diff --git a/internal/store/migrate_test.go b/internal/store/migrate_test.go
new file mode 100644
index 0000000..59fb304
--- /dev/null
+++ b/internal/store/migrate_test.go
@@ -0,0 +1,33 @@
+package store
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+)
+
+func TestMigration0013AlertsLastSeen(t *testing.T) {
+	t.Parallel()
+	dir := t.TempDir()
+	st, err := Open(context.Background(), filepath.Join(dir, "rm.db"))
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	defer st.Close()
+
+	// Column must exist after migration. Best signal: PRAGMA table_info.
+	rows, err := st.DB().Query(`SELECT name FROM pragma_table_info('alerts')`)
+	if err != nil {
+		t.Fatalf("pragma: %v", err)
+	}
+	defer rows.Close()
+	cols := map[string]bool{}
+	for rows.Next() {
+		var n string
+		_ = rows.Scan(&n)
+		cols[n] = true
+	}
+	if !cols["last_seen_at"] {
+		t.Fatalf("alerts.last_seen_at not present after migration; cols=%v", cols)
+	}
+}
diff --git a/internal/store/migrations/0013_alerts_last_seen.sql b/internal/store/migrations/0013_alerts_last_seen.sql
new file mode 100644
index 0000000..1acac1c
--- /dev/null
+++ b/internal/store/migrations/0013_alerts_last_seen.sql
@@ -0,0 +1,16 @@
+-- 0013_alerts_last_seen.sql
+--
+-- Add alerts.last_seen_at to support open-alert dedup with
+-- recurrence-tracking. The engine bumps this column on every tick
+-- where a rule still matches an existing open alert, so the UI can
+-- render "still happening · Ns ago" without sending a fresh
+-- notification.
+--
+-- Column-level ALTER per CLAUDE.md (no rebuild — alerts has inbound
+-- FK from acknowledged_by → users; rebuild would risk cascade).
+-- Backfill last_seen_at = created_at for any pre-existing rows so
+-- the column is non-null in practice (stays nullable in the schema
+-- for forwards-compat with rows that haven't been touched yet).
+
+ALTER TABLE alerts ADD COLUMN last_seen_at TEXT;
+UPDATE alerts SET last_seen_at = created_at WHERE last_seen_at IS NULL;