feat(cli,desktop): /plan enters plan mode directly

CHANGELOG.md

@@ -33,6 +33,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   the entries exact. A session restored from its log has only the text the
   model saw, so replayed cards degrade to today's plain-text body.
 
+- **`/plan`** enters plan mode directly (`/plan off` leaves it) — the same
+  switch `/mode plan` already threw, one word shorter and matching what Claude
+  Code users type. A trailing prompt (`/plan do X`) is refused with a pointer
+  rather than silently dropped, because half-obeying a command teaches the
+  wrong lesson about what it did.
+
 - **Persistent shells now last a whole CLI session, and `/shells` shows them.**
   The registry landed in #273 owned by a single `runAgent` call, which meant a
   shell opened in one turn was gone by the next — a slower `Bash` with extra

apps/cli/src/commands.test.ts

@@ -119,6 +119,25 @@ describe('built-in command behavior', () => {
     expect(ctx.mode).toBe('plan');
   });
 
+  it('/plan enters plan mode and /plan off leaves it', async () => {
+    const reg = new CommandRegistry();
+    const ctx = makeContext();
+    await reg.match('/plan')!.cmd.run([], ctx);
+    expect(ctx.mode).toBe('plan');
+    await reg.match('/plan off')!.cmd.run(['off'], ctx);
+    expect(ctx.mode).toBe('default');
+  });
+
+  it('/plan refuses a trailing prompt instead of silently dropping it', async () => {
+    const reg = new CommandRegistry();
+    const ctx = makeContext();
+    const out = await reg
+      .match('/plan refactor the auth flow')!
+      .cmd.run(['refactor', 'the', 'auth', 'flow'], ctx);
+    expect(out.join('\n')).toMatch(/takes no prompt/);
+    expect(ctx.mode).toBe('default'); // unchanged — nothing half-happened
+  });
+
   it('/effort switches when valid', async () => {
     const reg = new CommandRegistry();
     const ctx = makeContext();

apps/cli/src/commands.ts

@@ -287,6 +287,23 @@ export const ModeCommand: SlashCommand = {
   },
 };
 
+export const PlanCommand: SlashCommand = {
+  name: '/plan',
+  description: 'Enter plan mode (read-only exploring; same as /mode plan).',
+  run(args, ctx) {
+    // `/plan off` reads as the obvious way back out, so honor it rather than
+    // teaching one more incantation. Anything else after /plan is a mistake —
+    // the prompt itself is typed as a normal message once the mode is set.
+    if (args.length > 0 && args[0] !== 'off') {
+      return ['/plan takes no prompt. Enter plan mode first, then type your message.'];
+    }
+    const next = args[0] === 'off' ? 'default' : 'plan';
+    if (ctx.mode === next) return [`Already in ${next} mode.`];
+    ctx.mode = next;
+    return [`Mode switched to ${next}.`];
+  },
+};
+
 // Effort tier UI metadata surfaced by `/effort` with no args.
 // why: the maxTokens/temperature numbers are NOT defined here — they are read
 // from EFFORT_PARAMS in @deepcode/core, the single source of truth the REPL and
@@ -1440,6 +1457,7 @@ export const BUILTIN_COMMANDS: SlashCommand[] = [
   StatusCommand,
   ModelCommand,
   ModeCommand,
+  PlanCommand,
   EffortCommand,
   CostCommand,
   ContextCommand,

apps/desktop/src/lib/slash-commands.test.ts

@@ -63,6 +63,14 @@ describe('parseSlash', () => {
     expect(parseSlash('/effort max')).toEqual({ kind: 'set-effort', value: 'max' });
   });
 
+  it('/plan enters plan mode, /plan off leaves, a trailing prompt is refused', () => {
+    expect(parseSlash('/plan')).toEqual({ kind: 'set-mode', value: 'plan' });
+    expect(parseSlash('/plan off')).toEqual({ kind: 'set-mode', value: 'default' });
+    const r = parseSlash('/plan refactor auth');
+    expect(r?.kind).toBe('error');
+    expect(r && 'message' in r && r.message).toContain('takes no prompt');
+  });
+
   it('rejects an invalid argument with the usage line, not silently', () => {
     const r = parseSlash('/effort ludicrous');
     expect(r?.kind).toBe('error');

apps/desktop/src/lib/slash-commands.ts

@@ -44,6 +44,7 @@ export const DESKTOP_COMMANDS: SlashCommand[] = [
     args: '<name>',
     summary: 'Switch approval mode (default, plan, acceptEdits, …)',
   },
+  { name: '/plan', args: '[off]', summary: 'Enter plan mode (read-only); /plan off leaves it' },
   { name: '/effort', args: '<tier>', summary: 'Switch effort tier (low … max)' },
   { name: '/cost', summary: 'Spend and token usage this conversation' },
   { name: '/context', summary: 'How much of the context window is used' },
@@ -122,6 +123,16 @@ export function parseSlash(input: string): SlashAction | null {
       return (MODES as string[]).includes(arg)
         ? { kind: 'set-mode', value: arg as AgentMode }
         : { kind: 'error', message: `Usage: /mode ${MODES.join(' | ')}` };
+    case '/plan':
+      // Same switch /mode plan throws, one word shorter. A trailing prompt is
+      // refused rather than silently dropped — type it as a normal message.
+      if (arg === '' || arg === 'off') {
+        return { kind: 'set-mode', value: arg === 'off' ? 'default' : 'plan' };
+      }
+      return {
+        kind: 'error',
+        message: '/plan takes no prompt — enter plan mode, then type your message.',
+      };
     case '/effort':
       return (EFFORTS as string[]).includes(arg)
         ? { kind: 'set-effort', value: arg as Effort }

docs/BEHAVIOR_PARITY.md

@@ -53,7 +53,7 @@ Legend: `✅` matches · `🟡` matches with caveats · `🔄` deferred · `⚠
 | `/batch`                   | ✓           | ✗               | 🔄 — batch-of-prompts not yet wired (use `/background` per prompt)                                                                                                                                |
 | `/tasks`                   | ✓           | ✓               | ✅ — lists this session's background tasks; `/tasks <id>` shows one's status + output                                                                                                             |
 | `/shells`                  | ✗           | ✓               | 🆕 DeepCode-only — lists this session's persistent shells; `/shells close <id>` closes one                                                                                                        |
-| `/plan`                    | ✓           | ✗               | 🔄 — set via `/mode plan` in DeepCode                                                                                                                                                             |
+| `/plan`                    | ✓           | ✓               | ✅ — `/plan` enters plan mode, `/plan off` leaves; `/mode plan` still works                                                                                                                       |
 | `/login` / `/logout`       | ✓           | ✓               | ✅ — /logout clears creds + exits; /login <key> stores a new key (next launch)                                                                                                                    |
 | `/export`                  | ✓           | ✓               | ✅ — writes the conversation to a markdown file                                                                                                                                                   |
 | `/bug` (alias `/feedback`) | ✓           | ✓               | ✅ — prints a prefilled GitHub issue link (model/mode/effort in the body)                                                                                                                         |