diff --git a/src/web_interface/error_handler.py b/src/web_interface/error_handler.py
index ea6423a4..706bd747 100644
--- a/src/web_interface/error_handler.py
+++ b/src/web_interface/error_handler.py
@@ -77,6 +77,25 @@ def describe_exception(exc: BaseException,
     """
     message = str(exc).strip()
     text = f"{type(exc).__name__}: {message}" if message else type(exc).__name__
+    return redact_text(text, max_length)
+
+
+def redact_text(text: str, max_length: int = _MAX_DETAIL_LENGTH) -> str:
+    """Make arbitrary text safe to hand back over HTTP.
+
+    Split out of describe_exception because exceptions are not the only thing
+    worth returning: a subprocess's stderr, or a message a helper script
+    printed, is just as useful to a user and just as capable of carrying a
+    token or a password in it.
+
+    Args:
+        text: The text to redact
+        max_length: Truncate beyond this many characters
+
+    Returns:
+        A single line, credentials replaced, length capped.
+    """
+    text = text or ''
     # Order matters: the URL and header forms are more specific than the
     # generic key=value pattern, which would otherwise chew the scheme.
     text = _REDACT_URL_USERINFO.sub(r'\1<redacted>\3', text)
diff --git a/test/web_interface/test_calendar_oauth_endpoints.py b/test/web_interface/test_calendar_oauth_endpoints.py
new file mode 100644
index 00000000..9520be5f
--- /dev/null
+++ b/test/web_interface/test_calendar_oauth_endpoints.py
@@ -0,0 +1,408 @@
+"""Tests the calendar plugin's OAuth and calendar-listing endpoints.
+
+The plugin's config UI advertised a three-step setup, but only step 1 existed
+on the server. Step 3's picker fetched /api/v3/plugins/calendar/list-calendars,
+which was never registered, so Flask fell through to the global 404 handler and
+the user saw "Resource not found" — with nothing to say which resource. Step 2
+had no endpoint either, and no field in the schema at all, even though the
+plugin ships calendar_registration.py written expressly for a web-driven
+two-step flow.
+
+These cover the two new routes: that they exist, that they fail with something
+actionable rather than a bare 404, and that the shapes the widgets consume are
+what the server actually sends.
+"""
+
+import json
+import pickle
+import sys
+from pathlib import Path
+
+import pytest
+
+project_root = Path(__file__).parent.parent.parent
+sys.path.insert(0, str(project_root))
+
+from web_interface.blueprints import api_v3 as mod  # noqa: E402
+
+
+@pytest.fixture
+def client(monkeypatch, tmp_path):
+    """A test client whose calendar plugin lives in tmp_path."""
+    from flask import Flask
+
+    plugin_dir = tmp_path / 'calendar'
+    plugin_dir.mkdir()
+
+    app = Flask(__name__)
+    app.register_blueprint(mod.api_v3, url_prefix='/api/v3')
+    app.config['TESTING'] = True
+    monkeypatch.setattr(mod, '_calendar_plugin_dir', lambda: plugin_dir)
+    with app.test_client() as c:
+        c.plugin_dir = plugin_dir
+        yield c
+
+
+@pytest.fixture
+def uninstalled(monkeypatch):
+    from flask import Flask
+
+    app = Flask(__name__)
+    app.register_blueprint(mod.api_v3, url_prefix='/api/v3')
+    app.config['TESTING'] = True
+    monkeypatch.setattr(mod, '_calendar_plugin_dir', lambda: None)
+    with app.test_client() as c:
+        yield c
+
+
+class TestTheRoutesExistAtAll:
+    """The original bug: the URLs the widgets call were not registered."""
+
+    def test_list_calendars_is_routed(self, client):
+        response = client.get('/api/v3/plugins/calendar/list-calendars')
+        # Reaching the handler is the whole point; what it then says about
+        # missing setup is TestItSaysWhatIsWrong's business.
+        assert response.status_code != 404, "still unrouted"
+        assert response.get_json()['message'] != 'Resource not found'
+
+    def test_authenticate_is_routed(self, client):
+        response = client.post('/api/v3/plugins/calendar/authenticate', json={})
+        assert response.status_code != 404, "still unrouted"
+        assert response.get_json()['message'] != 'Resource not found'
+
+    def test_both_urls_match_what_the_widgets_request(self):
+        # The widgets hardcode these; a rename on either side reintroduces the
+        # original bug silently.
+        picker = Path(project_root) / 'web_interface/static/v3/js/widgets/google-calendar-picker.js'
+        oauth = Path(project_root) / 'web_interface/static/v3/js/widgets/google-oauth.js'
+        assert '/api/v3/plugins/calendar/list-calendars' in picker.read_text(encoding='utf-8')
+        assert '/api/v3/plugins/calendar/authenticate' in oauth.read_text(encoding='utf-8')
+        source = (Path(project_root) / 'web_interface/blueprints/api_v3.py').read_text(encoding='utf-8')
+        assert "'/plugins/calendar/list-calendars'" in source
+        assert "'/plugins/calendar/authenticate'" in source
+
+    def test_the_oauth_widget_is_dispatched_not_rendered_as_a_text_box(self):
+        # The string branch of the config template dispatches on an allow-list
+        # of widget names; anything missing from it silently falls through to a
+        # plain <input type="text">. That produced two boxes on the calendar
+        # page -- the widget's own, and a stray one for the same field -- and
+        # no way to tell which to paste into.
+        template = (Path(project_root)
+                    / 'web_interface/templates/v3/partials/plugin_config.html'
+                    ).read_text(encoding='utf-8')
+        allow_list_line = [ln for ln in template.splitlines()
+                           if "str_widget in [" in ln]
+        assert allow_list_line, "the string widget allow-list moved"
+        assert "'google-oauth'" in allow_list_line[0], allow_list_line[0]
+
+    def test_the_widget_script_is_served(self):
+        base = (Path(project_root) / 'web_interface/templates/v3/base.html'
+                ).read_text(encoding='utf-8')
+        assert 'widgets/google-oauth.js' in base
+
+    def test_the_status_line_is_announced(self):
+        # Every message the widget gives arrives after an async call, so a
+        # screen reader hears nothing unless the element is a live region.
+        widget = (Path(project_root)
+                  / 'web_interface/static/v3/js/widgets/google-oauth.js'
+                  ).read_text(encoding='utf-8')
+        # Both attributes must be on the *status* element. Searching for them
+        # separately would pass with each on a different node, which announces
+        # nothing.
+        assert "status.setAttribute('role', 'status')" in widget, widget[:0]
+        assert "status.setAttribute('aria-live', 'polite')" in widget
+
+    def test_the_paste_box_has_an_accessible_name(self):
+        # A visible label is not enough on its own: without the association the
+        # input's only name is a placeholder, which vanishes on focus -- which
+        # is exactly when the value is being pasted.
+        widget = (Path(project_root)
+                  / 'web_interface/static/v3/js/widgets/google-oauth.js'
+                  ).read_text(encoding='utf-8')
+        # The binding is what matters, not that both lines exist: a `for` and
+        # an `id` that disagree leave the input just as anonymous. Both must
+        # go through the same identifier.
+        import re as _re
+        for_target = _re.search(r"codeLabel\.setAttribute\('for',\s*(\w+)\)", widget)
+        id_source = _re.search(r"codeInput\.id\s*=\s*(\w+)", widget)
+        assert for_target and id_source, (for_target, id_source)
+        assert for_target.group(1) == id_source.group(1), (
+            "label points at %r but the input is %r"
+            % (for_target.group(1), id_source.group(1)))
+
+    def test_the_failed_page_is_called_out_loudly(self):
+        # The loopback redirect lands on a browser error page at exactly the
+        # moment the user has to act. In small grey text it gets missed and the
+        # flow reads as broken while it is working.
+        widget = (Path(project_root)
+                  / 'web_interface/static/v3/js/widgets/google-oauth.js'
+                  ).read_text(encoding='utf-8')
+        assert 'expected' in widget.lower()
+        assert 'amber' in widget, "the warning is not visually distinguished"
+
+
+class TestItSaysWhatIsWrong:
+    def test_listing_without_a_token_asks_for_step_2(self, client):
+        response = client.get('/api/v3/plugins/calendar/list-calendars')
+        assert response.status_code == 400
+        body = response.get_json()
+        assert body['status'] == 'error'
+        assert 'step 2' in body['message'].lower(), body['message']
+
+    def test_authenticating_without_credentials_asks_for_step_1(self, client):
+        response = client.post('/api/v3/plugins/calendar/authenticate', json={})
+        assert response.status_code == 400
+        assert 'step 1' in response.get_json()['message'].lower()
+
+    def test_an_uninstalled_plugin_says_so(self, uninstalled):
+        for response in (
+            uninstalled.get('/api/v3/plugins/calendar/list-calendars'),
+            uninstalled.post('/api/v3/plugins/calendar/authenticate', json={}),
+        ):
+            assert response.status_code == 404
+            # A 404 here is honest -- but it must name the plugin, not read as
+            # the generic "Resource not found" that started this.
+            assert 'not installed' in response.get_json()['message'].lower()
+
+
+class TestTheScriptRunner:
+    def test_it_returns_the_json_the_script_prints(self, tmp_path):
+        script = tmp_path / 'calendar_registration.py'
+        script.write_text(
+            'print(\'{"status": "success", "auth_url": "https://x"}\')\n',
+            encoding='utf-8')
+        payload, error = mod._run_calendar_registration(tmp_path, '')
+        assert error is None
+        assert payload['auth_url'] == 'https://x'
+
+    def test_it_ignores_noise_before_the_json(self, tmp_path):
+        # An import warning or a library writing to stdout would otherwise
+        # make the last-line parse fail.
+        script = tmp_path / 'calendar_registration.py'
+        script.write_text(
+            'print("some library warning")\n'
+            'print(\'{"status": "success"}\')\n', encoding='utf-8')
+        payload, error = mod._run_calendar_registration(tmp_path, '')
+        assert error is None and payload['status'] == 'success'
+
+    def test_it_passes_stdin_through(self, tmp_path):
+        script = tmp_path / 'calendar_registration.py'
+        script.write_text(
+            'import sys, json\n'
+            'print(json.dumps({"status": "success", "got": sys.stdin.read().strip()}))\n',
+            encoding='utf-8')
+        payload, _ = mod._run_calendar_registration(tmp_path, 'http://127.0.0.1/?code=abc')
+        assert payload['got'] == 'http://127.0.0.1/?code=abc'
+
+    def test_a_missing_script_is_reported(self, tmp_path):
+        payload, error = mod._run_calendar_registration(tmp_path, '')
+        assert payload is None
+        assert 'script not found' in error.lower()
+
+    def test_output_that_is_not_json_is_reported_with_context(self, tmp_path):
+        script = tmp_path / 'calendar_registration.py'
+        script.write_text('import sys\nsys.stderr.write("boom\\n")\n', encoding='utf-8')
+        payload, error = mod._run_calendar_registration(tmp_path, '')
+        assert payload is None
+        assert 'no result' in error.lower()
+        assert 'boom' in error
+
+
+class TestListingShape:
+    """The picker reads cal.id, cal.summary and cal.primary."""
+
+    def _authenticate(self, client, monkeypatch, items):
+        creds = type('C', (), {'expired': False, 'refresh_token': None, 'valid': True})()
+        (client.plugin_dir / 'token.pickle').write_bytes(pickle.dumps({'x': 1}))
+        monkeypatch.setattr(mod.pickle if hasattr(mod, 'pickle') else pickle,
+                            'loads', lambda *a, **k: creds, raising=False)
+
+        import types
+        fake_pickle = types.SimpleNamespace(load=lambda f: creds, dump=lambda *a: None)
+        # Callers pass a flat list of calendars; the API returns them wrapped
+        # in a page. One page is all these cases need -- TestPagination builds
+        # its own multi-page sequences.
+        pages = [{'items': items}]
+
+        state = {'i': 0}
+
+        def fake_list(**kwargs):
+            page = pages[min(state['i'], len(pages) - 1)]
+            state['i'] += 1
+            return types.SimpleNamespace(execute=lambda: page)
+
+        def fake_build(*args, **kwargs):
+            return types.SimpleNamespace(
+                calendarList=lambda: types.SimpleNamespace(list=fake_list))
+
+        real_import = __builtins__['__import__'] if isinstance(__builtins__, dict) \
+            else __builtins__.__import__
+
+        def fake_import(name, *args, **kwargs):
+            if name == 'pickle':
+                return fake_pickle
+            if name == 'google.auth.transport.requests':
+                return types.SimpleNamespace(Request=object)
+            if name == 'googleapiclient.discovery':
+                return types.SimpleNamespace(build=fake_build)
+            return real_import(name, *args, **kwargs)
+
+        monkeypatch.setattr('builtins.__import__', fake_import)
+
+    def test_it_returns_id_summary_and_primary(self, client, monkeypatch):
+        self._authenticate(client, monkeypatch, [
+            {'id': 'b@x', 'summary': 'Work'},
+            {'id': 'a@x', 'summary': 'Personal', 'primary': True},
+        ])
+        body = client.get('/api/v3/plugins/calendar/list-calendars').get_json()
+        assert body['status'] == 'success'
+        assert {c['id'] for c in body['calendars']} == {'a@x', 'b@x'}
+        assert all(set(c) == {'id', 'summary', 'primary'} for c in body['calendars'])
+
+    def test_the_primary_calendar_comes_first(self, client, monkeypatch):
+        # Short list, but the one the user wants is almost always their own.
+        self._authenticate(client, monkeypatch, [
+            {'id': 'z@x', 'summary': 'Aardvarks'},
+            {'id': 'a@x', 'summary': 'Zebras', 'primary': True},
+        ])
+        body = client.get('/api/v3/plugins/calendar/list-calendars').get_json()
+        assert body['calendars'][0]['id'] == 'a@x'
+        assert body['calendars'][0]['primary'] is True
+
+    def test_a_calendar_without_a_name_still_lists(self, client, monkeypatch):
+        self._authenticate(client, monkeypatch, [{'id': 'noname@x'}])
+        body = client.get('/api/v3/plugins/calendar/list-calendars').get_json()
+        assert body['calendars'][0]['summary'] == 'noname@x'
+
+    def test_entries_without_an_id_are_dropped(self, client, monkeypatch):
+        # Nothing could be selected by such a row, and the checkbox value
+        # would be undefined.
+        self._authenticate(client, monkeypatch, [{'summary': 'ghost'}, {'id': 'real@x'}])
+        body = client.get('/api/v3/plugins/calendar/list-calendars').get_json()
+        assert [c['id'] for c in body['calendars']] == ['real@x']
+
+
+class TestPagination:
+    """calendarList.list pages at 250 and defaults to 100."""
+
+    def _paged(self, client, monkeypatch, pages):
+        import types
+        creds = type('C', (), {'expired': False, 'refresh_token': None, 'valid': True})()
+        (client.plugin_dir / 'token.pickle').write_bytes(b'x')
+        state = {'i': 0}
+        seen = []
+
+        def fake_list(**kwargs):
+            seen.append(kwargs)
+            page = pages[min(state['i'], len(pages) - 1)]
+            state['i'] += 1
+            return types.SimpleNamespace(execute=lambda: page)
+
+        def fake_build(*args, **kwargs):
+            return types.SimpleNamespace(
+                calendarList=lambda: types.SimpleNamespace(list=fake_list))
+
+        real_import = __builtins__['__import__'] if isinstance(__builtins__, dict) \
+            else __builtins__.__import__
+
+        def fake_import(name, *args, **kwargs):
+            if name == 'pickle':
+                return types.SimpleNamespace(load=lambda f: creds, dump=lambda *a: None)
+            if name == 'google.auth.transport.requests':
+                return types.SimpleNamespace(Request=object)
+            if name == 'googleapiclient.discovery':
+                return types.SimpleNamespace(build=fake_build)
+            return real_import(name, *args, **kwargs)
+
+        monkeypatch.setattr('builtins.__import__', fake_import)
+        return seen
+
+    def test_every_page_is_collected(self, client, monkeypatch):
+        # Taking only the first page would hide calendars from the picker with
+        # nothing to say the list was cut short.
+        self._paged(client, monkeypatch, [
+            {'items': [{'id': 'a@x', 'summary': 'A'}], 'nextPageToken': 't1'},
+            {'items': [{'id': 'b@x', 'summary': 'B'}], 'nextPageToken': 't2'},
+            {'items': [{'id': 'c@x', 'summary': 'C'}]},
+        ])
+        body = client.get('/api/v3/plugins/calendar/list-calendars').get_json()
+        assert [c['id'] for c in body['calendars']] == ['a@x', 'b@x', 'c@x']
+
+    def test_the_page_token_is_passed_back(self, client, monkeypatch):
+        seen = self._paged(client, monkeypatch, [
+            {'items': [{'id': 'a@x', 'summary': 'A'}], 'nextPageToken': 'tok'},
+            {'items': [{'id': 'b@x', 'summary': 'B'}]},
+        ])
+        client.get('/api/v3/plugins/calendar/list-calendars')
+        assert seen[0]['pageToken'] is None
+        assert seen[1]['pageToken'] == 'tok'
+        assert all(k['maxResults'] == 250 for k in seen)
+
+    def test_a_looping_token_cannot_spin_forever(self, client, monkeypatch):
+        # Every page claims another follows.
+        self._paged(client, monkeypatch, [
+            {'items': [{'id': 'a@x', 'summary': 'A'}], 'nextPageToken': 'same'},
+        ])
+        body = client.get('/api/v3/plugins/calendar/list-calendars').get_json()
+        assert body['status'] == 'success'
+        assert len(body['calendars']) <= mod._CALENDAR_LIST_MAX_PAGES
+
+
+class TestDiagnosticsAreRedacted:
+    def test_script_stderr_is_redacted_on_the_way_out(self, tmp_path):
+        script = tmp_path / 'calendar_registration.py'
+        script.write_text(
+            'import sys\n'
+            'sys.stderr.write("boom client_secret=hunter2 more\\n")\n',
+            encoding='utf-8')
+        payload, error = mod._run_calendar_registration(tmp_path, '')
+        assert payload is None
+        assert 'hunter2' not in error, error
+        assert '<redacted>' in error, error
+
+    def test_a_failing_script_payload_is_redacted(self, client):
+        (client.plugin_dir / 'credentials.json').write_text('{}', encoding='utf-8')
+        (client.plugin_dir / 'calendar_registration.py').write_text(
+            'import json\n'
+            'print(json.dumps({"status": "error", '
+            '"message": "Failed: client_secret=topsecret"}))\n',
+            encoding='utf-8')
+        body = client.post('/api/v3/plugins/calendar/authenticate',
+                           json={}).get_json()
+        assert body['status'] == 'error'
+        assert 'topsecret' not in json.dumps(body), body
+        assert '<redacted>' in body['message'], body
+
+    def test_an_unrunnable_script_is_reported_without_raw_exception_text(self,
+                                                                        tmp_path,
+                                                                        monkeypatch):
+        # OSError from the spawn carries the interpreter path and whatever the
+        # OS chose to say; it reaches the client through the redactor like
+        # everything else.
+        script = tmp_path / 'calendar_registration.py'
+        script.write_text('', encoding='utf-8')
+
+        def boom(*a, **k):
+            raise OSError("Exec format error: token=abcd1234 /usr/bin/python3")
+
+        monkeypatch.setattr(mod.subprocess, 'run', boom)
+        payload, error = mod._run_calendar_registration(tmp_path, '')
+        assert payload is None
+        assert 'abcd1234' not in error, error
+        assert 'OSError' in error, error
+
+    def test_a_missing_google_library_is_reported_without_raw_exception_text(
+            self, client, monkeypatch):
+        (client.plugin_dir / 'token.pickle').write_bytes(b'x')
+        real_import = __builtins__['__import__'] if isinstance(__builtins__, dict) \
+            else __builtins__.__import__
+
+        def fake_import(name, *args, **kwargs):
+            if name.startswith('google'):
+                raise ImportError("No module named 'google' password=hunter2")
+            return real_import(name, *args, **kwargs)
+
+        monkeypatch.setattr('builtins.__import__', fake_import)
+        body = client.get('/api/v3/plugins/calendar/list-calendars').get_json()
+        assert 'hunter2' not in json.dumps(body), body
+        assert 'requirements.txt' in body['message']
diff --git a/web_interface/blueprints/api_v3.py b/web_interface/blueprints/api_v3.py
index 06fef117..984637e8 100644
--- a/web_interface/blueprints/api_v3.py
+++ b/web_interface/blueprints/api_v3.py
@@ -22,7 +22,7 @@ logger = logging.getLogger(__name__)
 from src.web_interface.api_helpers import success_response, error_response, validate_request_json
 from src.web_interface.errors import ErrorCode
 from src.web_interface.secret_helpers import find_secret_fields, separate_secrets
-from src.web_interface.error_handler import describe_exception
+from src.web_interface.error_handler import describe_exception, redact_text
 from src.plugin_system.operation_types import OperationType
 from src.web_interface.validators import (
     validate_file_upload
@@ -7317,6 +7317,227 @@ def upload_calendar_credentials():
         logger.error('Error in upload_calendar_credentials', exc_info=True)
         return jsonify({'status': 'error', 'message': 'An error occurred; see logs for details', 'details': describe_exception(e)}), 500
 
+# calendarList.list pages at 250 entries maximum. Ten pages is far past any
+# real account and exists only so a malformed nextPageToken cannot spin here.
+_CALENDAR_LIST_MAX_PAGES = 10
+
+
+def _calendar_plugin_dir() -> Optional[Path]:
+    """Where the calendar plugin is installed, or None if it is not."""
+    if api_v3.plugin_manager:
+        plugin_dir = api_v3.plugin_manager.get_plugin_directory('calendar')
+    else:
+        plugin_dir = PROJECT_ROOT / 'plugins' / 'calendar'
+    if not plugin_dir:
+        return None
+    plugin_dir = Path(plugin_dir)
+    return plugin_dir if plugin_dir.exists() else None
+
+
+def _run_calendar_registration(plugin_dir: Path, stdin_payload: str):
+    """Run the plugin's OAuth script and return the JSON object it prints.
+
+    The script decides between web and terminal mode by whether stdin is a
+    tty, so it must be given a pipe. It emits one JSON object on stdout; the
+    last parsable line is taken, because an import warning or a library's
+    stderr redirection can land in front of it.
+
+    Returns (payload, error_message). Exactly one is None.
+    """
+    script = plugin_dir / 'calendar_registration.py'
+    if not script.exists():
+        return None, 'Authentication script not found in the calendar plugin'
+
+    try:
+        result = subprocess.run(  # nosec B603 - fixed script path inside the plugin dir
+            [sys.executable, str(script)],
+            input=stdin_payload,
+            capture_output=True,
+            text=True,
+            timeout=120,
+            cwd=str(plugin_dir),
+        )
+    except subprocess.TimeoutExpired:
+        return None, 'Authentication timed out after 120s'
+    except OSError as e:
+        logger.error('Could not run calendar_registration.py', exc_info=True)
+        return None, 'Could not run the authentication script: %s' % describe_exception(e)
+
+    for line in reversed((result.stdout or '').splitlines()):
+        line = line.strip()
+        if not line:
+            continue
+        try:
+            payload = json.loads(line)
+        except json.JSONDecodeError:
+            continue
+        if isinstance(payload, dict):
+            return payload, None
+
+    raw = (result.stderr or result.stdout or '').strip()
+    # The unredacted text goes to the log, where it is worth having in full.
+    # What comes back over HTTP is redacted: this is a script that handles
+    # OAuth client secrets, and its stderr can quote them.
+    if raw:
+        logger.error('calendar_registration.py failed (exit %s): %s',
+                     result.returncode, raw)
+    return None, 'Authentication script produced no result%s' % (
+        ': %s' % redact_text(raw) if raw else '')
+
+
+@api_v3.route('/plugins/calendar/authenticate', methods=['POST'])
+def authenticate_calendar():
+    """Google OAuth for the calendar plugin, in the two steps it requires.
+
+    Step 1 (no body) returns the consent URL to open. Step 2 posts back the
+    URL Google redirected to -- it fails to load, because the redirect points
+    at a loopback address nothing is listening on, but the address bar carries
+    the authorization code -- and the script exchanges it for a token.
+
+    Two calls rather than one because the user has to visit Google in between.
+    The script persists the PKCE verifier from step 1 for step 2 to reuse; the
+    exchange fails with "Missing code verifier" otherwise.
+    """
+    try:
+        plugin_dir = _calendar_plugin_dir()
+        if plugin_dir is None:
+            return jsonify({
+                'status': 'error',
+                'message': 'The calendar plugin is not installed'
+            }), 404
+
+        if not (plugin_dir / 'credentials.json').exists():
+            return jsonify({
+                'status': 'error',
+                'message': ('No credentials.json yet. Upload your Google OAuth '
+                            'client file first (Step 1).')
+            }), 400
+
+        data = request.get_json(silent=True) or {}
+        redirect_url = (data.get('redirect_url') or data.get('code') or '').strip()
+
+        payload, error = _run_calendar_registration(plugin_dir, redirect_url)
+        if error:
+            return jsonify({'status': 'error', 'message': error}), 500
+        if payload.get('status') != 'success':
+            # The script's own diagnosis is more useful than anything that
+            # could be reconstructed here -- but it interpolates exceptions
+            # into its messages, so it reaches the client redacted and the
+            # original goes to the log.
+            logger.error('calendar authentication failed: %s', payload)
+            safe = dict(payload)
+            safe['message'] = redact_text(str(payload.get('message', '')
+                                              or 'Authentication failed'))
+            return jsonify(safe), 400
+        return jsonify(payload)
+
+    except Exception as e:
+        logger.error('Error in authenticate_calendar', exc_info=True)
+        return jsonify({'status': 'error',
+                        'message': 'An error occurred; see logs for details',
+                        'details': describe_exception(e)}), 500
+
+
+@api_v3.route('/plugins/calendar/list-calendars', methods=['GET'])
+def list_calendar_calendars():
+    """The calendars this account can see, for the config picker.
+
+    Reads the token the OAuth flow wrote rather than shelling out again: the
+    picker is used interactively and a subprocess per click is slower than the
+    API call it would be wrapping.
+    """
+    try:
+        plugin_dir = _calendar_plugin_dir()
+        if plugin_dir is None:
+            return jsonify({
+                'status': 'error',
+                'message': 'The calendar plugin is not installed'
+            }), 404
+
+        token_file = plugin_dir / 'token.pickle'
+        if not token_file.exists():
+            return jsonify({
+                'status': 'error',
+                'message': ('Not authenticated with Google yet. Complete Step 2 '
+                            'first, then load your calendars.')
+            }), 400
+
+        try:
+            import pickle
+            from google.auth.transport.requests import Request as GoogleRequest
+            from googleapiclient.discovery import build as build_google_service
+        except ImportError as e:
+            return jsonify({
+                'status': 'error',
+                # The name of the missing module is the whole diagnosis, but it
+                # arrives as an exception, so it goes through the redactor like
+                # any other -- an ImportError can quote a path.
+                'message': ('The Google API libraries are not installed. Install '
+                            "the calendar plugin's requirements.txt. (%s)"
+                            % describe_exception(e))
+            }), 500
+
+        with open(token_file, 'rb') as handle:
+            # Written only by this plugin's own OAuth flow, into its own
+            # directory, and read here exactly as the plugin itself reads it.
+            creds = pickle.load(handle)  # nosec B301 - locally generated token
+
+        if creds and creds.expired and creds.refresh_token:
+            creds.refresh(GoogleRequest())
+            with open(token_file, 'wb') as handle:
+                pickle.dump(creds, handle)
+            os.chmod(token_file, 0o600)
+
+        if not creds or not creds.valid:
+            return jsonify({
+                'status': 'error',
+                'message': ('Stored Google credentials are no longer valid. '
+                            'Run Step 2 again to re-authenticate.')
+            }), 400
+
+        service = build_google_service('calendar', 'v3', credentials=creds)
+
+        # calendarList.list returns 100 entries per page by default and caps at
+        # 250, handing back a nextPageToken when there are more. Taking only
+        # the first page would silently hide calendars from the picker, and the
+        # user would have no way to tell the list was truncated.
+        entries = []
+        page_token = None
+        for _ in range(_CALENDAR_LIST_MAX_PAGES):
+            response = service.calendarList().list(
+                maxResults=250, pageToken=page_token).execute()
+            entries.extend(response.get('items', []))
+            page_token = response.get('nextPageToken')
+            if not page_token:
+                break
+        else:
+            # 2500 calendars in, something is wrong with the account or the
+            # token is looping; show what was collected rather than spin.
+            logger.warning(
+                'calendarList paging stopped at %d pages with more remaining',
+                _CALENDAR_LIST_MAX_PAGES)
+
+        calendars = [{
+            'id': entry.get('id'),
+            # The picker labels each row with summary and falls back to the id
+            # only in its own display, so send something either way.
+            'summary': entry.get('summary') or entry.get('id'),
+            'primary': bool(entry.get('primary', False)),
+        } for entry in entries if entry.get('id')]
+
+        # Primary first, then alphabetically: the list is usually short but the
+        # one the user wants is almost always their own calendar.
+        calendars.sort(key=lambda c: (not c['primary'], c['summary'].lower()))
+
+        return jsonify({'status': 'success', 'calendars': calendars})
+
+    except Exception as e:
+        logger.error('Error in list_calendar_calendars', exc_info=True)
+        return jsonify({'status': 'error',
+                        'message': 'An error occurred; see logs for details',
+                        'details': describe_exception(e)}), 500
+
+
 @api_v3.route('/plugins/assets/delete', methods=['POST'])
 def delete_plugin_asset():
     """Delete an asset file for a plugin"""
diff --git a/web_interface/static/v3/js/widgets/google-oauth.js b/web_interface/static/v3/js/widgets/google-oauth.js
new file mode 100644
index 00000000..f920b7f9
--- /dev/null
+++ b/web_interface/static/v3/js/widgets/google-oauth.js
@@ -0,0 +1,196 @@
+/**
+ * Google OAuth Widget
+ *
+ * Step 2 of the calendar plugin's setup, between uploading the OAuth client
+ * file and picking calendars. Google will not let a headless device complete
+ * consent on its own, so the flow is necessarily two calls with a human in
+ * between:
+ *
+ *   1. POST /api/v3/plugins/calendar/authenticate with no body
+ *      -> { auth_url } to open in a browser
+ *   2. the browser lands on a loopback address that fails to load; its URL
+ *      carries the authorization code. POST it back as redirect_url
+ *      -> the server exchanges it and writes token.pickle
+ *
+ * The failed page in step 2 is expected and is worth saying out loud, because
+ * it looks exactly like something went wrong.
+ *
+ * @module GoogleOAuthWidget
+ */
+
+(function () {
+    'use strict';
+
+    if (typeof window.LEDMatrixWidgets === 'undefined') {
+        console.error('[GoogleOAuthWidget] LEDMatrixWidgets registry not found. Load registry.js first.');
+        return;
+    }
+
+    const ENDPOINT = '/api/v3/plugins/calendar/authenticate';
+
+    window.LEDMatrixWidgets.register('google-oauth', {
+        name: 'Google OAuth Widget',
+        version: '1.0.0',
+
+        /**
+         * @param {HTMLElement} container
+         * @param {Object} config  - schema config (unused)
+         * @param {*}      value   - unused; this widget stores nothing
+         * @param {Object} options - { fieldId, pluginId, name }
+         */
+        render: function (container, config, value, options) {
+            const fieldId = options.fieldId;
+
+            // Nothing is stored in config by this step -- the result is
+            // token.pickle on the device -- but the form still expects a field.
+            const hidden = document.createElement('input');
+            hidden.type = 'hidden';
+            hidden.id = fieldId + '_hidden';
+            hidden.name = options.name;
+            hidden.value = value || '';
+
+            const startBtn = document.createElement('button');
+            startBtn.type = 'button';
+            startBtn.className = 'px-3 py-1.5 text-sm rounded-md bg-blue-600 hover:bg-blue-700 text-white';
+            startBtn.innerHTML = '<i class="fas fa-key"></i> Connect Google Account';
+
+            const status = document.createElement('p');
+            status.className = 'text-xs text-gray-400 mt-2';
+            // Every message this widget gives -- the consent link is ready,
+            // the exchange failed -- arrives here after an async call, so a
+            // screen reader is told nothing unless it is a live region.
+            status.setAttribute('role', 'status');
+            status.setAttribute('aria-live', 'polite');
+
+            const step2 = document.createElement('div');
+            step2.className = 'mt-3 hidden';
+
+            const link = document.createElement('a');
+            link.target = '_blank';
+            link.rel = 'noopener noreferrer';
+            link.className = 'text-blue-400 underline text-sm break-all';
+            link.textContent = 'Open the Google consent screen';
+
+            // Deliberately loud. After consent the browser is redirected to a
+            // loopback address nothing is listening on, so it lands on a
+            // browser error page -- which reads as a failure at exactly the
+            // moment the user has to act on it. Said quietly in grey it gets
+            // missed, and the flow looks broken when it is working.
+            const hint = document.createElement('div');
+            hint.className =
+                'mt-3 p-3 rounded-md border border-amber-500/60 bg-amber-500/10';
+            hint.innerHTML =
+                '<p class="text-sm text-amber-300 font-semibold">'
+                + '<i class="fas fa-triangle-exclamation"></i> '
+                + 'The next page will fail to load. That is expected.</p>'
+                + '<p class="text-xs text-amber-200/90 mt-1">'
+                + 'After you approve access, Google sends your browser to '
+                + '<code>127.0.0.1</code>, where nothing is running \u2014 so you will see '
+                + '"This site can\u2019t be reached" or similar. Nothing has gone wrong. '
+                + 'Copy the <strong>entire address</strong> out of the address bar '
+                + '(it contains <code>?code=...</code>) and paste it in the box below.</p>';
+
+            const codeInputId = fieldId + '_redirect_url';
+
+            const codeLabel = document.createElement('label');
+            codeLabel.className = 'block text-xs text-gray-300 mt-3';
+            codeLabel.textContent = 'Paste the address from that failed page here:';
+            // The label was visible but not associated, so the input still had
+            // no accessible name -- a placeholder is not one, and it vanishes
+            // on focus, which is exactly when the value is being pasted.
+            codeLabel.setAttribute('for', codeInputId);
+
+            const codeInput = document.createElement('input');
+            codeInput.type = 'text';
+            codeInput.id = codeInputId;
+            codeInput.placeholder = 'http://127.0.0.1/?code=...';
+            codeInput.className =
+                'mt-2 block w-full px-3 py-2 text-sm border border-gray-600 '
+                + 'rounded-md bg-gray-800 text-gray-100';
+
+            const finishBtn = document.createElement('button');
+            finishBtn.type = 'button';
+            finishBtn.className = 'mt-2 px-3 py-1.5 text-sm rounded-md bg-green-600 hover:bg-green-700 text-white';
+            finishBtn.innerHTML = '<i class="fas fa-check"></i> Finish Authentication';
+
+            function say(message, kind) {
+                status.textContent = message;
+                status.className = 'text-xs mt-2 ' + (
+                    kind === 'error' ? 'text-red-400'
+                        : kind === 'success' ? 'text-green-400'
+                            : 'text-gray-400');
+            }
+
+            function post(body) {
+                return fetch(ENDPOINT, {
+                    method: 'POST',
+                    headers: { 'Content-Type': 'application/json' },
+                    body: JSON.stringify(body || {})
+                }).then(function (r) {
+                    return r.json().catch(function () {
+                        // A non-JSON body here means the request never reached
+                        // the handler -- worth saying so rather than "undefined".
+                        return { status: 'error', message: 'Server returned ' + r.status };
+                    });
+                });
+            }
+
+            startBtn.addEventListener('click', function () {
+                startBtn.disabled = true;
+                say('Requesting a consent link...');
+                post({}).then(function (data) {
+                    startBtn.disabled = false;
+                    if (data.status !== 'success' || !data.auth_url) {
+                        say(data.message || 'Could not start authentication.', 'error');
+                        return;
+                    }
+                    link.href = data.auth_url;
+                    step2.classList.remove('hidden');
+                    say(data.message || 'Open the link, approve, then paste the address back.');
+                }).catch(function (err) {
+                    startBtn.disabled = false;
+                    say('Request failed: ' + err.message, 'error');
+                });
+            });
+
+            finishBtn.addEventListener('click', function () {
+                const pasted = codeInput.value.trim();
+                if (!pasted) {
+                    say('Paste the address your browser was redirected to.', 'error');
+                    return;
+                }
+                finishBtn.disabled = true;
+                say('Exchanging the code with Google...');
+                post({ redirect_url: pasted }).then(function (data) {
+                    finishBtn.disabled = false;
+                    if (data.status !== 'success') {
+                        say(data.message || 'Authentication failed.', 'error');
+                        return;
+                    }
+                    say(data.message || 'Authenticated.', 'success');
+                    step2.classList.add('hidden');
+                    codeInput.value = '';
+                }).catch(function (err) {
+                    finishBtn.disabled = false;
+                    say('Request failed: ' + err.message, 'error');
+                });
+            });
+
+            step2.appendChild(link);
+            step2.appendChild(hint);
+            step2.appendChild(codeLabel);
+            step2.appendChild(codeInput);
+            step2.appendChild(finishBtn);
+
+            container.appendChild(hidden);
+            container.appendChild(startBtn);
+            container.appendChild(status);
+            container.appendChild(step2);
+        },
+
+        getValue: function (fieldId) {
+            const hidden = document.getElementById(fieldId + '_hidden');
+            return hidden ? hidden.value : '';
+        }
+    });
+})();
diff --git a/web_interface/templates/v3/base.html b/web_interface/templates/v3/base.html
index 28ed48b9..fb4cfa6a 100644
--- a/web_interface/templates/v3/base.html
+++ b/web_interface/templates/v3/base.html
@@ -987,6 +987,7 @@
     <script src="{{ url_for('static', filename='v3/js/widgets/custom-feeds.js') }}" defer></script>
     <script src="{{ url_for('static', filename='v3/js/widgets/array-table.js') }}" defer></script>
     <script src="{{ url_for('static', filename='v3/js/widgets/google-calendar-picker.js') }}" defer></script>
+    <script src="{{ url_for('static', filename='v3/js/widgets/google-oauth.js') }}" defer></script>
     <script src="{{ url_for('static', filename='v3/js/widgets/day-selector.js') }}" defer></script>
     <script src="{{ url_for('static', filename='v3/js/widgets/time-range.js') }}" defer></script>
     <script src="{{ url_for('static', filename='v3/js/widgets/time-picker.js') }}" defer></script>
diff --git a/web_interface/templates/v3/partials/plugin_config.html b/web_interface/templates/v3/partials/plugin_config.html
index 41651fb8..f789fd5b 100644
--- a/web_interface/templates/v3/partials/plugin_config.html
+++ b/web_interface/templates/v3/partials/plugin_config.html
@@ -815,7 +815,7 @@
                     <i class="fas fa-info-circle mr-1"></i>
                     Changes in the file manager save immediately — no need to click Save Configuration.
                 </p>
-            {% elif str_widget in ['text-input', 'textarea', 'select-dropdown', 'toggle-switch', 'radio-group', 'date-picker', 'time-picker', 'slider', 'color-picker', 'email-input', 'url-input', 'password-input', 'font-selector', 'file-upload-single', 'plugin-file-manager'] %}
+            {% elif str_widget in ['text-input', 'textarea', 'select-dropdown', 'toggle-switch', 'radio-group', 'date-picker', 'time-picker', 'slider', 'color-picker', 'email-input', 'url-input', 'password-input', 'font-selector', 'file-upload-single', 'plugin-file-manager', 'google-oauth'] %}
                 {# Render widget container #}
                 <div id="{{ field_id }}_container" class="{{ str_widget }}-container"></div>
                 <script>