diff --git a/ide/auth.py b/ide/auth.py
index 41d429dd..4f8b528a 100644
--- a/ide/auth.py
+++ b/ide/auth.py
@@ -7,6 +7,7 @@
 from flask import Flask, url_for, session, request, make_response, redirect
 from flask import render_template, redirect
 from authlib.integrations.flask_client import OAuth
+from authlib.integrations.base_client.errors import OAuthError
 from authlib.common.security import generate_token
 import json, base64
 import os
@@ -209,14 +210,31 @@ def auth():
             newURL = urlunparse((scheme,dstHost) + oldURL[2:])  # build the final URL
             return redirect(newURL)
     else:
+        # A bare /google/auth with no state — a bookmark or a manually edited
+        # URL. Processing it can only crash; send them home to start over.
         app.logger.info("Yikes! No state found. This shouldn't happen.")
+        return redirect('/')
 
     #
     # If we get to here it means we're the final server. Go ahead and process.
     #
         
     oauth = authNamespace.get('oauth') or fillAuthNamespace()
-    token = oauth.google.authorize_access_token()
+    try:
+        token = oauth.google.authorize_access_token()
+    except OAuthError as err:
+        # The state is single-use and lives in the session cookie, so this is
+        # reached by two real populations, both harmless and both unrecoverable
+        # on THIS request:
+        #   - a refresh/replay of the callback URL (the state was consumed on
+        #     the first attempt) — observed live as one machine retrying a dead
+        #     callback 68 times, each retry a bare GAE 500 page;
+        #   - a browser that refused the session cookie, so no state exists.
+        # Production ran at a steady 1-2% of sign-ins failing this way. A retry
+        # of the same URL can never succeed, so the only useful answer is a
+        # clean landing where the user can simply sign in again.
+        app.logger.warning("OAuth callback failed (%s); sending user home to retry", err.error)
+        return redirect('/')
     user = token['userinfo']
     
     if check_auth_host_for_preview(auth_host): # are we in a preview version?
diff --git a/tests/test_auth_callback.py b/tests/test_auth_callback.py
new file mode 100644
index 00000000..8b5d8eff
--- /dev/null
+++ b/tests/test_auth_callback.py
@@ -0,0 +1,34 @@
+import base64
+import json
+
+# A replayed or cookie-less OAuth callback must not 500.
+#
+# The authlib state is single-use and lives in the session cookie. Two real
+# populations therefore hit MismatchingStateError on /google/auth:
+#   - anyone who REFRESHES the callback URL (the state was consumed on the
+#     first attempt) — observed live: one classroom machine retried a dead
+#     callback 68 times on 2026-09-01, each retry rendering GAE's bare
+#     "500 Server Error" page;
+#   - browsers that refuse the session cookie, so no state is ever stored.
+# Production logs show a steady 1-2% of sign-ins failing this way for at least
+# a month. The failure is unrecoverable BY DESIGN — retrying the same URL can
+# only ever fail — so the only useful response is a clean landing page where
+# the user can start over.
+
+def _state(host):
+    return base64.b64encode(json.dumps({'dstHost': host, 'salt': 'x'}).encode()).decode()
+
+def test_replayed_callback_redirects_home_instead_of_500(client):
+    # No session state exists (fresh client), which is exactly the replay /
+    # blocked-cookie shape: authlib raises MismatchingStateError.
+    resp = client.get('/google/auth?state=' + _state('localhost') + '&code=junk')
+
+    assert resp.status_code == 302, (
+        'a dead callback should land the user somewhere useful, got %s' % resp.status_code)
+    assert resp.headers['Location'].startswith('/'), resp.headers['Location']
+
+def test_callback_with_no_state_at_all_redirects_home(client):
+    # A bookmarked /google/auth with no parameters — the "Yikes!" branch.
+    resp = client.get('/google/auth')
+    assert resp.status_code == 302
+    assert resp.headers['Location'].startswith('/')