diff --git a/src/client/api.js b/src/client/api.js
index 8778d19..ec9efee 100644
--- a/src/client/api.js
+++ b/src/client/api.js
@@ -4,6 +4,7 @@ import fs from 'node:fs';
 import crypto from 'node:crypto';
 import { Readable } from 'node:stream';
 import { pipeline } from 'node:stream/promises';
+import { ownBuild } from './update.js';
 
 export class Api {
   constructor(server, token) {
@@ -14,7 +15,7 @@ export class Api {
   async request(method, path, body) {
     const res = await fetch(this.server + path, {
       method,
-      headers: { 'content-type': 'application/json', ...(this.token ? { authorization: `Bearer ${this.token}` } : {}) },
+      headers: { 'content-type': 'application/json', 'x-td-build': await ownBuild(), ...(this.token ? { authorization: `Bearer ${this.token}` } : {}) },
       body: body === undefined ? undefined : JSON.stringify(body),
     });
     const text = await res.text();
@@ -40,7 +41,7 @@ export class Api {
   /** GET that sends the ETag of a cached copy: null when unchanged (304), else { data, etag }. */
   async getIfChanged(p, etag) {
     const res = await fetch(this.server + p, {
-      headers: { ...(this.token ? { authorization: `Bearer ${this.token}` } : {}), ...(etag ? { 'if-none-match': etag } : {}) },
+      headers: { 'x-td-build': await ownBuild(), ...(this.token ? { authorization: `Bearer ${this.token}` } : {}), ...(etag ? { 'if-none-match': etag } : {}) },
     });
     if (res.status === 304) return null;
     if (!res.ok) throw new Error(`${res.status} ${res.statusText}`);
diff --git a/src/client/update.js b/src/client/update.js
index 428642d..1b4b93a 100644
--- a/src/client/update.js
+++ b/src/client/update.js
@@ -44,6 +44,25 @@ async function ownHash(exe, state) {
   return hash;
 }
 
+/**
+ * This td's build: the SHA-256 of the standalone executable (cached by size and mtime), or 'dev'
+ * when it runs from a checkout. Sent with every request so the server can tell an outdated td.
+ */
+let buildCache = null;
+export async function ownBuild() {
+  if (buildCache) return buildCache;
+  try {
+    const sea = await import('node:sea').catch(() => null);
+    if (!sea?.isSea?.()) return (buildCache = 'dev');
+    const state = readState();
+    buildCache = await ownHash(process.execPath, state);
+    writeState(state);
+  } catch {
+    buildCache = 'unknown';
+  }
+  return buildCache;
+}
+
 /**
  * Checks for and installs a newer td. Returns a message when an update was installed (it takes
  * effect on the next start), otherwise null. Never throws: an update must not break a command.
diff --git a/src/server/app.js b/src/server/app.js
index 35eb510..3594b05 100644
--- a/src/server/app.js
+++ b/src/server/app.js
@@ -13,7 +13,7 @@ import { Verifier } from './verify.js';
 import { Merger } from './merge.js';
 import { PeerReviews } from './peer.js';
 import { Originals } from './original.js';
-import { downloadsDir, installPs1, installSh } from './install.js';
+import { downloadsDir, installPs1, installSh, publishedBuilds } from './install.js';
 import { ServerGhidra } from './ghidra.js';
 import { functionRegions } from './split.js';
 import { CheckQueue, WorkerUnavailable } from './checks.js';
@@ -788,7 +788,7 @@ export function createApp(ctx) {
       const m = router.match(req.method, url.pathname);
       if (!m) throw new HttpError(404, 'not found');
       const result = await m.handler({ req, res, params: m.params, query: url.searchParams });
-      if (result !== undefined) sendJson(res, 200, result);
+      if (result !== undefined) sendJson(res, 200, withUpdateNote(req, result));
     } catch (e) {
       if (res.headersSent) return res.end();
       if (e instanceof HttpError) sendJson(res, e.status, { error: e.message, details: e.details });
@@ -804,6 +804,30 @@ export function createApp(ctx) {
 // Public downloads of the td binaries (tools/build-td output copied to <dataDir>/downloads).
 const DOWNLOADS = new Set(['td-windows-x64.exe', 'td-linux-x64', 'SHA256SUMS']);
 let downloadsRoot = null;
+// Agents read the guidance in work responses; one whose td is not the current build is told to
+// update there (td from before builds were reported sends none). Read per request, so a new
+// publish takes effect without a restart.
+function withUpdateNote(req, result) {
+  if (!result || typeof result !== 'object' || typeof result.guidance !== 'string' || !downloadsRoot) return result;
+  const build = String(req.headers['x-td-build'] ?? '');
+  if (build === 'dev') return result;
+  let current;
+  try {
+    current = Object.values(publishedBuilds({ dataDir: path.dirname(downloadsRoot) }));
+  } catch {
+    return result;
+  }
+  if (!current.length || current.includes(build)) return result;
+  return {
+    ...result,
+    guidance:
+      result.guidance +
+      ` UPDATE NEEDED: your td is not the current build (${build ? build.slice(0, 12) : 'it does not report one'}; current ${current.map((h) => h.slice(0, 12)).join(' / ')}).` +
+      ' Tell the user to restart the AI app (td updates itself when it starts) or to run "td update" and reconnect.' +
+      ' Older td lacks td_mark_no_decompile, td_place and bulk verdicts in td_submit_peer_review.',
+  };
+}
+
 function serveDownload(name, req, res) {
   const file = DOWNLOADS.has(name) && downloadsRoot ? path.join(downloadsRoot, name) : null;
   if (!file || !fs.existsSync(file)) {
diff --git a/src/server/install.js b/src/server/install.js
index f8b3956..0d2bd78 100644
--- a/src/server/install.js
+++ b/src/server/install.js
@@ -11,6 +11,11 @@ export function downloadsDir(config) {
   return path.join(config.dataDir, 'downloads');
 }
 
+/** The SHA-256 of every published td binary (download name -> hash). */
+export function publishedBuilds(config) {
+  return checksums(config);
+}
+
 function checksums(config) {
   const file = path.join(downloadsDir(config), 'SHA256SUMS');
   const out = {};
diff --git a/test/server.test.js b/test/server.test.js
index b01d0d0..87a0fd7 100644
--- a/test/server.test.js
+++ b/test/server.test.js
@@ -748,6 +748,24 @@ test('verification: unwritten functions stay claimed, failing ones go back alone
   }
 });
 
+test('agents with an outdated td are told to update in their guidance', async () => {
+  const token = await activeUser('oldtd');
+  const dir = path.join(ctx.config.dataDir, 'downloads');
+  fs.mkdirSync(dir, { recursive: true });
+  const current = 'a'.repeat(64);
+  fs.writeFileSync(path.join(dir, 'SHA256SUMS'), `${current}  td-windows-x64.exe\n${'b'.repeat(64)}  td-linux-x64\n`);
+  const task = ctx.store.get('SELECT id FROM tasks LIMIT 1').id;
+  const ask = async (build) => {
+    const res = await fetch(`${base}/api/tasks/${task}`, { headers: { authorization: `Bearer ${token}`, ...(build ? { 'x-td-build': build } : {}) } });
+    return (await res.json()).guidance ?? '';
+  };
+  assert.match(await ask(null), /UPDATE NEEDED/, 'td from before builds were reported');
+  assert.match(await ask('c'.repeat(64)), /UPDATE NEEDED/, 'an older build');
+  assert.doesNotMatch(await ask(current), /UPDATE NEEDED/, 'the current build');
+  assert.doesNotMatch(await ask('dev'), /UPDATE NEEDED/, 'a checkout');
+  fs.rmSync(path.join(dir, 'SHA256SUMS'));
+});
+
 test('the crew view is public and shows no account details', async () => {
   const token = await activeUser('crewview');
   await call('GET', '/api/tasks', null, token); // the agent checks in