diff --git a/CHANGELOG.md b/CHANGELOG.md
index 970c478..9b24fcb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,16 @@ Notable changes to kubestronaut-sim, newest first. A version is the `VERSION` st
 (year.month.day.n); `./ksim help` and the top bar of the web page show the one you are running.
 Every entry names the commit it came in.
 
+## Unreleased
+
+### Fixed
+
+- Choosing *Teach a domain* could start a full exam. `./ksim web` reads `web/index.html` afresh on
+  every request but keeps running the code it started with, so a server from before the update
+  read the new request as its default, a full exam. The server now reports when its code has
+  changed on disk since it started (a server from before this check counts as changed). The
+  lobby then asks you to restart it and starts nothing until you do.
+
 ## 2026.10.26.2 — 2026-10-26
 
 Everything below came in `e852419`.
diff --git a/ksim b/ksim
index 6699c91..2bfbc88 100755
--- a/ksim
+++ b/ksim
@@ -2165,9 +2165,17 @@ def cmd_reset(args):
     print(("rebuilt " if not problem else "problem rebuilding ") + f"Q{q['id']:02d}: {q['title']}")
 
 # --------------------------------------------------------------- web ----
+def code_stamp():
+    """When the simulator's own code (ksim, sim/*.py) last changed on disk."""
+    sim_dir = os.path.join(ROOT, "sim")
+    return max(os.path.getmtime(f) for f in [os.path.join(ROOT, "ksim")]
+               + [os.path.join(sim_dir, n) for n in os.listdir(sim_dir) if n.endswith(".py")])
+
+
 class Api(http.server.BaseHTTPRequestHandler):
     """Serves web/index.html plus a small JSON API. State lives in the same files the CLI uses."""
     lock = threading.Lock()
+    started_code = None   # code_stamp() when ./ksim web started
     log_lines = []
     seeding = False
     seed_error = None
@@ -2345,6 +2353,10 @@ class Api(http.server.BaseHTTPRequestHandler):
             "k8s": k8s_version(), "pending": len(PENDING), "drilled": drilled_domains(), "sets": {k: {"name": v, "count": len(set_questions(k))} for k, v in SETS.items()},
             "seen": sorted(last_seen()),
             "seeding": Api.seeding, "seed_error": Api.seed_error,
+            # index.html is read from disk on every request, the server's code only when it started:
+            # after an update the page may ask for what this server does not know (an old one read
+            # "teach a domain" as a full exam), so the page starts nothing until it is restarted
+            "stale": Api.started_code is not None and code_stamp() > Api.started_code,
             "session": None,
         }
         payload.update(self.path_payload())
@@ -2444,6 +2456,7 @@ def cmd_web(args):
     except envs.EnvError as e:
         Api.desktop_error = str(e)
         print(yellow(f"  no desktop: {e}"))
+    Api.started_code = code_stamp()
     http.server.ThreadingHTTPServer.allow_reuse_address = True
     srv = http.server.ThreadingHTTPServer(("127.0.0.1", port), Api)  # type: ignore[arg-type]
     print(bold(f"\n  {TRACK_ID.upper()} exam UI:  http://127.0.0.1:{port}   (ksim {VERSION})\n")
diff --git a/web/index.html b/web/index.html
index 86b6414..7bf5bc7 100644
--- a/web/index.html
+++ b/web/index.html
@@ -397,6 +397,7 @@ try { const t = localStorage.getItem('ksim.theme'); if (t === 'light' || t === '
     <div class="scrollhint">SCROLL FOR YOUR PATH AND OBJECTIVES</div>
   </section>
   <main class="wrap">
+    <div class="alert alert-warning hidden" id="staleNote"></div>
     <div class="alert alert-danger hidden" id="lobbyErr"></div>
     <div class="alert alert-info hidden" id="lobbyNote"></div>
     <section class="card" id="pathCard">
@@ -596,6 +597,9 @@ try { const t = localStorage.getItem('ksim.theme'); if (t === 'light' || t === '
   const fmtHMS = s => { s = Math.max(0, Math.floor(s)); return [s / 3600, s % 3600 / 60, s % 60].map(pad).join(':'); };
   const esc = s => String(s).replace(/[&<>"]/g, c => ({'&': '&amp;', '<': '&lt;', '>': '&gt;', '"': '&quot;'}[c]));
   const oldServer = what => `This ./ksim web was started before "${what}" existed. Stop it (Ctrl-C) and start it again.`;
+  // a server that sends no "stale" predates the check, so it is older than this page as well
+  const STALE = 'The simulator was updated after this ./ksim web started, and the server still runs the old code: it would misread what this page asks for (an old one starts a full exam instead of a domain\'s lessons). Stop it (Ctrl-C), start it again, and reload this page.';
+  const stale = () => S && S.stale !== false;
   const openModal = id => $(id).classList.remove('hidden');
   const closeModal = id => $(id).classList.add('hidden');
   const live = () => !!(S && S.session && !S.session.ended_at);
@@ -683,6 +687,7 @@ try { const t = localStorage.getItem('ksim.theme'); if (t === 'light' || t === '
     $$('#trackPick [data-track]').forEach(el => el.onclick = () => switchTrack(el.dataset.track));
     $('#ver').textContent = 'ksim ' + (S.version || '?') + (S.pending ? ` · ${S.pending} task(s) held back` : '');
     $('#foot').textContent = `ksim ${S.version || '?'} · Apache-2.0 · not affiliated with the CNCF or the Linux Foundation`;
+    $('#staleNote').textContent = STALE; $('#staleNote').classList.toggle('hidden', !stale());
     $('#heroTitle').textContent = T.name;
     $('#heroSub').textContent = `Kubernetes ${T.k8s}. Every task runs on its own cluster that you reach with ssh from a remote desktop, and every session runs against the clock.`;
     $('#hardHero').checked = hard;
@@ -891,6 +896,7 @@ try { const t = localStorage.getItem('ksim.theme'); if (t === 'light' || t === '
     }
   }
   async function launch(body) {
+    if (stale()) { closeModal('#mPicker'); return lobbyError(STALE); }
     closeModal('#mPicker'); $('#lobbyErr').classList.add('hidden'); $('#lobbyNote').classList.add('hidden');
     $('#prepMsg').textContent = 'starting…'; $('#prepLog').textContent = ''; openModal('#mPrep'); polling = true;
     try { await api('/api/start', body); await waitSeeded(); ready(true); }