feat(cli): /resume <id> switches the live session in-REPL

apps/cli/src/commands.ts

@@ -415,9 +415,25 @@ export const AddDirCommand: SlashCommand = {
 
 export const ResumeCommand: SlashCommand = {
   name: '/resume',
-  description: 'List recent sessions.',
-  async run(_args, ctx) {
+  description: 'List recent sessions, or `/resume <id|number>` to switch live.',
+  async run(args, ctx) {
     const sessions = await ctx.sessions.list();
+    if (args[0]) {
+      // Accept a full id, or a 1-based index into the recent list below.
+      let id = args[0].trim();
+      const n = Number(id);
+      if (Number.isInteger(n) && n >= 1 && n <= sessions.length) id = sessions[n - 1]!.id;
+      const loaded = await ctx.sessions.load(id);
+      if (!loaded) return [`Session ${id} not found. Run /resume to list recent sessions.`];
+      // Swap the live conversation (REPL applies newHistory) + the append target.
+      ctx.sessionId = id;
+      ctx.newHistory = loaded.messages;
+      const c = loaded.messages.length;
+      return [
+        `↻ Switched to session ${id} (${c} message${c === 1 ? '' : 's'}).`,
+        'New messages now append to this session.',
+      ];
+    }
     if (sessions.length === 0) return ['No previous sessions.'];
     const top = sessions.slice(0, 10);
     return [
@@ -426,7 +442,7 @@ export const ResumeCommand: SlashCommand = {
         (s, i) => `  ${String(i + 1).padStart(2)}. ${s.id}  ${s.title ?? s.cwd}  (${s.updatedAt})`,
       ),
       '',
-      'To resume: deepcode --resume <id> (M2 picker in next iteration).',
+      'Switch live with `/resume <id-or-number>`, or `deepcode --resume <id>` at launch.',
     ];
   },
 };

apps/cli/src/parity-commands.test.ts

@@ -182,3 +182,29 @@ describe('/config set', () => {
     expect(out.join('\n')).toMatch(/Usage: \/config set/);
   });
 });
+
+describe('/resume <id> (live switch)', () => {
+  it('switches the live session: sets sessionId + newHistory', async () => {
+    const sm = new SessionManager({ root: await tmpHome() });
+    const s = await sm.create('/proj', { title: 'old chat' });
+    await sm.append(s.id, { role: 'user', content: [{ type: 'text', text: 'hi' }] });
+    const c = ctx({ sessions: sm, sessionId: 'current-session' });
+    const out = await reg.match('/resume')!.cmd.run([s.id], c);
+    expect(out.join('\n')).toMatch(/Switched to session/);
+    expect(c.sessionId).toBe(s.id);
+    expect(c.newHistory).toHaveLength(1);
+  });
+
+  it('errors on an unknown id', async () => {
+    const sm = new SessionManager({ root: await tmpHome() });
+    const out = await reg.match('/resume')!.cmd.run(['nope-xyz'], ctx({ sessions: sm }));
+    expect(out.join('\n')).toMatch(/not found/i);
+  });
+
+  it('lists sessions with no args', async () => {
+    const sm = new SessionManager({ root: await tmpHome() });
+    await sm.create('/proj', { title: 'a' });
+    const out = await reg.match('/resume')!.cmd.run([], ctx({ sessions: sm }));
+    expect(out.join('\n')).toMatch(/Recent sessions/);
+  });
+});

apps/cli/src/repl.ts

@@ -609,7 +609,9 @@ export async function startRepl(opts: ReplOpts): Promise<number> {
       temperature,
       maxTurns: opts.maxTurns,
       cwd: ctx.cwd,
-      session: { manager: sessions, id: session.id },
+      // ctx.sessionId (not the launch `session.id`) so a live `/resume <id>`
+      // switch redirects new messages to the resumed session.
+      session: { manager: sessions, id: ctx.sessionId },
       mode: ctx.mode as Mode,
       permissions: settings.permissions,
       hooks,

docs/BEHAVIOR_PARITY.md

@@ -33,7 +33,7 @@ Legend: `✅` matches · `🟡` matches with caveats · `🔄` deferred · `⚠
 | `/cost` / `/usage`         | ✓           | ✓                  | ✅                                                                                                                                   |
 | `/context`                 | ✓           | ✓                  | ✅                                                                                                                                   |
 | `/config`                  | ✓           | ✓                  | 🟡 — dumps merged settings + `/config set <key> <value>` (dotted keys, JSON values) writes user settings; no full arrow-key editor   |
-| `/resume`                  | ✓           | ✓ (list only)      | 🟡 — Claude Code has fuzzy picker; ours lists; pick via `--resume <id>`                                                              |
+| `/resume`                  | ✓           | ✓                  | ✅ — lists recent sessions; `/resume <id\|number>` switches the live session in-REPL; `--resume <id>` / `-r` at launch               |
 | `/init`                    | ✓           | ✓                  | ✅ — interactive 3-phase REPL flow (scan → draft → approve-write `AGENTS.md`)                                                        |
 | `/mcp`                     | ✓           | ✓                  | ✅                                                                                                                                   |
 | `/add-dir`                 | ✓           | ✓ (records intent) | 🟡 — M3 will enforce                                                                                                                 |