fix: perform auth server metadata discovery fallbacks on any 4xx

src/mcp/client/auth.py

@@ -526,8 +526,8 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                                 break
                             except ValidationError:
                                 continue
-                        elif oauth_metadata_response.status_code != 404:
-                            break  # Non-404 error, stop trying
+                        elif oauth_metadata_response.status_code < 400 or oauth_metadata_response.status_code >= 500:
+                            break  # Non-4XX error, stop trying
 
                     # Step 3: Register client if needed
                     registration_request = await self._register_client()

tests/client/test_auth.py

@@ -261,6 +261,109 @@ async def test_oauth_discovery_fallback_order(self, oauth_provider):
             "https://api.example.com/v1/mcp/.well-known/openid-configuration",
         ]
 
+    @pytest.mark.anyio
+    async def test_oauth_discovery_fallback_conditions(self, oauth_provider):
+        """Test the conditions during which an AS metadata discovery fallback will be attempted."""
+        # Ensure no tokens are stored
+        oauth_provider.context.current_tokens = None
+        oauth_provider.context.token_expiry_time = None
+        oauth_provider._initialized = True
+
+        # Mock client info to skip DCR
+        oauth_provider.context.client_info = OAuthClientInformationFull(
+            client_id="existing_client",
+            redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+        )
+
+        # Create a test request
+        test_request = httpx.Request("GET", "https://api.example.com/v1/mcp")
+
+        # Mock the auth flow
+        auth_flow = oauth_provider.async_auth_flow(test_request)
+
+        # First request should be the original request without auth header
+        request = await auth_flow.__anext__()
+        assert "Authorization" not in request.headers
+
+        # Send a 401 response to trigger the OAuth flow
+        response = httpx.Response(
+            401,
+            headers={
+                "WWW-Authenticate": 'Bearer resource_metadata="https://api.example.com/.well-known/oauth-protected-resource"'
+            },
+            request=test_request,
+        )
+
+        # Next request should be to discover protected resource metadata
+        discovery_request = await auth_flow.asend(response)
+        assert str(discovery_request.url) == "https://api.example.com/.well-known/oauth-protected-resource"
+        assert discovery_request.method == "GET"
+
+        # Send a successful discovery response with minimal protected resource metadata
+        discovery_response = httpx.Response(
+            200,
+            content=b'{"resource": "https://api.example.com/v1/mcp", "authorization_servers": ["https://auth.example.com/v1/mcp"]}',
+            request=discovery_request,
+        )
+
+        # Next request should be to discover OAuth metadata
+        oauth_metadata_request_1 = await auth_flow.asend(discovery_response)
+        assert (
+            str(oauth_metadata_request_1.url)
+            == "https://auth.example.com/.well-known/oauth-authorization-server/v1/mcp"
+        )
+        assert oauth_metadata_request_1.method == "GET"
+
+        # Send a 404 response
+        oauth_metadata_response_1 = httpx.Response(
+            404,
+            content=b"Not Found",
+            request=oauth_metadata_request_1,
+        )
+
+        # Next request should be to discover OAuth metadata at the next endpoint
+        oauth_metadata_request_2 = await auth_flow.asend(oauth_metadata_response_1)
+        assert str(oauth_metadata_request_2.url) == "https://auth.example.com/.well-known/oauth-authorization-server"
+        assert oauth_metadata_request_2.method == "GET"
+
+        # Send a 400 response
+        oauth_metadata_response_2 = httpx.Response(
+            400,
+            content=b"Bad Request",
+            request=oauth_metadata_request_2,
+        )
+
+        # Next request should be to discover OAuth metadata at the next endpoint
+        oauth_metadata_request_3 = await auth_flow.asend(oauth_metadata_response_2)
+        assert str(oauth_metadata_request_3.url) == "https://auth.example.com/.well-known/openid-configuration/v1/mcp"
+        assert oauth_metadata_request_3.method == "GET"
+
+        # Send a 500 response
+        oauth_metadata_response_3 = httpx.Response(
+            500,
+            content=b"Internal Server Error",
+            request=oauth_metadata_request_3,
+        )
+
+        # Mock the authorization process to minimize unnecessary state in this test
+        oauth_provider._perform_authorization = mock.AsyncMock(return_value=("test_auth_code", "test_code_verifier"))
+
+        # Next request should fall back to legacy behavior and auth with the RS (mocked /authorize, next is /token)
+        token_request = await auth_flow.asend(oauth_metadata_response_3)
+        assert str(token_request.url) == "https://api.example.com/token"
+        assert token_request.method == "POST"
+
+        # Send a successful token response
+        token_response = httpx.Response(
+            200,
+            content=(
+                b'{"access_token": "new_access_token", "token_type": "Bearer", "expires_in": 3600, '
+                b'"refresh_token": "new_refresh_token"}'
+            ),
+            request=token_request,
+        )
+        token_request = await auth_flow.asend(token_response)
+
     @pytest.mark.anyio
     async def test_handle_metadata_response_success(self, oauth_provider):
         """Test successful metadata response handling."""