind, dedup_key, severity, message, created_at, last_seen_at)
+		 VALUES (?, NULL, ?, ?, ?, ?, ?, ?)`,
+		id, kind, dedupKey, severity, message, whenStr, whenStr)
+	if err != nil {
+		return "", false, fmt.Errorf("store: insert alert: %w", err)
+	}
+	if err := tx.Commit(); err != nil {
+		return "", false, err
+	}
+	return id, true, nil
+}
+
 // refreshHostOpenAlertCount recomputes hosts.open_alert_count from the
 // alerts table for one host. Self-healing: idempotent and survives
 // out-of-order edits. Best-effort — errors are returned but callers
diff --git a/internal/store/fleet_updates.go b/internal/store/fleet_updates.go
new file mode 100644
index 0000000..ae9fec2
--- /dev/null
+++ b/internal/store/fleet_updates.go
@@ -0,0 +1,258 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+	"fmt"
+	"time"
+)
+
+// ErrFleetUpdateRunning is returned by CreateFleetUpdate if another
+// fleet update is already in 'running' state. The HTTP layer surfaces
+// this as a 409 with a structured error code.
+var ErrFleetUpdateRunning = errors.New("store: fleet update already running")
+
+// CreateFleetUpdate inserts the parent row and one pending child per
+// hostID, in the order given (position = index). Returns
+// ErrFleetUpdateRunning if a fleet update is already in flight.
+func (st *Store) CreateFleetUpdate(ctx context.Context, fu FleetUpdate, hostIDs []string) error {
+	if fu.ID == "" || fu.StartedByUserID == "" || fu.TargetVersion == "" {
+		return errors.New("store: fleet update id, user_id, target_version required")
+	}
+	if fu.Status == "" {
+		fu.Status = "running"
+	}
+	if fu.StartedAt.IsZero() {
+		fu.StartedAt = time.Now().UTC()
+	}
+	tx, err := st.db.BeginTx(ctx, nil)
+	if err != nil {
+		return fmt.Errorf("store: begin: %w", err)
+	}
+	defer func() { _ = tx.Rollback() }()
+
+	var existing string
+	if err := tx.QueryRowContext(ctx,
+		`SELECT id FROM fleet_updates WHERE status = 'running' LIMIT 1`).
+		Scan(&existing); err == nil {
+		return fmt.Errorf("%w: %s", ErrFleetUpdateRunning, existing)
+	} else if !errors.Is(err, sql.ErrNoRows) {
+		return fmt.Errorf("store: check active fleet update: %w", err)
+	}
+
+	if _, err := tx.ExecContext(ctx,
+		`INSERT INTO fleet_updates (id, started_at, started_by_user_id, target_version, status)
+		 VALUES (?, ?, ?, ?, ?)`,
+		fu.ID, fu.StartedAt.UTC().Format(time.RFC3339Nano), fu.StartedByUserID, fu.TargetVersion, fu.Status,
+	); err != nil {
+		return fmt.Errorf("store: insert fleet_updates: %w", err)
+	}
+	for i, hid := range hostIDs {
+		if _, err := tx.ExecContext(ctx,
+			`INSERT INTO fleet_update_hosts (fleet_update_id, host_id, position, status)
+			 VALUES (?, ?, ?, 'pending')`,
+			fu.ID, hid, i,
+		); err != nil {
+			return fmt.Errorf("store: insert fleet_update_hosts: %w", err)
+		}
+	}
+	return tx.Commit()
+}
+
+// ActiveFleetUpdate returns the currently-running fleet update or nil.
+func (st *Store) ActiveFleetUpdate(ctx context.Context) (*FleetUpdate, error) {
+	var fu FleetUpdate
+	var startedAt string
+	var current sql.NullString
+	var halted sql.NullString
+	var completedAt sql.NullString
+	err := st.db.QueryRowContext(ctx,
+		`SELECT id, started_at, started_by_user_id, target_version, status,
+		   current_host_id, halted_reason, completed_at
+		 FROM fleet_updates WHERE status = 'running' LIMIT 1`).
+		Scan(&fu.ID, &startedAt, &fu.StartedByUserID, &fu.TargetVersion, &fu.Status,
+			&current, &halted, &completedAt)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("store: active fleet update: %w", err)
+	}
+	fu.StartedAt, _ = time.Parse(time.RFC3339Nano, startedAt)
+	fu.CurrentHostID = current.String
+	fu.HaltedReason = halted.String
+	if completedAt.Valid {
+		t, _ := time.Parse(time.RFC3339Nano, completedAt.String)
+		fu.CompletedAt = &t
+	}
+	return &fu, nil
+}
+
+// GetFleetUpdate hydrates parent + ordered child rows. Returns
+// ErrNotFound on missing id.
+func (st *Store) GetFleetUpdate(ctx context.Context, id string) (*FleetUpdate, []FleetUpdateHost, error) {
+	var fu FleetUpdate
+	var startedAt string
+	var current sql.NullString
+	var halted sql.NullString
+	var completedAt sql.NullString
+	err := st.db.QueryRowContext(ctx,
+		`SELECT id, started_at, started_by_user_id, target_version, status,
+		   current_host_id, halted_reason, completed_at
+		 FROM fleet_updates WHERE id = ?`, id).
+		Scan(&fu.ID, &startedAt, &fu.StartedByUserID, &fu.TargetVersion, &fu.Status,
+			&current, &halted, &completedAt)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, nil, ErrNotFound
+	}
+	if err != nil {
+		return nil, nil, fmt.Errorf("store: get fleet update: %w", err)
+	}
+	fu.StartedAt, _ = time.Parse(time.RFC3339Nano, startedAt)
+	fu.CurrentHostID = current.String
+	fu.HaltedReason = halted.String
+	if completedAt.Valid {
+		t, _ := time.Parse(time.RFC3339Nano, completedAt.String)
+		fu.CompletedAt = &t
+	}
+
+	rows, err := st.db.QueryContext(ctx,
+		`SELECT host_id, position, status, COALESCE(job_id, ''), COALESCE(failed_reason, '')
+		 FROM fleet_update_hosts
+		 WHERE fleet_update_id = ?
+		 ORDER BY position`, id)
+	if err != nil {
+		return nil, nil, fmt.Errorf("store: list fleet hosts: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+	out := []FleetUpdateHost{}
+	for rows.Next() {
+		fh := FleetUpdateHost{FleetUpdateID: id}
+		if err := rows.Scan(&fh.HostID, &fh.Position, &fh.Status, &fh.JobID, &fh.FailedReason); err != nil {
+			return nil, nil, fmt.Errorf("store: scan fleet host: %w", err)
+		}
+		out = append(out, fh)
+	}
+	return &fu, out, rows.Err()
+}
+
+// ListPendingFleetUpdateHosts returns rows with status='pending' for
+// this fleet update, in position order. The worker calls this to
+// pick the next host to dispatch.
+func (st *Store) ListPendingFleetUpdateHosts(ctx context.Context, fuID string) ([]FleetUpdateHost, error) {
+	rows, err := st.db.QueryContext(ctx,
+		`SELECT host_id, position, status, COALESCE(job_id, ''), COALESCE(failed_reason, '')
+		 FROM fleet_update_hosts
+		 WHERE fleet_update_id = ? AND status = 'pending'
+		 ORDER BY position`, fuID)
+	if err != nil {
+		return nil, fmt.Errorf("store: list pending fleet hosts: %w", err)
+	}
+	defer func() { _ = rows.Close() }()
+	out := []FleetUpdateHost{}
+	for rows.Next() {
+		fh := FleetUpdateHost{FleetUpdateID: fuID}
+		if err := rows.Scan(&fh.HostID, &fh.Position, &fh.Status, &fh.JobID, &fh.FailedReason); err != nil {
+			return nil, err
+		}
+		out = append(out, fh)
+	}
+	return out, rows.Err()
+}
+
+// SetFleetUpdateHostStatus moves one row through pending → running →
+// {succeeded, failed, skipped}. failedReason and jobID may be empty
+// (e.g. on succeeded). Empty values are stored as NULL so subsequent
+// reads round-trip cleanly via COALESCE.
+func (st *Store) SetFleetUpdateHostStatus(ctx context.Context, fuID, hostID, status, failedReason, jobID string) error {
+	_, err := st.db.ExecContext(ctx,
+		`UPDATE fleet_update_hosts
+		 SET status = ?, failed_reason = ?, job_id = COALESCE(?, job_id)
+		 WHERE fleet_update_id = ? AND host_id = ?`,
+		status, nullableString(failedReason), nullableString(jobID),
+		fuID, hostID,
+	)
+	if err != nil {
+		return fmt.Errorf("store: set fleet host status: %w", err)
+	}
+	return nil
+}
+
+// SetFleetUpdateCurrentHost stamps which host the worker is actively
+// waiting on. Pass empty string to clear.
+func (st *Store) SetFleetUpdateCurrentHost(ctx context.Context, fuID, hostID string) error {
+	_, err := st.db.ExecContext(ctx,
+		`UPDATE fleet_updates SET current_host_id = ? WHERE id = ?`,
+		nullableString(hostID), fuID,
+	)
+	if err != nil {
+		return fmt.Errorf("store: set fleet current host: %w", err)
+	}
+	return nil
+}
+
+// HaltFleetUpdate flips status to 'halted', stamps the reason, and
+// clears current_host_id.
+func (st *Store) HaltFleetUpdate(ctx context.Context, fuID, reason string, when time.Time) error {
+	_, err := st.db.ExecContext(ctx,
+		`UPDATE fleet_updates
+		 SET status = 'halted', halted_reason = ?, current_host_id = NULL,
+		     completed_at = ?
+		 WHERE id = ? AND status = 'running'`,
+		reason, when.UTC().Format(time.RFC3339Nano), fuID,
+	)
+	if err != nil {
+		return fmt.Errorf("store: halt fleet update: %w", err)
+	}
+	return nil
+}
+
+// CancelFleetUpdate flips status to 'cancelled'. Caller checks that
+// the row is still 'running' before calling.
+func (st *Store) CancelFleetUpdate(ctx context.Context, fuID string, when time.Time) error {
+	_, err := st.db.ExecContext(ctx,
+		`UPDATE fleet_updates
+		 SET status = 'cancelled', current_host_id = NULL, completed_at = ?
+		 WHERE id = ? AND status = 'running'`,
+		when.UTC().Format(time.RFC3339Nano), fuID,
+	)
+	if err != nil {
+		return fmt.Errorf("store: cancel fleet update: %w", err)
+	}
+	return nil
+}
+
+// CompleteFleetUpdate flips status to 'completed' once every host has
+// reached a terminal state.
+func (st *Store) CompleteFleetUpdate(ctx context.Context, fuID string, when time.Time) error {
+	_, err := st.db.ExecContext(ctx,
+		`UPDATE fleet_updates
+		 SET status = 'completed', current_host_id = NULL, completed_at = ?
+		 WHERE id = ? AND status = 'running'`,
+		when.UTC().Format(time.RFC3339Nano), fuID,
+	)
+	if err != nil {
+		return fmt.Errorf("store: complete fleet update: %w", err)
+	}
+	return nil
+}
+
+// RunningUpdateJobForHost returns the id of any in-flight (queued or
+// running) `update` job for hostID, or "" + nil if none. Used by the
+// host-update HTTP handler to refuse double-dispatch and by the
+// fleet worker to dedupe on retry.
+func (st *Store) RunningUpdateJobForHost(ctx context.Context, hostID string) (string, error) {
+	var id string
+	err := st.db.QueryRowContext(ctx,
+		`SELECT id FROM jobs
+		 WHERE host_id = ? AND kind = 'update' AND status IN ('queued','running')
+		 ORDER BY created_at DESC LIMIT 1`, hostID).Scan(&id)
+	if errors.Is(err, sql.ErrNoRows) {
+		return "", nil
+	}
+	if err != nil {
+		return "", fmt.Errorf("store: running update job: %w", err)
+	}
+	return id, nil
+}
diff --git a/internal/store/fleet_updates_test.go b/internal/store/fleet_updates_test.go
new file mode 100644
index 0000000..9942411
--- /dev/null
+++ b/internal/store/fleet_updates_test.go
@@ -0,0 +1,180 @@
+package store
+
+import (
+	"context"
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/oklog/ulid/v2"
+)
+
+func ptrStr(s string) *string { return &s }
+
+func seedFleetUser(t *testing.T, s *Store) string {
+	t.Helper()
+	id := ulid.Make().String()
+	if err := s.CreateUser(context.Background(), User{
+		ID: id, Username: "u-" + id[:6], PasswordHash: "x", Role: RoleAdmin,
+	}); err != nil {
+		t.Fatalf("create user: %v", err)
+	}
+	return id
+}
+
+func seedFleetHost(t *testing.T, s *Store, name string) string {
+	t.Helper()
+	id := ulid.Make().String()
+	if err := s.CreateHost(context.Background(), Host{
+		ID: id, Name: name, OS: "linux", Arch: "amd64",
+		EnrolledAt: time.Now().UTC(),
+	}, "tokenhash-"+id[:6], ""); err != nil {
+		t.Fatalf("create host: %v", err)
+	}
+	return id
+}
+
+func TestCreateFleetUpdate_RefusesIfRunning(t *testing.T) {
+	t.Parallel()
+	s := openTestStore(t)
+	uid := seedFleetUser(t, s)
+	h1 := seedFleetHost(t, s, "h1")
+
+	fu1 := FleetUpdate{ID: ulid.Make().String(), StartedByUserID: uid, TargetVersion: "v1"}
+	if err := s.CreateFleetUpdate(context.Background(), fu1, []string{h1}); err != nil {
+		t.Fatalf("create #1: %v", err)
+	}
+	fu2 := FleetUpdate{ID: ulid.Make().String(), StartedByUserID: uid, TargetVersion: "v2"}
+	err := s.CreateFleetUpdate(context.Background(), fu2, []string{h1})
+	if !errors.Is(err, ErrFleetUpdateRunning) {
+		t.Fatalf("want ErrFleetUpdateRunning, got %v", err)
+	}
+}
+
+func TestCreateFleetUpdate_HydrateRoundTrip(t *testing.T) {
+	t.Parallel()
+	s := openTestStore(t)
+	uid := seedFleetUser(t, s)
+	h1 := seedFleetHost(t, s, "h1")
+	h2 := seedFleetHost(t, s, "h2")
+
+	fu := FleetUpdate{ID: ulid.Make().String(), StartedByUserID: uid, TargetVersion: "v1.2.3"}
+	if err := s.CreateFleetUpdate(context.Background(), fu, []string{h1, h2}); err != nil {
+		t.Fatal(err)
+	}
+
+	got, hosts, err := s.GetFleetUpdate(context.Background(), fu.ID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got.Status != "running" || got.TargetVersion != "v1.2.3" {
+		t.Fatalf("parent: %+v", got)
+	}
+	if len(hosts) != 2 || hosts[0].Position != 0 || hosts[1].Position != 1 {
+		t.Fatalf("hosts: %+v", hosts)
+	}
+	if hosts[0].Status != "pending" || hosts[1].Status != "pending" {
+		t.Fatalf("hosts status: %+v", hosts)
+	}
+}
+
+func TestSetFleetUpdateHostStatus_ProgressesAndStoresJobID(t *testing.T) {
+	t.Parallel()
+	s := openTestStore(t)
+	uid := seedFleetUser(t, s)
+	h := seedFleetHost(t, s, "h1")
+	fu := FleetUpdate{ID: ulid.Make().String(), StartedByUserID: uid, TargetVersion: "v1"}
+	_ = s.CreateFleetUpdate(context.Background(), fu, []string{h})
+
+	jobID := ulid.Make().String()
+	if err := s.CreateJob(context.Background(), Job{
+		ID: jobID, HostID: h, Kind: "update",
+		ActorKind: "user", ActorID: ptrStr(uid), CreatedAt: time.Now().UTC(),
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := s.SetFleetUpdateHostStatus(context.Background(), fu.ID, h, "running", "", ""); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.SetFleetUpdateHostStatus(context.Background(), fu.ID, h, "succeeded", "", jobID); err != nil {
+		t.Fatal(err)
+	}
+	_, hs, _ := s.GetFleetUpdate(context.Background(), fu.ID)
+	if hs[0].Status != "succeeded" || hs[0].JobID != jobID {
+		t.Fatalf("after succeed: %+v", hs[0])
+	}
+
+	pending, _ := s.ListPendingFleetUpdateHosts(context.Background(), fu.ID)
+	if len(pending) != 0 {
+		t.Fatalf("pending should be empty: %+v", pending)
+	}
+}
+
+func TestHaltAndCompleteFleetUpdate(t *testing.T) {
+	t.Parallel()
+	s := openTestStore(t)
+	uid := seedFleetUser(t, s)
+	h := seedFleetHost(t, s, "h1")
+
+	fu1 := FleetUpdate{ID: ulid.Make().String(), StartedByUserID: uid, TargetVersion: "v1"}
+	_ = s.CreateFleetUpdate(context.Background(), fu1, []string{h})
+	if err := s.HaltFleetUpdate(context.Background(), fu1.ID, "boom", time.Now().UTC()); err != nil {
+		t.Fatal(err)
+	}
+	got, _, _ := s.GetFleetUpdate(context.Background(), fu1.ID)
+	if got.Status != "halted" || got.HaltedReason != "boom" {
+		t.Fatalf("after halt: %+v", got)
+	}
+	if got.CompletedAt == nil {
+		t.Fatal("halted must stamp completed_at")
+	}
+	if active, _ := s.ActiveFleetUpdate(context.Background()); active != nil {
+		t.Fatalf("halted should clear active: %+v", active)
+	}
+
+	// Now a fresh run can start.
+	fu2 := FleetUpdate{ID: ulid.Make().String(), StartedByUserID: uid, TargetVersion: "v2"}
+	if err := s.CreateFleetUpdate(context.Background(), fu2, []string{h}); err != nil {
+		t.Fatalf("create after halt: %v", err)
+	}
+	if err := s.CompleteFleetUpdate(context.Background(), fu2.ID, time.Now().UTC()); err != nil {
+		t.Fatal(err)
+	}
+	got, _, _ = s.GetFleetUpdate(context.Background(), fu2.ID)
+	if got.Status != "completed" {
+		t.Fatalf("after complete: %+v", got)
+	}
+}
+
+func TestRunningUpdateJobForHost(t *testing.T) {
+	t.Parallel()
+	s := openTestStore(t)
+	h := seedFleetHost(t, s, "h1")
+
+	got, err := s.RunningUpdateJobForHost(context.Background(), h)
+	if err != nil || got != "" {
+		t.Fatalf("empty case: got=%q err=%v", got, err)
+	}
+
+	jobID := ulid.Make().String()
+	if err := s.CreateJob(context.Background(), Job{
+		ID: jobID, HostID: h, Kind: "update",
+		ActorKind: "user", ActorID: ptrStr("u-1"), CreatedAt: time.Now().UTC(),
+	}); err != nil {
+		t.Fatal(err)
+	}
+	got, err = s.RunningUpdateJobForHost(context.Background(), h)
+	if err != nil || got != jobID {
+		t.Fatalf("queued case: got=%q err=%v", got, err)
+	}
+
+	// Mark succeeded → no longer "in flight".
+	if err := s.MarkJobFinished(context.Background(), jobID, "succeeded", 0, nil, "", time.Now().UTC()); err != nil {
+		t.Fatal(err)
+	}
+	got, err = s.RunningUpdateJobForHost(context.Background(), h)
+	if err != nil || got != "" {
+		t.Fatalf("after succeed: got=%q err=%v", got, err)
+	}
+}
diff --git a/internal/store/migrations/0021_jobs_update_kind.sql b/internal/store/migrations/0021_jobs_update_kind.sql
new file mode 100644
index 0000000..241bdc0
--- /dev/null
+++ b/internal/store/migrations/0021_jobs_update_kind.sql
@@ -0,0 +1,57 @@
+-- 0021_jobs_update_kind.sql
+--
+-- Add 'update' to the jobs.kind CHECK constraint so the agent
+-- self-update flow (P6-01) can persist its job rows. SQLite can't
+-- ALTER a CHECK in place, so we rebuild the table.
+--
+-- Same safe rebuild pattern as 0012:
+--   1. Stash job_logs into a temp table BEFORE rebuilding jobs.
+--   2. Create jobs_new with the wider CHECK; copy data; DROP jobs;
+--      RENAME jobs_new TO jobs.
+--   3. Restore job_logs (cascade-trap defence — see CLAUDE.md).
+--
+-- jobs_new mirrors the live schema *including* post-0012 column
+-- additions (0015 added source_group_id). When adding a new
+-- migration that touches this table, mirror the latest column set.
+
+CREATE TEMPORARY TABLE _job_logs_backup AS
+  SELECT job_id, seq, ts, stream, payload FROM job_logs;
+
+CREATE TABLE jobs_new (
+  id              TEXT PRIMARY KEY,
+  host_id         TEXT NOT NULL REFERENCES hosts(id) ON DELETE CASCADE,
+  kind            TEXT NOT NULL CHECK (kind IN
+                       ('backup','init','forget','prune','check','unlock','restore','diff','update')),
+  status          TEXT NOT NULL CHECK (status IN ('queued','running','succeeded','failed','cancelled')),
+  scheduled_id    TEXT REFERENCES schedules(id) ON DELETE SET NULL,
+  actor_kind      TEXT NOT NULL CHECK (actor_kind IN ('user','schedule','system')),
+  actor_id        TEXT,
+  started_at      TEXT,
+  finished_at     TEXT,
+  exit_code       INTEGER,
+  stats           TEXT,
+  error           TEXT,
+  created_at      TEXT NOT NULL,
+  source_group_id TEXT REFERENCES source_groups(id) ON DELETE SET NULL
+);
+
+INSERT INTO jobs_new
+  SELECT id, host_id, kind, status, scheduled_id, actor_kind, actor_id,
+         started_at, finished_at, exit_code, stats, error, created_at,
+         source_group_id
+  FROM jobs;
+
+DROP TABLE jobs;
+ALTER TABLE jobs_new RENAME TO jobs;
+
+CREATE INDEX jobs_host_id ON jobs(host_id);
+CREATE INDEX jobs_status ON jobs(status);
+CREATE INDEX jobs_created_at ON jobs(created_at);
+CREATE INDEX jobs_source_group_id ON jobs(source_group_id);
+
+-- Defensive: restore job_logs from the temp backup. INSERT OR IGNORE
+-- so a re-run is harmless. Same shape as 0012's safety net.
+INSERT OR IGNORE INTO job_logs (job_id, seq, ts, stream, payload)
+  SELECT job_id, seq, ts, stream, payload FROM _job_logs_backup;
+
+DROP TABLE _job_logs_backup;
diff --git a/internal/store/migrations/0022_fleet_updates.sql b/internal/store/migrations/0022_fleet_updates.sql
new file mode 100644
index 0000000..a57242f
--- /dev/null
+++ b/internal/store/migrations/0022_fleet_updates.sql
@@ -0,0 +1,35 @@
+-- 0022_fleet_updates.sql
+--
+-- Tables backing the rolling fleet-update worker (P6-02). One row in
+-- fleet_updates per "update all" invocation, a child row per host so
+-- the worker can iterate in position order, report progress, and
+-- record per-host outcome. Halt-on-fail semantics live in the worker
+-- (internal/server/fleetupdate); this schema just captures state.
+
+CREATE TABLE fleet_updates (
+  id                 TEXT PRIMARY KEY,
+  started_at         TEXT NOT NULL,
+  started_by_user_id TEXT NOT NULL REFERENCES users(id),
+  target_version     TEXT NOT NULL,
+  status             TEXT NOT NULL CHECK (status IN
+                          ('running','completed','halted','cancelled')),
+  current_host_id    TEXT REFERENCES hosts(id),
+  halted_reason      TEXT,
+  completed_at       TEXT
+);
+
+CREATE INDEX fleet_updates_status ON fleet_updates(status);
+
+CREATE TABLE fleet_update_hosts (
+  fleet_update_id TEXT NOT NULL REFERENCES fleet_updates(id) ON DELETE CASCADE,
+  host_id         TEXT NOT NULL REFERENCES hosts(id) ON DELETE CASCADE,
+  position        INTEGER NOT NULL,
+  status          TEXT NOT NULL CHECK (status IN
+                       ('pending','running','succeeded','failed','skipped')),
+  job_id          TEXT REFERENCES jobs(id) ON DELETE SET NULL,
+  failed_reason   TEXT,
+  PRIMARY KEY (fleet_update_id, host_id)
+);
+
+CREATE INDEX fleet_update_hosts_position
+  ON fleet_update_hosts(fleet_update_id, position);
diff --git a/internal/store/types.go b/internal/store/types.go
index 5b16294..cc60e48 100644
--- a/internal/store/types.go
+++ b/internal/store/types.go
@@ -211,6 +211,33 @@ type PendingRun struct {
 	LastError     string
 }
 
+// FleetUpdate captures one "update all" invocation. Status moves
+// running → one of {completed, halted, cancelled}. CurrentHostID
+// tracks the host the worker is actively waiting on; cleared (empty)
+// outside an active dispatch.
+type FleetUpdate struct {
+	ID              string
+	StartedAt       time.Time
+	StartedByUserID string
+	TargetVersion   string
+	Status          string
+	CurrentHostID   string
+	HaltedReason    string
+	CompletedAt     *time.Time
+}
+
+// FleetUpdateHost is one host's slot in a fleet update. Position is
+// the iteration order. JobID is set once the worker has dispatched
+// command.update for this host; FailedReason on a failed/halted row.
+type FleetUpdateHost struct {
+	FleetUpdateID string
+	HostID        string
+	Position      int
+	Status        string
+	JobID         string
+	FailedReason  string
+}
+
 // EnrollmentToken is the issuer's view of a one-time token.
 type EnrollmentToken struct {
 	Raw       string
diff --git a/internal/version/version.go b/internal/version/version.go
new file mode 100644
index 0000000..3a5fb70
--- /dev/null
+++ b/internal/version/version.go
@@ -0,0 +1,16 @@
+// Package version exposes build-time identifying constants. Both the
+// server and agent link this package; their values are set via
+// -ldflags during the build. An unset Version falls back to "dev"
+// so source builds without ldflags still run.
+package version
+
+var (
+	// Version is the human-facing release string, e.g. "v1.2.3" or
+	// "v1.2.3-dirty". Compared byte-for-byte between agent and
+	// server to drive the "out of date" signal.
+	Version = "dev"
+
+	// Commit is the short git SHA. Informational only; surfaced via
+	// /api/version but not used for any comparison.
+	Commit = ""
+)
diff --git a/tasks.md b/tasks.md
index b67c288..088179f 100644
--- a/tasks.md
+++ b/tasks.md
@@ -344,8 +344,33 @@ Sizes: **S** = under a day, **M** = 1–3 days, **L** = 3–7 days.
 
 > Deferred from Phase 4 on 2026-05-05 — operator-experience polish that doesn't gate a working v1.
 
-- [ ] **P6-01** (S) Agent self-update from the server's bundled binaries. P5-03 already bakes matching `agent-{linux-amd64,linux-arm64,windows-amd64}` into the server image under `/opt/restic-manager/dist/`, served by `/agent/binary`. Add a `restic-manager-agent update` subcommand (and a server-dispatched `command.update` WS envelope) that fetches `$RM_SERVER/agent/binary?os=…&arch=…`, verifies sha256 against a digest the server advertises alongside the binary, atomic-renames over the running binary (`tmp+fsync+rename`), and asks the service manager to restart (`systemctl restart` on Linux, SCM restart on Windows). Version pinning is automatic — the server only ever serves the agent that matches its own release. No apt repo, no Chocolatey, no third-party signing infra. _(Was P4-01; original apt/choco plan dropped after the P5-03 Docker pivot made the server the natural distribution point.)_
-- [ ] **P6-02** (M) Agent version reporting + fleet update on dashboard. Server already knows its own build version and each agent's `agent_version` from the WS hello. Surface "N hosts behind" on the dashboard, a per-host "out of date" chip, and an admin-only **Update all** action that fans out `command.update` to every online host (offline hosts queue via `pending_runs`-style retry on reconnect). Per-host **Update** button on host detail for one-shot upgrades. Audit-logged. _(Was P4-02.)_
+- [x] **P6-01** (S) Agent self-update from the server's bundled binaries. Server-dispatched `command.update` WS envelope; agent fetches `$RM_SERVER/agent/binary?os=…&arch=…` to `<bin>.new`, copies running binary to `<bin>.old` (M1 — keep one revision back), atomic-rename, exit cleanly. Linux relies on systemd `Restart=always`; Windows writes a detached `update.cmd` helper that waits 3s, `sc stop`s, renames, `sc start`s. No sha256 digest verification — TLS already covers corruption-in-transit (decision deferred per spec §4). _(Was P4-01.)_
+- [x] **P6-02** (M) Agent version reporting + fleet update on dashboard. `internal/version` package + Makefile ldflags injection so server and agent are comparable byte-for-byte. Out-of-date chip on host rows + detail header (amber, format `out of date · A → B`). Hero tile "N hosts behind" with `?updates=behind` filter. Per-host **Update agent** button on host detail. Admin `/settings/fleet-update` page drives a rolling worker (`internal/server/fleetupdate`) that updates one host at a time, polls for hello-with-target-version up to 95s, halts on first failure with `fleet_update_halted` alert. Per-host `update_failed` alerts auto-resolve when the agent reconnects at the right version. `host.update_dispatched/_succeeded/_failed` and `fleet.update_started/_completed/_halted/_cancelled` audit actions. _(Was P4-02.)_
+
+> **As shipped (2026-05-06, branch `p6-agent-self-update`):**
+> Spec `docs/superpowers/specs/2026-05-06-p6-01-02-agent-self-update-design.md`,
+> plan `docs/superpowers/plans/2026-05-06-p6-01-02-agent-self-update.md`.
+> Schema: migration 0021 widens `jobs.kind` CHECK to include `update`;
+> 0022 creates `fleet_updates` + `fleet_update_hosts`. Agent: new
+> `internal/agent/updater` package (build-tag split unix/windows);
+> dispatcher case `MsgCommandUpdate` in `cmd/agent/update_dispatch.go`
+> emits `job.started` + `log.stream` updates before exit. Server: WS
+> update-watcher (`internal/server/ws/update_watch.go`) tracks in-flight
+> dispatches, marks succeeded on hello-with-matching-version, fails after
+> 90s timeout (covers both no-show and rollback cases per spec §3.2).
+> Endpoint `POST /api/hosts/{id}/update` (admin, JSON) + `POST /hosts/{id}/update`
+> (HTMX, `HX-Redirect: /jobs/{id}`); pre-checks for offline / already
+> up-to-date / update_in_progress. Fleet worker exposes `Start` /
+> `Cancel` and runs at most one rolling sequence at a time. Alert kinds
+> `update_failed` and `fleet_update_halted` plug into the P3-05 engine.
+>
+> **Smoke caught + fixed mid-sweep:** the systemd unit's
+> `ProtectSystem=full` made `/usr/local/bin` read-only, blocking the
+> .new staging file. Added `/usr/local/bin` to `ReadWritePaths`. With
+> the fix in place: end-to-end Update agent took the host from
+> `v0.9.0-11-gccaccd8-dirty` → `v9.9.9-smoke` in <5s; `.old` preserved
+> on disk; chip and hero tile cleared on reconnect; audit row landed.
+> Screenshots in `_diag/p6-update-sweep/`.
 - [ ] **P6-03** (M) Repo size trend graphs (sparkline on host card, full chart on repo page). _(Was P4-06.)_
 - [ ] **P6-04** (M) Prometheus `/metrics` endpoint: per-host gauges (last backup timestamp, last backup status, repo size, snapshot count, agent online), server gauges (active alerts, build info), job duration histograms; protected by bearer token or IP allow-list. _(Was P4-08.)_
 - [ ] **P6-05** (S) Document Prometheus integration + sample Grafana dashboard JSON. _(Was P4-09.)_
diff --git a/web/static/css/styles.css b/web/static/css/styles.css
index d72d394..37ca83f 100644
--- a/web/static/css/styles.css
+++ b/web/static/css/styles.css
@@ -1,3 +1,3 @@
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250)}.p-0{padding:0}.p-2{padding:.5rem}.p-3{padding:.75rem}.p-3\.5{padding:.875rem}.p-4{padding:1rem}.p-5{padding:1.25rem}.p-6{padding:1.5rem}.p-7{padding:1.75rem}.p-8{padding:2rem}.p-\[18px\]{padding:18px}.p-\[3px\]{padding:3px}.px-1{padding-left:.25rem;padding-right:.25rem}.px-2{padding-left:.5rem;padding-right:.5rem}.px-2\.5{padding-left:.625rem;padding-right:.625rem}.px-3{padding-left:.75rem;padding-right:.75rem}.px-3\.5{padding-left:.875rem;padding-right:.875rem}.px-4{padding-left:1rem;padding-right:1rem}.px-5{padding-left:1.25rem;padding-right:1.25rem}.px-7{padding-left:1.75rem;padding-right:1.75rem}.px-8{padding-left:2rem;padding-right:2rem}.px-\[18px\]{padding-left:18px;padding-right:18px}.py-0\.5{padding-bottom:.125rem;padding-top:.125rem}.py-1{padding-bottom:.25rem;padding-top:.25rem}.py-1\.5{padding-bottom:.375rem;padding-top:.375rem}.py-12{padding-bottom:3rem;padding-top:3rem}.py-2{padding-bottom:.5rem;padding-top:.5rem}.py-2\.5{padding-bottom:.625rem;padding-top:.625rem}.py-3{padding-bottom:.75rem;padding-top:.75rem}.py-3\.5{padding-bottom:.875rem;padding-top:.875rem}.py-4{padding-bottom:1rem;padding-top:1rem}.py-5{padding-bottom:1.25rem;padding-top:1.25rem}.py-6{padding-bottom:1.5rem;padding-top:1.5rem}.py-7{padding-bottom:1.75rem;padding-top:1.75rem}.py-8{padding-bottom:2rem;padding-top:2rem}.py-\[14px\]{padding-bottom:14px;padding-top:14px}.py-\[5px\]{padding-bottom:5px;padding-top:5px}.pb-14{padding-bottom:3.5rem}.pb-2{padding-bottom:.5rem}.pb-24{padding-bottom:6rem}.pb-3{padding-bottom:.75rem}.pb-4{padding-bottom:1rem}.pb-\[18px\]{padding-bottom:18px}.pl-5{padding-left:1.25rem}.pl-6{padding-left:1.5rem}.pl-9{padding-left:2.25rem}.pr-4{padding-right:1rem}.pt-0\.5{padding-top:.125rem}.pt-1{padding-top:.25rem}.pt-14{padding-top:3.5rem}.pt-2{padding-top:.5rem}.pt-20{padding-top:5rem}.pt-4{padding-top:1rem}.pt-5{padding-top:1.25rem}.pt-6{padding-top:1.5rem}.pt-7{padding-top:1.75rem}.pt-9{padding-top:2.25rem}.pt-\[1px\]{padding-top:1px}.text-left{text-align:left}.text-center{text-align:center}.text-right{text-align:right}.text-2xl{font-size:1.5rem;line-height:2rem}.text-\[10\.5px\]{font-size:10.5px}.text-\[10px\]{font-size:10px}.text-\[11\.5px\]{font-size:11.5px}.text-\[11px\]{font-size:11px}.text-\[12\.5px\]{font-size:12.5px}.text-\[12px\]{font-size:12px}.text-\[13px\]{font-size:13px}.text-\[14px\]{font-size:14px}.text-\[16px\]{font-size:16px}.text-\[18px\]{font-size:18px}.text-\[19px\]{font-size:19px}.text-\[20px\]{font-size:20px}.text-\[22px\]{font-size:22px}.text-\[26px\]{font-size:26px}.text-\[28px\]{font-size:28px}.text-base{font-size:1rem;line-height:1.5rem}.text-lg{font-size:1.125rem;line-height:1.75rem}.text-sm{font-size:.875rem;line-height:1.25rem}.text-xs{font-size:.75rem;line-height:1rem}.font-medium{font-weight:500}.font-normal{font-weight:400}.font-semibold{font-weight:600}.uppercase{text-transform:uppercase}.normal-case{text-transform:none}.italic{font-style:italic}.leading-\[1\.55\]{line-height:1.55}.leading-\[1\.5\]{line-height:1.5}.leading-\[1\.65\]{line-height:1.65}.leading-\[1\.6\]{line-height:1.6}.leading-\[1\.7\]{line-height:1.7}.leading-\[20px\]{line-height:20px}.leading-none{line-height:1}.tracking-\[-0\.005em\]{letter-spacing:-.005em}.tracking-\[-0\.012em\]{letter-spacing:-.012em}.tracking-\[-0\.01em\]{letter-spacing:-.01em}.tracking-\[-0\.02em\]{letter-spacing:-.02em}.tracking-\[0\.005em\]{letter-spacing:.005em}.tracking-\[0\.01em\]{letter-spacing:.01em}.tracking-\[0\.02em\]{letter-spacing:.02em}.tracking-\[0\.08em\]{letter-spacing:.08em}.tracking-\[0\.1em\]{letter-spacing:.1em}.text-accent{color:oklch(.82 .12 195)}.text-bad{color:oklch(.7 .2 25)}.text-ink{color:oklch(.96 .005 250)}.text-ink-fade{color:oklch(.42 .006 250)}.text-ink-mid{color:oklch(.78 .005 250)}.text-ink-mute{color:oklch(.58 .006 250)}.text-ok{color:oklch(.78 .14 155)}.text-warn{color:oklch(.82 .13 80)}.underline{text-decoration-line:underline}.no-underline{text-decoration-line:none}.decoration-line{text-decoration-color:oklch(.27 .01 250)}.underline-offset-4{text-underline-offset:4px}.opacity-40{opacity:.4}.filter{filter:var(--tw-blur) var(--tw-brightness) var(--tw-contrast) var(--tw-grayscale) var(--tw-hue-rotate) var(--tw-invert) var(--tw-saturate) var(--tw-sepia) var(--tw-drop-shadow)}.transition{transition-duration:.15s;transition-property:color,background-color,border-color,text-decoration-color,fill,stroke,opacity,box-shadow,transform,filter,-webkit-backdrop-filter;transition-property:color,background-color,border-color,text-decoration-color,fill,stroke,opacity,box-shadow,transform,filter,backdrop-filter;transition-property:color,background-color,border-color,text-decoration-color,fill,stroke,opacity,box-shadow,transform,filter,backdrop-filter,-webkit-backdrop-filter;transition-timing-function:cubic-bezier(.4,0,.2,1)}.hover\:text-ink:hover{color:oklch(.96 .005 250)}.hover\:text-ink-mid:hover{color:oklch(.78 .005 250)}.hover\:underline:hover{text-decoration-line:underline}
diff --git a/web/styles/input.css b/web/styles/input.css
index 4420168..fb27b08 100644
--- a/web/styles/input.css
+++ b/web/styles/input.css
@@ -104,6 +104,65 @@
   .btn-lg { font-size: 13px; padding: 9px 14px; }
   .btn-block { width: 100%; justify-content: center; }
 
+  /* Amber action — used for the per-host "Update agent" button and
+     the fleet-update Start button. Same warning palette as the
+     update-chip below. */
+  .btn-amber {
+    color: oklch(0.18 0.01 80);
+    background: var(--warn);
+    border-color: var(--warn);
+  }
+  .btn-amber:hover { filter: brightness(1.08); }
+  .btn-amber:disabled, .btn-amber[disabled] {
+    opacity: 0.45; cursor: not-allowed; pointer-events: none;
+  }
+
+  /* Update-available chip — small amber pill rendered next to a host's
+     agent version (in the row OS column and in the host detail
+     header). Hidden when the host is up to date. */
+  .update-chip {
+    display: inline-flex; align-items: center; gap: 4px;
+    padding: 1px 6px;
+    border-radius: 3px;
+    font-size: 10px; font-weight: 500;
+    line-height: 1.4;
+    color: oklch(0.18 0.01 80);
+    background: color-mix(in oklch, var(--warn), transparent 30%);
+    border: 1px solid color-mix(in oklch, var(--warn), transparent 50%);
+    white-space: nowrap;
+  }
+
+  /* Hero tile — large, clickable summary card on the dashboard.
+     Today only used by the "N hosts behind" tile; the existing
+     four summary boxes use bespoke grid markup. Add more variants
+     as adjacent dashboard tiles adopt this. */
+  .hero-tile {
+    display: flex; flex-direction: column; gap: 4px;
+    padding: 14px 16px;
+    border-radius: 7px;
+    border: 1px solid var(--line-soft);
+    background: var(--panel);
+    text-decoration: none;
+    transition: filter 120ms ease, background 120ms ease;
+  }
+  .hero-tile:hover { filter: brightness(1.08); }
+  .hero-tile .hero-num {
+    font-family: 'JetBrains Mono', ui-monospace, monospace;
+    font-size: 22px; font-weight: 500;
+    letter-spacing: -0.01em;
+    color: var(--ink);
+  }
+  .hero-tile .hero-label {
+    font-size: 11.5px;
+    color: var(--ink-mute);
+  }
+  .hero-tile--amber {
+    background: color-mix(in oklch, var(--warn), transparent 88%);
+    border-color: color-mix(in oklch, var(--warn), transparent 60%);
+  }
+  .hero-tile--amber .hero-num   { color: oklch(0.86 0.13 80); }
+  .hero-tile--amber .hero-label { color: oklch(0.78 0.08 80); }
+
   /* ---------- nav tabs ---------- */
   .nav-tab {
     font-size: 13px; padding: 18px 0;
diff --git a/web/templates/pages/dashboard.html b/web/templates/pages/dashboard.html
index b4ffb23..e29dbc8 100644
--- a/web/templates/pages/dashboard.html
+++ b/web/templates/pages/dashboard.html
@@ -66,6 +66,16 @@
       </div>
     </div>
 
+    {{/* ---------- Hosts-behind hero tile (P6-18) ---------- */}}
+    {{if gt $page.UpdatesBehind 0}}
+      <div class="pt-4">
+        <a href="?updates=behind" class="hero-tile hero-tile--amber" style="display:inline-flex;">
+          <span class="hero-num">{{$page.UpdatesBehind}}</span>
+          <span class="hero-label">{{if eq $page.UpdatesBehind 1}}host behind{{else}}hosts behind{{end}} · review →</span>
+        </a>
+      </div>
+    {{end}}
+
     {{/* ---------- Pending hosts (announce-and-approve queue) ---------- */}}
     {{if gt (len $page.PendingHosts) 0}}
       <div class="pt-6">
diff --git a/web/templates/pages/fleet_update.html b/web/templates/pages/fleet_update.html
new file mode 100644
index 0000000..3373e0d
--- /dev/null
+++ b/web/templates/pages/fleet_update.html
@@ -0,0 +1,32 @@
+{{define "title"}}Fleet update · restic-manager{{end}}
+
+{{define "content"}}
+{{$page := .Page}}
+<div class="max-w-[1280px] mx-auto px-8 pb-14">
+
+  {{/* breadcrumbs */}}
+  <div class="crumbs pt-6">
+    <a href="/">Dashboard</a><span class="sep">/</span>
+    <a href="/settings">Settings</a><span class="sep">/</span>
+    <span class="text-ink-mid">fleet update</span>
+  </div>
+
+  {{/* page header */}}
+  <div class="flex items-baseline justify-between mt-3.5">
+    <div>
+      <h1 class="text-[22px] font-medium tracking-[-0.005em]">
+        Fleet update
+        <span class="text-ink-fade font-normal text-[14px] ml-2 mono">target {{$page.TargetVersion}}</span>
+      </h1>
+      <p class="text-ink-mute text-[12px] mt-1 max-w-[760px] leading-[1.55]">
+        Rolling, sequential agent self-update. One host at a time, halts on first failure,
+        cancellable mid-roll. Only online hosts whose <span class="mono">agent_version</span>
+        differs from the server are eligible.
+      </p>
+    </div>
+  </div>
+
+  {{template "fleet_update_inner" .}}
+
+</div>
+{{end}}
diff --git a/web/templates/pages/host_detail.html b/web/templates/pages/host_detail.html
index fdd9890..4d3b134 100644
--- a/web/templates/pages/host_detail.html
+++ b/web/templates/pages/host_detail.html
@@ -78,6 +78,26 @@
         </p>
       </div>
 
+      {{if and $page.CanAdmin $page.UpdateAvailable}}
+      <div class="panel rounded-[7px] px-4 py-3.5">
+        <div class="text-[11px] text-ink-fade uppercase tracking-[0.1em] mb-2.5">Agent update</div>
+        <p class="text-[12px] text-ink-mute leading-[1.55] mb-3">
+          Agent at <span class="mono text-ink-mid">{{$host.AgentVersion}}</span> ·
+          server at <span class="mono text-ink-mid">{{$page.TargetVersion}}</span>.
+          Pushes a self-update command; the agent re-launches into the new binary
+          and reconnects.
+        </p>
+        <form hx-post="/hosts/{{$host.ID}}/update" hx-swap="none">
+          <button class="btn btn-amber btn-block"
+                  {{if not $page.Online}}disabled title="Agent must be online"
+                  {{else if $page.UpdateInProgress}}disabled title="Update already in progress"
+                  {{end}}>
+            Update agent
+          </button>
+        </form>
+      </div>
+      {{end}}
+
       <div class="panel rounded-[7px] px-4 py-3.5">
         <div class="text-[11px] text-ink-fade uppercase tracking-[0.1em] mb-2.5">Restore</div>
         <p class="text-[12px] text-ink-mute leading-[1.55] mb-3">
diff --git a/web/templates/partials/fleet_update_inner.html b/web/templates/partials/fleet_update_inner.html
new file mode 100644
index 0000000..357881f
--- /dev/null
+++ b/web/templates/partials/fleet_update_inner.html
@@ -0,0 +1,171 @@
+{{/*
+  fleet_update_inner — inner panel for /settings/fleet-update.
+  Rendered both as part of the full page and as the htmx polling
+  fragment via /settings/fleet-update/partial.
+
+  Expects .Page to be a fleetUpdatePage struct (see fleet_update.go).
+*/}}
+{{define "fleet_update_inner"}}
+{{$page := .Page}}
+<div id="fleet-update-panel" class="mt-5"
+     hx-get="{{$page.PollURL}}"
+     hx-trigger="every 3s [document.visibilityState==='visible']"
+     hx-select="#fleet-update-panel"
+     hx-swap="outerHTML">
+
+{{if and $page.Active (eq $page.Active.Status "running")}}
+
+  {{/* ---------- running state ---------- */}}
+  <div class="panel rounded-[7px] px-5 py-4">
+    <div class="flex items-baseline justify-between">
+      <div>
+        <span class="mono text-[12px] text-ink-fade">fleet update</span>
+        <span class="mono text-[12px] text-accent ml-2">running</span>
+        <span class="mono text-[11px] text-ink-fade ml-2">{{$page.Active.ID}}</span>
+      </div>
+      <form hx-post="/api/fleet-updates/{{$page.Active.ID}}/cancel" hx-swap="none">
+        <button class="btn btn-danger" type="submit"
+                onclick="return confirm('Cancel this fleet update? Hosts already updated stay updated; pending hosts will be skipped.');">
+          Cancel
+        </button>
+      </form>
+    </div>
+    <div class="text-[11.5px] text-ink-mute mt-1">
+      target <span class="mono text-ink-mid">{{$page.Active.TargetVersion}}</span>
+      · started <span class="mono text-ink-mid">{{relTime $page.Active.StartedAt}}</span>
+      {{if $page.Active.CurrentHostID}}
+        · waiting on <span class="mono text-ink-mid">{{index $page.HostNames $page.Active.CurrentHostID}}</span>
+      {{end}}
+    </div>
+  </div>
+
+  {{template "fleet_update_rows" $page}}
+
+{{else if $page.Active}}
+
+  {{/* ---------- terminal state (completed / halted / cancelled) ---------- */}}
+  <div class="panel rounded-[7px] px-5 py-4">
+    <div class="flex items-baseline justify-between">
+      <div>
+        <span class="mono text-[12px] text-ink-fade">last fleet update</span>
+        {{if eq $page.Active.Status "completed"}}
+          <span class="mono text-[12px] text-ok ml-2">completed</span>
+        {{else if eq $page.Active.Status "halted"}}
+          <span class="mono text-[12px] text-bad ml-2">halted</span>
+        {{else if eq $page.Active.Status "cancelled"}}
+          <span class="mono text-[12px] text-warn ml-2">cancelled</span>
+        {{else}}
+          <span class="mono text-[12px] text-ink-mid ml-2">{{$page.Active.Status}}</span>
+        {{end}}
+        <span class="mono text-[11px] text-ink-fade ml-2">{{$page.Active.ID}}</span>
+      </div>
+    </div>
+    <div class="text-[11.5px] text-ink-mute mt-1">
+      target <span class="mono text-ink-mid">{{$page.Active.TargetVersion}}</span>
+      · started <span class="mono text-ink-mid">{{relTime $page.Active.StartedAt}}</span>
+      {{if $page.Active.CompletedAt}} · finished <span class="mono text-ink-mid">{{relTime $page.Active.CompletedAt}}</span>{{end}}
+    </div>
+    {{if $page.Active.HaltedReason}}
+      <div class="text-[12px] text-bad mt-2">{{$page.Active.HaltedReason}}</div>
+    {{end}}
+  </div>
+
+  {{template "fleet_update_rows" $page}}
+
+  {{if gt (len $page.OutOfDateHosts) 0}}
+    <div class="mt-5">
+      {{template "fleet_update_idle_panel" $page}}
+    </div>
+  {{end}}
+
+{{else}}
+
+  {{template "fleet_update_idle_panel" $page}}
+
+{{end}}
+</div>
+{{end}}
+
+{{define "fleet_update_rows"}}
+{{$page := .}}
+<div class="panel mt-3 rounded-[7px] overflow-hidden">
+  <div class="hairline grid items-baseline px-4 py-2.5 text-[11px] text-ink-fade uppercase tracking-[0.08em]"
+       style="grid-template-columns: 0.4fr 1.5fr 0.8fr 1.2fr 1.5fr; column-gap: 18px;">
+    <div>#</div>
+    <div>Host</div>
+    <div>Status</div>
+    <div>Job</div>
+    <div>Detail</div>
+  </div>
+  {{range $page.ActiveRows}}
+    <div class="grid items-center px-4 py-2.5 text-[12.5px] hairline"
+         style="grid-template-columns: 0.4fr 1.5fr 0.8fr 1.2fr 1.5fr; column-gap: 18px;">
+      <div class="mono text-ink-fade">{{.Position}}</div>
+      <div class="mono text-ink">{{if .HostName}}{{.HostName}}{{else}}{{.HostID}}{{end}}</div>
+      <div>
+        {{if eq .Status "pending"}}<span class="text-ink-fade">pending</span>
+        {{else if eq .Status "running"}}<span class="text-accent">running…</span>
+        {{else if eq .Status "succeeded"}}<span class="text-ok">succeeded</span>
+        {{else if eq .Status "failed"}}<span class="text-bad font-medium">failed</span>
+        {{else if eq .Status "skipped"}}<span class="text-ink-mute">skipped</span>
+        {{else}}<span class="text-ink-mute">{{.Status}}</span>{{end}}
+      </div>
+      <div>
+        {{if .JobID}}<a class="link mono text-[11.5px]" href="/jobs/{{.JobID}}">{{.JobID}}</a>{{else}}<span class="text-ink-fade">—</span>{{end}}
+      </div>
+      <div class="mono text-[11.5px] text-ink-mute truncate" title="{{.FailedReason}}">{{.FailedReason}}</div>
+    </div>
+  {{end}}
+</div>
+{{end}}
+
+{{define "fleet_update_idle_panel"}}
+{{$page := .}}
+<div class="panel rounded-[7px] px-5 py-4">
+  {{if eq (len $page.OutOfDateHosts) 0}}
+    <div class="flex items-center gap-3">
+      <span class="dot dot-online"></span>
+      <div>
+        <div class="text-ink text-[14px] font-medium">All hosts are up to date.</div>
+        <div class="text-ink-mute text-[12px] mt-0.5">
+          Every online agent matches server version <span class="mono">{{$page.TargetVersion}}</span>.
+        </div>
+      </div>
+    </div>
+  {{else}}
+    <div class="flex items-baseline justify-between">
+      <h2 class="text-[14px] font-medium">{{len $page.OutOfDateHosts}} host{{if ne (len $page.OutOfDateHosts) 1}}s{{end}} out of date</h2>
+      <span class="mono text-[11px] text-ink-fade">target {{$page.TargetVersion}}</span>
+    </div>
+    <ul class="mt-3 space-y-1 text-[12px]">
+      {{range $page.OutOfDateHosts}}
+        <li class="flex items-center gap-3">
+          <span class="dot dot-online"></span>
+          <span class="mono text-ink">{{.Name}}</span>
+          <span class="mono text-ink-mute">{{if .AgentVersion}}{{.AgentVersion}}{{else}}—{{end}} → {{$page.TargetVersion}}</span>
+        </li>
+      {{end}}
+    </ul>
+
+    <form id="fleet-update-start-form" class="mt-4 flex items-center gap-3"
+          hx-post="/api/fleet/update"
+          hx-headers='{"Content-Type":"application/json"}'
+          hx-vals='{}'
+          hx-swap="none"
+          hx-on::after-request="if(event.detail.successful) location.reload()">
+      <label class="text-[11.5px] text-ink-mute">
+        Type the count
+        <span class="mono text-ink-mid">({{len $page.OutOfDateHosts}})</span>
+        to enable Start:
+      </label>
+      <input type="text" id="fleet-update-confirm" class="field mono text-[12.5px]"
+             style="width: 80px; padding: 5px 8px;"
+             oninput="document.getElementById('fleet-update-start-btn').disabled = (this.value !== '{{len $page.OutOfDateHosts}}');"
+             autocomplete="off" />
+      <button type="submit" id="fleet-update-start-btn" class="btn btn-amber" disabled>
+        Start fleet update
+      </button>
+    </form>
+  {{end}}
+</div>
+{{end}}
diff --git a/web/templates/partials/host_chrome.html b/web/templates/partials/host_chrome.html
index dad2c56..2f02f0b 100644
--- a/web/templates/partials/host_chrome.html
+++ b/web/templates/partials/host_chrome.html
@@ -83,7 +83,7 @@
       <div class="flex items-center gap-3 mt-3 text-[13px] text-ink-mute">
         <span class="mono text-ink-mid">{{$host.OS}}/{{$host.Arch}}</span>
         <span class="text-ink-fade">·</span>
-        <span>agent <span class="mono text-ink-mid">{{if $host.AgentVersion}}{{$host.AgentVersion}}{{else}}—{{end}}</span></span>
+        <span>agent <span class="mono text-ink-mid">{{if $host.AgentVersion}}{{$host.AgentVersion}}{{else}}—{{end}}</span>{{if $page.UpdateAvailable}} {{template "host_update_chip" $page}}{{end}}</span>
         <span class="text-ink-fade">·</span>
         <span>restic <span class="mono text-ink-mid">{{if $host.ResticVersion}}{{$host.ResticVersion}}{{else}}—{{end}}</span></span>
         <span class="text-ink-fade">·</span>
diff --git a/web/templates/partials/host_row.html b/web/templates/partials/host_row.html
index 9c7799e..128d417 100644
--- a/web/templates/partials/host_row.html
+++ b/web/templates/partials/host_row.html
@@ -14,7 +14,7 @@
     {{- end -}}
   </div>
   <div class="mono {{if eq $h.Status "offline"}}text-ink-mid{{else}}text-ink{{end}} font-medium">{{$h.Name}}</div>
-  <div class="mono text-ink-mid text-[12px]">{{$h.OS}}/{{$h.Arch}}</div>
+  <div class="mono text-ink-mid text-[12px]">{{$h.OS}}/{{$h.Arch}}{{if .UpdateAvailable}} {{template "host_update_chip" .}}{{end}}</div>
   <div class="text-xs text-ink-mid">
     {{- if $h.CurrentJobID -}}
       <span class="text-accent">backup running…</span><br>
diff --git a/web/templates/partials/host_update_chip.html b/web/templates/partials/host_update_chip.html
new file mode 100644
index 0000000..b2d7837
--- /dev/null
+++ b/web/templates/partials/host_update_chip.html
@@ -0,0 +1,11 @@
+{{/*
+  host_update_chip — small amber chip rendered when the agent version
+  on a host is behind the server's. Expects:
+    .UpdateAvailable bool
+    .TargetVersion   string
+    .Host            store.Host (for AgentVersion)
+  Hidden entirely when UpdateAvailable is false.
+*/}}
+{{define "host_update_chip"}}
+{{if .UpdateAvailable}}<span class="update-chip" title="Agent at {{.Host.AgentVersion}}; server at {{.TargetVersion}}">out of date · {{.Host.AgentVersion}} → {{.TargetVersion}}</span>{{end}}
+{{end}}