diff --git a/CHANGELOG.md b/CHANGELOG.md
index de48ddf..32fe13e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,9 @@
 
 ## Unreleased
 
+### Added
+- **Responder-lock enforcement in the MCP room tools (#42)**: `room_post` and `alert_recipient` now refuse when another live session holds the machine's room-responder lock — a PASSIVE session that ignores its injected instructions still cannot post as the agent through IAK tooling. Lock semantics mirror the SessionStart hook exactly (pid + process-start-time identity, stale locks ignored, fail-open on mechanics); the owning session is recognized by finding the lock's pid in the caller's process-ancestor chain. Read-only room tools (`room_recent`) are unaffected. Raw HTTP posting against GroupMind is out of scope client-side and tracked in issue #42's server-side follow-up.
+
 ### Fixed
 - **Duplicate room-responder voices**: `session-bootstrap.sh` now enforces a single room responder per notification file via a lock file (`<notification file>.responder.lock`). The first session claims the lock and gets the full bootstrap; any later session on the same machine gets PASSIVE instructions (serve the user directly, never answer the room) instead of booting a second copy of the same handle. The lock stores the owner's pid + session id + process start time, so the owner reclaims it across resume/compact, a lock whose owner process died is stolen automatically, and a recycled pid never masquerades as the owner (adversarial review by @codexmb: initial claim is `O_EXCL`-atomic, aliveness uses `ps` so a cross-user EPERM never reads as dead, and every winner re-verifies ownership after a settle window so concurrent starts elect exactly one responder). Disable with `IAK_RESPONDER_LOCK=off`, relocate with `IAK_RESPONDER_LOCK=<path>`. Motivation: on 2026-07-16 a stray `claude rc` session on the Mac mini booted the same bootstrap and posted to the room as a second @claudemm, producing conflicting answers.
 
diff --git a/src/mcp-server.mjs b/src/mcp-server.mjs
index 8ee1b45..7e8fef6 100644
--- a/src/mcp-server.mjs
+++ b/src/mcp-server.mjs
@@ -33,6 +33,7 @@ import {
 import { nudgeTmux } from './common/notify.mjs';
 import { tmuxRun } from './ide/tmux-runner.mjs';
 import { loadConfig } from './config.mjs';
+import { assertRoomVoice } from './responder-lock.mjs';
 import {
   createIntent,
   decideIntent,
@@ -710,6 +711,11 @@ export async function runMcpServer({ configPath } = {}) {
         }
         case 'room_post': {
           if (!roomToolsEnabled) return err('room_post: room API is not configured.');
+          // #42: a PASSIVE session (another live session holds the machine's
+          // room-responder lock) is refused here even if it ignores its
+          // injected instructions. Read-only room tools stay available.
+          const voice = assertRoomVoice({ config });
+          if (!voice.allowed) return err(`room_post refused: ${voice.reason}`);
           const posted = await postRoomMessage({
             config,
             room: args.room,
@@ -727,6 +733,8 @@ export async function runMcpServer({ configPath } = {}) {
           if (!roomToolsEnabled) return err('alert_recipient: room API is not configured.');
           if (!args.handle) return err('alert_recipient: handle is required');
           if (!args.body) return err('alert_recipient: body is required');
+          const voice = assertRoomVoice({ config });
+          if (!voice.allowed) return err(`alert_recipient refused: ${voice.reason}`);
           const handle = String(args.handle).startsWith('@') ? String(args.handle) : `@${args.handle}`;
           const posted = await postRoomMessage({
             config,
diff --git a/src/responder-lock.mjs b/src/responder-lock.mjs
new file mode 100644
index 0000000..7a7e453
--- /dev/null
+++ b/src/responder-lock.mjs
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+//
+// Enforcement half of the single room-responder lock (#40 wrote it, #42
+// enforces it): scripts/session-bootstrap.sh gives PASSIVE sessions
+// instructions not to answer the room, but instructions are model-followed.
+// This module lets the MCP room-post path REFUSE the post when another live
+// session holds the machine's responder lock, so a session that ignores its
+// PASSIVE context still cannot speak as the agent through IAK tooling.
+// (Raw curl against the GroupMind API is out of scope here — that needs
+// server-side per-session keys; see issue #42.)
+//
+// Semantics mirror the bash hook exactly:
+//   - lock file: $IAK_RESPONDER_LOCK, else <notification file>.responder.lock;
+//     the literal value "off" disables enforcement.
+//   - owner identity: pid + process start time (a recycled pid is stale).
+//   - fail OPEN on mechanics (missing/corrupt lock, ps failures): a machine
+//     with a broken lock keeps its room voice; the hook is the same way.
+//   - "we hold the lock": the lock's pid is this process or one of its
+//     ancestors — the MCP server is spawned by the session process, so the
+//     owning session appears in our parent chain.
+
+import { readFileSync } from 'node:fs';
+import { execFileSync } from 'node:child_process';
+
+export function lockPathFor(config, env = process.env) {
+  const explicit = env.IAK_RESPONDER_LOCK;
+  if (explicit) return explicit; // may be the literal 'off'
+  const newFile =
+    env.IAK_NEW_FILE ||
+    config?.poller?.notification_file ||
+    '/tmp/iak-new-messages.txt';
+  return `${newFile}.responder.lock`;
+}
+
+export function readResponderLock(lockPath) {
+  try {
+    const raw = readFileSync(lockPath, 'utf8');
+    const get = (k) => {
+      const m = raw.match(new RegExp(`^${k}=(.*)$`, 'm'));
+      return m ? m[1].trim() : '';
+    };
+    const pid = parseInt(get('pid'), 10);
+    return {
+      pid: Number.isInteger(pid) && pid > 0 ? pid : null,
+      sid: get('sid'),
+      pstart: get('pstart'),
+    };
+  } catch {
+    return null;
+  }
+}
+
+function procStart(pid, exec) {
+  try {
+    return exec('ps', ['-p', String(pid), '-o', 'lstart='], { encoding: 'utf8' }).trim();
+  } catch {
+    return '';
+  }
+}
+
+// Returns the parent pid, 0 when the chain legitimately ended (ps ran and
+// the pid is gone), or null when the LOOKUP ITSELF failed (spawn-level
+// error) — the caller must treat null as unknown, never as "not an
+// ancestor", or a transient ps failure would refuse a legitimate owner
+// (codex review on #45).
+function parentOf(pid, exec) {
+  try {
+    const out = exec('ps', ['-p', String(pid), '-o', 'ppid='], { encoding: 'utf8' }).trim();
+    const parent = parseInt(out, 10);
+    return Number.isInteger(parent) && parent > 0 ? parent : 0;
+  } catch (e) {
+    // execFileSync sets .status when the command RAN and exited nonzero —
+    // for ps that means "no such pid": the chain ended. No .status means
+    // ps never ran (ENOENT, EAGAIN, …): mechanics failure -> unknown.
+    if (e && typeof e.status === 'number') return 0;
+    return null;
+  }
+}
+
+// true / false / null(unknown — caller fails open).
+function isSelfOrAncestor(targetPid, fromPid, exec) {
+  let pid = fromPid;
+  // The owning session process is normally our direct parent; walk a few
+  // levels to cover wrapper processes between the session and this server.
+  for (let hops = 0; hops < 10 && pid && pid > 1; hops++) {
+    if (pid === targetPid) return true;
+    pid = parentOf(pid, exec);
+    if (pid === null) return null;
+  }
+  return false;
+}
+
+/**
+ * Decide whether THIS process may post to the room as the agent.
+ * Returns {allowed, reason}. Never throws.
+ *
+ * Injectables (tests): env, selfPid (start of the ancestor walk), exec.
+ */
+export function assertRoomVoice({
+  config,
+  env = process.env,
+  selfPid = process.pid,
+  exec = execFileSync,
+} = {}) {
+  try {
+    const lockPath = lockPathFor(config, env);
+    if (lockPath === 'off') return { allowed: true, reason: 'responder lock disabled' };
+
+    const lock = readResponderLock(lockPath);
+    if (!lock || !lock.pid) return { allowed: true, reason: 'no responder lock' };
+
+    const start = procStart(lock.pid, exec);
+    if (!start) return { allowed: true, reason: 'lock owner process is gone (stale lock)' };
+    if (lock.pstart && start !== lock.pstart) {
+      return { allowed: true, reason: 'lock owner pid was recycled (stale lock)' };
+    }
+
+    const ancestry = isSelfOrAncestor(lock.pid, selfPid, exec);
+    if (ancestry === null) {
+      return { allowed: true, reason: 'ancestor lookup failed mid-walk (fail open)' };
+    }
+    if (ancestry) {
+      return { allowed: true, reason: 'this session holds the responder lock' };
+    }
+
+    return {
+      allowed: false,
+      reason:
+        `another live session (pid ${lock.pid}${lock.sid ? `, session ${lock.sid}` : ''}) ` +
+        `holds the room-responder lock (${lockPath}). This session is PASSIVE and must not ` +
+        `post to the room — duplicate agent voices confuse the team. Takeover only on the ` +
+        `user's explicit request: delete the lock file, then re-run the standard bootstrap.`,
+    };
+  } catch {
+    // Mechanics failure: fail open, same philosophy as the bash hook.
+    return { allowed: true, reason: 'lock check failed (fail open)' };
+  }
+}
diff --git a/test/responder-lock.test.mjs b/test/responder-lock.test.mjs
new file mode 100644
index 0000000..72a9a80
--- /dev/null
+++ b/test/responder-lock.test.mjs
@@ -0,0 +1,182 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { describe, it, afterEach } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { mkdtempSync, writeFileSync, rmSync, realpathSync } from 'node:fs';
+import { join } from 'node:path';
+import { tmpdir } from 'node:os';
+import { assertRoomVoice, lockPathFor, readResponderLock } from '../src/responder-lock.mjs';
+
+const tempPaths = [];
+function tempDir() {
+  const dir = realpathSync(mkdtempSync(join(tmpdir(), 'iak-voice-')));
+  tempPaths.push(dir);
+  return dir;
+}
+afterEach(() => {
+  while (tempPaths.length > 0) rmSync(tempPaths.pop(), { recursive: true, force: true });
+});
+
+/**
+ * Fake process table for the injectable exec: {pid: {lstart, ppid}}.
+ * Throws (like execFileSync) for unknown pids.
+ */
+function fakeExec(table) {
+  return (cmd, args) => {
+    const pid = parseInt(args[1], 10);
+    const col = args[3];
+    const row = table[pid];
+    if (!row) {
+      // Mimic execFileSync when ps RAN and found no such pid: nonzero exit.
+      const e = new Error(`no such process ${pid}`);
+      e.status = 1;
+      throw e;
+    }
+    if (row === 'SPAWN_FAILURE') {
+      // Mimic a spawn-level failure: ps never ran, no .status on the error.
+      throw new Error('spawn ps EAGAIN');
+    }
+    if (col === 'lstart=') return `${row.lstart}\n`;
+    if (col === 'ppid=') return `${row.ppid}\n`;
+    throw new Error(`unexpected ps column ${col}`);
+  };
+}
+
+function writeLock(dir, { pid, sid = 'sess-owner', pstart = '' }) {
+  const path = join(dir, 'new.txt.responder.lock');
+  writeFileSync(path, `pid=${pid}\nsid=${sid}\n${pstart ? `pstart=${pstart}\n` : ''}`);
+  return path;
+}
+
+const START = 'Thu Jul 16 11:08:28 2026';
+
+describe('lockPathFor', () => {
+  it('derives from the notification file and honors env overrides', () => {
+    assert.equal(lockPathFor({}, {}), '/tmp/iak-new-messages.txt.responder.lock');
+    assert.equal(lockPathFor({ poller: { notification_file: '/x/n.txt' } }, {}), '/x/n.txt.responder.lock');
+    assert.equal(lockPathFor({}, { IAK_NEW_FILE: '/y/n.txt' }), '/y/n.txt.responder.lock');
+    assert.equal(lockPathFor({}, { IAK_RESPONDER_LOCK: 'off' }), 'off');
+    assert.equal(lockPathFor({}, { IAK_RESPONDER_LOCK: '/z/lock' }), '/z/lock');
+  });
+});
+
+describe('readResponderLock', () => {
+  it('parses the hook lock format and tolerates junk', () => {
+    const dir = tempDir();
+    const path = writeLock(dir, { pid: 42, sid: 's1', pstart: START });
+    assert.deepEqual(readResponderLock(path), { pid: 42, sid: 's1', pstart: START });
+    assert.equal(readResponderLock(join(dir, 'missing')), null);
+    const junk = join(dir, 'junk');
+    writeFileSync(junk, 'pid=not-a-number\n%%%\n');
+    assert.equal(readResponderLock(junk).pid, null);
+  });
+});
+
+describe('assertRoomVoice', () => {
+  it('allows when the lock is disabled, absent, or unreadable', () => {
+    const dir = tempDir();
+    assert.equal(assertRoomVoice({ env: { IAK_RESPONDER_LOCK: 'off' } }).allowed, true);
+    assert.equal(assertRoomVoice({
+      env: { IAK_RESPONDER_LOCK: join(dir, 'nope') }, exec: fakeExec({}),
+    }).allowed, true);
+  });
+
+  it('allows when the owner process is dead (stale lock)', () => {
+    const dir = tempDir();
+    const lock = writeLock(dir, { pid: 999, pstart: START });
+    const verdict = assertRoomVoice({
+      env: { IAK_RESPONDER_LOCK: lock }, selfPid: 10, exec: fakeExec({ 10: { lstart: 'x', ppid: 1 } }),
+    });
+    assert.equal(verdict.allowed, true);
+    assert.match(verdict.reason, /stale/);
+  });
+
+  it('allows when the owner pid was recycled (pstart mismatch)', () => {
+    const dir = tempDir();
+    const lock = writeLock(dir, { pid: 999, pstart: 'Thu Jan  1 00:00:00 1970' });
+    const verdict = assertRoomVoice({
+      env: { IAK_RESPONDER_LOCK: lock },
+      selfPid: 10,
+      exec: fakeExec({ 999: { lstart: START, ppid: 1 }, 10: { lstart: 'x', ppid: 1 } }),
+    });
+    assert.equal(verdict.allowed, true);
+    assert.match(verdict.reason, /recycled/);
+  });
+
+  it('allows the session that holds the lock (owner in the ancestor chain)', () => {
+    const dir = tempDir();
+    const lock = writeLock(dir, { pid: 500, pstart: START });
+    // self 10 -> parent 20 -> parent 500 (the owning session process)
+    const table = {
+      10: { lstart: 'a', ppid: 20 },
+      20: { lstart: 'b', ppid: 500 },
+      500: { lstart: START, ppid: 1 },
+    };
+    const verdict = assertRoomVoice({ env: { IAK_RESPONDER_LOCK: lock }, selfPid: 10, exec: fakeExec(table) });
+    assert.equal(verdict.allowed, true);
+    assert.match(verdict.reason, /holds the responder lock/);
+  });
+
+  it('REFUSES a passive session while the owner is alive elsewhere', () => {
+    const dir = tempDir();
+    const lock = writeLock(dir, { pid: 500, sid: 'sess-owner', pstart: START });
+    // self 10 -> 20 -> 1: owner 500 alive but NOT in our chain.
+    const table = {
+      10: { lstart: 'a', ppid: 20 },
+      20: { lstart: 'b', ppid: 1 },
+      500: { lstart: START, ppid: 1 },
+    };
+    const verdict = assertRoomVoice({ env: { IAK_RESPONDER_LOCK: lock }, selfPid: 10, exec: fakeExec(table) });
+    assert.equal(verdict.allowed, false);
+    assert.match(verdict.reason, /pid 500/);
+    assert.match(verdict.reason, /sess-owner/);
+    assert.match(verdict.reason, /PASSIVE/);
+  });
+
+  it('legacy lock without pstart still refuses on a live owner pid', () => {
+    const dir = tempDir();
+    const lock = writeLock(dir, { pid: 500 });
+    const table = { 10: { lstart: 'a', ppid: 1 }, 500: { lstart: START, ppid: 1 } };
+    const verdict = assertRoomVoice({ env: { IAK_RESPONDER_LOCK: lock }, selfPid: 10, exec: fakeExec(table) });
+    assert.equal(verdict.allowed, false);
+  });
+
+  it('fails OPEN when an intermediate ancestor lookup fails at spawn level (codex regression)', () => {
+    const dir = tempDir();
+    const lock = writeLock(dir, { pid: 500, pstart: START });
+    // Owner 500 is alive and IS our grandparent, but the walk's lookup of
+    // pid 20's parent fails at spawn level — must fail open, not refuse.
+    const table = {
+      10: { lstart: 'a', ppid: 20 },
+      20: 'SPAWN_FAILURE',
+      500: { lstart: START, ppid: 1 },
+    };
+    const verdict = assertRoomVoice({ env: { IAK_RESPONDER_LOCK: lock }, selfPid: 10, exec: fakeExec(table) });
+    assert.equal(verdict.allowed, true);
+    assert.match(verdict.reason, /fail open/);
+  });
+
+  it('a walk ancestor that exited (ps ran, pid gone) ends the chain and refuses', () => {
+    const dir = tempDir();
+    const lock = writeLock(dir, { pid: 500, pstart: START });
+    // self 10 -> parent 77 which is GONE (ps exits 1): chain ends without
+    // reaching the live owner 500 -> refused, not crashed.
+    const table = {
+      10: { lstart: 'a', ppid: 77 },
+      500: { lstart: START, ppid: 1 },
+    };
+    const verdict = assertRoomVoice({ env: { IAK_RESPONDER_LOCK: lock }, selfPid: 10, exec: fakeExec(table) });
+    assert.equal(verdict.allowed, false);
+  });
+
+  it('fails open when ps itself explodes', () => {
+    const dir = tempDir();
+    const lock = writeLock(dir, { pid: 500, pstart: START });
+    const verdict = assertRoomVoice({
+      env: { IAK_RESPONDER_LOCK: lock },
+      selfPid: 10,
+      exec: () => { throw new Error('ps unavailable'); },
+    });
+    assert.equal(verdict.allowed, true);
+  });
+});