diff --git a/client/cmd/root.go b/client/cmd/root.go
index 72de67f..697a781 100644
--- a/client/cmd/root.go
+++ b/client/cmd/root.go
@@ -38,6 +38,9 @@ var rootCmd = &cobra.Command{
 		if len(args) == 0 {
 			return cmd.Help()
 		}
+		if len(args) == 1 {
+			return fmt.Errorf("unknown command %q\nTo add a note, use: kb \"%s ...\" or pass multiple words", args[0], args[0])
+		}
 		note := strings.Join(args, " ")
 		tags, _ := cmd.Flags().GetString("tags")
 		client := api.NewClient()
@@ -48,8 +51,8 @@ var rootCmd = &cobra.Command{
 func init() {
 	api.SetVersionInfo(Version, MinEngineVersion)
 	rootCmd.Version = Version
-	rootCmd.SetUsageTemplate(`Quick note taking:
-  kb "note text" [flags]
+	rootCmd.SetUsageTemplate(`Quick note taking (must be more than one word):
+  kb "note text here" [flags]
 
 Normal usage:
   kb [command] [flags]{{if .HasAvailableSubCommands}}
diff --git a/client/cmd/root_test.go b/client/cmd/root_test.go
new file mode 100644
index 0000000..0d3be51
--- /dev/null
+++ b/client/cmd/root_test.go
@@ -0,0 +1,54 @@
+package cmd
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestRootCmd_SingleWordRejected(t *testing.T) {
+	rootCmd.SetArgs([]string{"infow"})
+
+	var stderr bytes.Buffer
+	rootCmd.SetErr(&stderr)
+
+	err := rootCmd.Execute()
+	if err == nil {
+		t.Fatal("expected error for single bare word, got nil")
+	}
+
+	errMsg := err.Error()
+	if !strings.Contains(errMsg, `unknown command "infow"`) {
+		t.Errorf("expected error to mention unknown command, got: %s", errMsg)
+	}
+	if !strings.Contains(errMsg, "multiple words") {
+		t.Errorf("expected error to suggest multiple words, got: %s", errMsg)
+	}
+}
+
+func TestRootCmd_MultipleWordsNotRejected(t *testing.T) {
+	rootCmd.SetArgs([]string{"remember", "to", "update", "dns"})
+
+	err := rootCmd.Execute()
+	// Will fail at API call (no server), but should NOT be the "unknown command" error
+	if err != nil && strings.Contains(err.Error(), "unknown command") {
+		t.Errorf("multi-word input should not be rejected as unknown command, got: %s", err.Error())
+	}
+}
+
+func TestRootCmd_NoArgs_ShowsHelp(t *testing.T) {
+	rootCmd.SetArgs([]string{})
+
+	var stdout bytes.Buffer
+	rootCmd.SetOut(&stdout)
+
+	err := rootCmd.Execute()
+	if err != nil {
+		t.Fatalf("expected no error for zero args, got: %v", err)
+	}
+
+	output := stdout.String()
+	if !strings.Contains(output, "Available Commands") {
+		t.Errorf("expected help output, got: %s", output)
+	}
+}
diff --git a/openspec/specs/go-client/spec.md b/openspec/specs/go-client/spec.md
index ea030d0..ff9b42d 100644
--- a/openspec/specs/go-client/spec.md
+++ b/openspec/specs/go-client/spec.md
@@ -70,7 +70,7 @@ The client SHALL provide a `kb search <query>` command that sends the query to t
 
 ### Requirement: Implicit note shorthand
 
-The client SHALL treat bare string arguments (with no subcommand) as an implicit note. `kb "my note"` SHALL behave identically to submitting a note via `POST /api/v1/jobs`. All persistent flags (`--format`, `--engine`, `--api-key`) and the root `--tags` flag SHALL work with the shorthand form.
+The client SHALL treat bare string arguments (with no subcommand) as an implicit note only when **more than one argument** is provided. `kb "my note"` SHALL behave identically to submitting a note via `POST /api/v1/jobs`. All persistent flags (`--format`, `--engine`, `--api-key`) and the root `--tags` flag SHALL work with the shorthand form. A single bare word SHALL be rejected with an error message.
 
 #### Scenario: Quick note via bare argument
 - **WHEN** the user runs `kb "remember to update DNS"`
@@ -92,6 +92,10 @@ The client SHALL treat bare string arguments (with no subcommand) as an implicit
 - **WHEN** the user runs `kb remember to update dns` (without quotes)
 - **THEN** the client SHALL join all arguments into a single note string and submit it
 
+#### Scenario: Single bare word rejected
+- **WHEN** the user runs `kb infow` (a single unrecognized word)
+- **THEN** the client SHALL print to stderr: `Unknown command "infow". Run 'kb --help' for available commands.` followed by a hint about note usage, and exit with a non-zero code
+
 #### Scenario: No interference with subcommands
 - **WHEN** the user runs `kb search "query"` or any other existing subcommand
 - **THEN** the client SHALL route to the subcommand as before — the implicit note shorthand SHALL NOT interfere