fix: restore eager OAuth discovery to avoid slow unauthenticated roundtrip (#1274)

When the client has no valid tokens, perform OAuth discovery and
authorization BEFORE sending the MCP request. This restores the eager
behavior from v1.11.0 that was removed in v1.12.0, eliminating the
unnecessary unauthenticated roundtrip that servers like Notion handle
slowly (~10s latency per operation).

Both the eager (pre-request) and reactive (post-401) paths now share
a single `_perform_oauth_discovery_and_auth()` helper, keeping the
code DRY while preserving RFC 9728 WWW-Authenticate header support
on the 401 path.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: BabyChrist666

src/mcp/client/auth/oauth2.py

@@ -496,6 +496,102 @@ async def _validate_resource_match(self, prm: ProtectedResourceMetadata) -> None
         if not check_resource_allowed(requested_resource=default_resource, configured_resource=prm_resource):
             raise OAuthFlowError(f"Protected resource {prm_resource} does not match expected {default_resource}")
 
+    async def _perform_oauth_discovery_and_auth(
+        self,
+        www_auth_response: httpx.Response | None = None,
+    ) -> AsyncGenerator[httpx.Request, httpx.Response]:
+        """Perform the full OAuth discovery, registration, and authorization flow.
+
+        This is extracted as a helper to allow both eager (pre-request) and
+        reactive (post-401) OAuth flows to share the same implementation.
+
+        Args:
+            www_auth_response: Optional 401 response to extract WWW-Authenticate
+                header from for RFC 9728 resource_metadata discovery. When None,
+                falls back to well-known URL discovery.
+        """
+        www_auth_resource_metadata_url = (
+            extract_resource_metadata_from_www_auth(www_auth_response) if www_auth_response else None
+        )
+
+        # Step 1: Discover protected resource metadata (SEP-985 with fallback support)
+        prm_discovery_urls = build_protected_resource_metadata_discovery_urls(
+            www_auth_resource_metadata_url, self.context.server_url
+        )
+
+        for url in prm_discovery_urls:  # pragma: no branch
+            discovery_request = create_oauth_metadata_request(url)
+
+            discovery_response = yield discovery_request  # sending request
+
+            prm = await handle_protected_resource_response(discovery_response)
+            if prm:
+                # Validate PRM resource matches server URL (RFC 8707)
+                await self._validate_resource_match(prm)
+                self.context.protected_resource_metadata = prm
+
+                # todo: try all authorization_servers to find the OASM
+                assert (
+                    len(prm.authorization_servers) > 0
+                )  # this is always true as authorization_servers has a min length of 1
+
+                self.context.auth_server_url = str(prm.authorization_servers[0])
+                break
+            else:
+                logger.debug(f"Protected resource metadata discovery failed: {url}")
+
+        asm_discovery_urls = build_oauth_authorization_server_metadata_discovery_urls(
+            self.context.auth_server_url, self.context.server_url
+        )
+
+        # Step 2: Discover OAuth Authorization Server Metadata (OASM)
+        for url in asm_discovery_urls:  # pragma: no branch
+            oauth_metadata_request = create_oauth_metadata_request(url)
+            oauth_metadata_response = yield oauth_metadata_request
+
+            ok, asm = await handle_auth_metadata_response(oauth_metadata_response)
+            if not ok:
+                break
+            if ok and asm:
+                self.context.oauth_metadata = asm
+                break
+            else:
+                logger.debug(f"OAuth metadata discovery failed: {url}")
+
+        # Step 3: Apply scope selection strategy
+        self.context.client_metadata.scope = get_client_metadata_scopes(
+            extract_scope_from_www_auth(www_auth_response) if www_auth_response else None,
+            self.context.protected_resource_metadata,
+            self.context.oauth_metadata,
+        )
+
+        # Step 4: Register client or use URL-based client ID (CIMD)
+        if not self.context.client_info:
+            if should_use_client_metadata_url(self.context.oauth_metadata, self.context.client_metadata_url):
+                # Use URL-based client ID (CIMD)
+                logger.debug(f"Using URL-based client ID (CIMD): {self.context.client_metadata_url}")
+                client_information = create_client_info_from_metadata_url(
+                    self.context.client_metadata_url,  # type: ignore[arg-type]
+                    redirect_uris=self.context.client_metadata.redirect_uris,
+                )
+                self.context.client_info = client_information
+                await self.context.storage.set_client_info(client_information)
+            else:
+                # Fallback to Dynamic Client Registration
+                registration_request = create_client_registration_request(
+                    self.context.oauth_metadata,
+                    self.context.client_metadata,
+                    self.context.get_authorization_base_url(self.context.server_url),
+                )
+                registration_response = yield registration_request
+                client_information = await handle_registration_response(registration_response)
+                self.context.client_info = client_information
+                await self.context.storage.set_client_info(client_information)
+
+        # Step 5: Perform authorization and complete token exchange
+        token_response = yield await self._perform_authorization()
+        await self._handle_token_response(token_response)
+
     async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.Request, httpx.Response]:
         """HTTPX auth flow integration."""
         async with self.context.lock:
@@ -514,96 +610,39 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                     # Refresh failed, need full re-authentication
                     self._initialized = False
 
+            # Eager OAuth: if we have no valid token and can't refresh, perform the
+            # full OAuth discovery flow BEFORE sending the MCP request.  This avoids
+            # an unnecessary unauthenticated round-trip that some servers (e.g. Notion)
+            # handle slowly, causing ~10 s latency per request.  See #1274.
+            if not self.context.is_token_valid():
+                try:
+                    oauth_gen = self._perform_oauth_discovery_and_auth()
+                    oauth_request = await oauth_gen.__anext__()
+                    while True:
+                        oauth_response = yield oauth_request
+                        oauth_request = await oauth_gen.asend(oauth_response)
+                except StopAsyncIteration:
+                    pass
+                except Exception:  # pragma: no cover
+                    logger.exception("OAuth flow error")
+                    raise
+
             if self.context.is_token_valid():
                 self._add_auth_header(request)
 
             response = yield request
 
             if response.status_code == 401:
-                # Perform full OAuth flow
+                # Perform full OAuth flow (reactive path — uses WWW-Authenticate
+                # header from the 401 response for RFC 9728 discovery)
                 try:
-                    # OAuth flow must be inline due to generator constraints
-                    www_auth_resource_metadata_url = extract_resource_metadata_from_www_auth(response)
-
-                    # Step 1: Discover protected resource metadata (SEP-985 with fallback support)
-                    prm_discovery_urls = build_protected_resource_metadata_discovery_urls(
-                        www_auth_resource_metadata_url, self.context.server_url
-                    )
-
-                    for url in prm_discovery_urls:  # pragma: no branch
-                        discovery_request = create_oauth_metadata_request(url)
-
-                        discovery_response = yield discovery_request  # sending request
-
-                        prm = await handle_protected_resource_response(discovery_response)
-                        if prm:
-                            # Validate PRM resource matches server URL (RFC 8707)
-                            await self._validate_resource_match(prm)
-                            self.context.protected_resource_metadata = prm
-
-                            # todo: try all authorization_servers to find the OASM
-                            assert (
-                                len(prm.authorization_servers) > 0
-                            )  # this is always true as authorization_servers has a min length of 1
-
-                            self.context.auth_server_url = str(prm.authorization_servers[0])
-                            break
-                        else:
-                            logger.debug(f"Protected resource metadata discovery failed: {url}")
-
-                    asm_discovery_urls = build_oauth_authorization_server_metadata_discovery_urls(
-                        self.context.auth_server_url, self.context.server_url
-                    )
-
-                    # Step 2: Discover OAuth Authorization Server Metadata (OASM) (with fallback for legacy servers)
-                    for url in asm_discovery_urls:  # pragma: no branch
-                        oauth_metadata_request = create_oauth_metadata_request(url)
-                        oauth_metadata_response = yield oauth_metadata_request
-
-                        ok, asm = await handle_auth_metadata_response(oauth_metadata_response)
-                        if not ok:
-                            break
-                        if ok and asm:
-                            self.context.oauth_metadata = asm
-                            break
-                        else:
-                            logger.debug(f"OAuth metadata discovery failed: {url}")
-
-                    # Step 3: Apply scope selection strategy
-                    self.context.client_metadata.scope = get_client_metadata_scopes(
-                        extract_scope_from_www_auth(response),
-                        self.context.protected_resource_metadata,
-                        self.context.oauth_metadata,
-                    )
-
-                    # Step 4: Register client or use URL-based client ID (CIMD)
-                    if not self.context.client_info:
-                        if should_use_client_metadata_url(
-                            self.context.oauth_metadata, self.context.client_metadata_url
-                        ):
-                            # Use URL-based client ID (CIMD)
-                            logger.debug(f"Using URL-based client ID (CIMD): {self.context.client_metadata_url}")
-                            client_information = create_client_info_from_metadata_url(
-                                self.context.client_metadata_url,  # type: ignore[arg-type]
-                                redirect_uris=self.context.client_metadata.redirect_uris,
-                            )
-                            self.context.client_info = client_information
-                            await self.context.storage.set_client_info(client_information)
-                        else:
-                            # Fallback to Dynamic Client Registration
-                            registration_request = create_client_registration_request(
-                                self.context.oauth_metadata,
-                                self.context.client_metadata,
-                                self.context.get_authorization_base_url(self.context.server_url),
-                            )
-                            registration_response = yield registration_request
-                            client_information = await handle_registration_response(registration_response)
-                            self.context.client_info = client_information
-                            await self.context.storage.set_client_info(client_information)
-
-                    # Step 5: Perform authorization and complete token exchange
-                    token_response = yield await self._perform_authorization()
-                    await self._handle_token_response(token_response)
+                    oauth_gen = self._perform_oauth_discovery_and_auth(www_auth_response=response)
+                    oauth_request = await oauth_gen.__anext__()
+                    while True:
+                        oauth_response = yield oauth_request
+                        oauth_request = await oauth_gen.asend(oauth_response)
+                except StopAsyncIteration:
+                    pass
                 except Exception:  # pragma: no cover
                     logger.exception("OAuth flow error")
                     raise