diff --git a/engine/auth/browser_pkce.py b/engine/auth/browser_pkce.py
index bc19f67..0bcb079 100644
--- a/engine/auth/browser_pkce.py
+++ b/engine/auth/browser_pkce.py
@@ -17,6 +17,31 @@
 ``is_available()`` is False on a headless server (no browser / no loopback), so
 the UI falls back to :class:`~engine.auth.paste.PasteTokenProvider`.
 
+NOT WIRED END TO END -- do not enable this provider by setting the Supabase
+environment variables alone. Steps 2-4 above each depend on a hosted contract
+that ``app.openadapt.ai`` does not implement today:
+
+* ``GET /login`` reads only ``checkout_session_id`` and ``next`` (clamped to a
+  same-origin ``/dashboard`` destination). It ignores ``redirect_to``,
+  ``code_challenge``, ``code_challenge_method`` and ``state``, so the browser
+  never navigates to the loopback listener and step 3 cannot happen.
+* The Supabase ``uri_allow_list`` is rewritten on every production deploy and
+  contains no loopback entry, so Supabase would reject a ``127.0.0.1`` redirect
+  even if the page forwarded one.
+* ``POST /api/ingest-tokens`` authenticates the browser session cookie and
+  requires a ``name`` field. It rejects a ``Authorization: Bearer <supabase
+  access token>`` with 401, so step 4 cannot mint.
+
+Because ``_exchange_code`` is only reached AFTER the loopback wait, leaving the
+provider enabled makes ``login`` open a browser tab and then block for the full
+``timeout`` before failing over to token paste. ``is_available()`` therefore
+requires the Supabase configuration up front: the provider reports unavailable
+rather than costing the operator a silent multi-minute stall. Restoring it is a
+hosted-contract change, not a client change -- see the three bullets above.
+
+The working, shipped path to a credential is the one-time pairing flow
+(:mod:`engine.auth.pairing`), which the hosted control plane does implement.
+
 Reconciliation note: the shared store (:mod:`engine.auth.store`) keys ONE active
 credential per host and ``auth_header()`` returns exactly one bearer. We
 therefore fold the Supabase session and the minted ingest token into a single
@@ -161,7 +186,17 @@ def _default_open_browser(url: str) -> None:
         webbrowser.open(url)
 
     def is_available(self) -> bool:
-        """False on a headless box (no browser / no display)."""
+        """False on a headless box, and False while the flow is unconfigured.
+
+        The Supabase check is deliberately first. ``_exchange_code`` cannot run
+        without a project URL and anon key, but it is only reached after the
+        loopback wait -- so an unconfigured provider would open a browser tab
+        and stall for the whole ``timeout`` before failing over to token paste.
+        Reporting unavailable up front keeps ``login`` responsive and keeps the
+        provider chain honest about what it can actually complete.
+        """
+        if not (self._supabase_url and self._supabase_anon_key):
+            return False
         if os.environ.get("OPENADAPT_HEADLESS", "").strip():
             return False
         if sys.platform.startswith("linux"):
diff --git a/tests/test_engine/test_auth_browser_pkce.py b/tests/test_engine/test_auth_browser_pkce.py
index 81165ee..4247505 100644
--- a/tests/test_engine/test_auth_browser_pkce.py
+++ b/tests/test_engine/test_auth_browser_pkce.py
@@ -49,22 +49,65 @@ def _deliver():
         assert receiver.state == "s1"
 
 
+def _configured(**kwargs) -> BrowserPkceProvider:
+    """A provider whose Supabase project is configured, as in a wired deployment."""
+    kwargs.setdefault("supabase_url", "https://project.supabase.co")
+    kwargs.setdefault("supabase_anon_key", "anon_key")
+    return BrowserPkceProvider(**kwargs)
+
+
 class TestIsAvailable:
     def test_headless_env_false(self, monkeypatch) -> None:
         monkeypatch.setenv("OPENADAPT_HEADLESS", "1")
-        assert BrowserPkceProvider().is_available() is False
+        assert _configured().is_available() is False
 
     def test_linux_without_display_false(self, monkeypatch) -> None:
         monkeypatch.delenv("OPENADAPT_HEADLESS", raising=False)
         monkeypatch.setattr("engine.auth.browser_pkce.sys.platform", "linux")
         monkeypatch.delenv("DISPLAY", raising=False)
         monkeypatch.delenv("WAYLAND_DISPLAY", raising=False)
+        assert _configured().is_available() is False
+
+    def test_macos_true_when_configured(self, monkeypatch) -> None:
+        monkeypatch.delenv("OPENADAPT_HEADLESS", raising=False)
+        monkeypatch.setattr("engine.auth.browser_pkce.sys.platform", "darwin")
+        assert _configured().is_available() is True
+
+    def test_unconfigured_is_unavailable_even_on_a_desktop(self, monkeypatch) -> None:
+        """An unconfigured provider must not lead the chain and stall the login.
+
+        Without a Supabase project the exchange cannot succeed, but it is only
+        attempted after the loopback wait -- so reporting available here would
+        open a browser tab and block for the full timeout before falling back
+        to token paste. Regression guard for that stall.
+        """
+        monkeypatch.delenv("OPENADAPT_HEADLESS", raising=False)
+        monkeypatch.delenv("OPENADAPT_SUPABASE_URL", raising=False)
+        monkeypatch.delenv("OPENADAPT_SUPABASE_ANON_KEY", raising=False)
+        monkeypatch.setattr("engine.auth.browser_pkce.sys.platform", "darwin")
         assert BrowserPkceProvider().is_available() is False
 
-    def test_macos_true(self, monkeypatch) -> None:
+    def test_partial_configuration_is_unavailable(self, monkeypatch) -> None:
         monkeypatch.delenv("OPENADAPT_HEADLESS", raising=False)
         monkeypatch.setattr("engine.auth.browser_pkce.sys.platform", "darwin")
-        assert BrowserPkceProvider().is_available() is True
+        url_only = BrowserPkceProvider(
+            supabase_url="https://project.supabase.co", supabase_anon_key=""
+        )
+        key_only = BrowserPkceProvider(supabase_url="", supabase_anon_key="anon_key")
+        assert url_only.is_available() is False
+        assert key_only.is_available() is False
+
+    def test_unconfigured_provider_never_opens_a_browser(self, monkeypatch) -> None:
+        """The stall is user-visible as a stray tab; assert none is opened."""
+        monkeypatch.delenv("OPENADAPT_HEADLESS", raising=False)
+        monkeypatch.delenv("OPENADAPT_SUPABASE_URL", raising=False)
+        monkeypatch.delenv("OPENADAPT_SUPABASE_ANON_KEY", raising=False)
+        monkeypatch.setattr("engine.auth.browser_pkce.sys.platform", "darwin")
+        opened: list[str] = []
+        provider = BrowserPkceProvider(open_browser=opened.append)
+        with pytest.raises(RuntimeError, match="unavailable"):
+            provider.login()
+        assert opened == []
 
 
 class TestLogin:
@@ -84,7 +127,7 @@ def _deliver():
 
             threading.Thread(target=_deliver, daemon=True).start()
 
-        provider = BrowserPkceProvider(host="https://app.openadapt.ai", open_browser=_open_browser)
+        provider = _configured(host="https://app.openadapt.ai", open_browser=_open_browser)
         provider._exchange_code = lambda code, verifier, redirect_uri: {  # type: ignore[assignment]
             "access_token": "supabase_access",
             "refresh_token": "supabase_refresh",