Accept additional response_types values from OAuth servers

OAuth servers may return additional response_types beyond what the
client requested (e.g., ["code", "none"] instead of just ["code"]).
Per RFC 7591 Section 3.2.1, servers can modify registration metadata
and return all registered values including server-provisioned fields.
For example, Keycloak returns ["code", "none"] even when just ["code"]
was requested.

- Changed response_types field from list[Literal["code"]] to list[str]
- Added validation in registration handler to ensure "code" is present
- Added tests for response_types flexibility

This fixes compatibility issues with OAuth servers that return additional
response_types while maintaining MCP's requirement for the "code" flow.

Author: jonshea

src/mcp/server/auth/handlers/register.py

@@ -77,6 +77,17 @@ async def handle(self, request: Request) -> Response:
                 status_code=400,
             )
 
+        # The MCP spec requires servers to use the authorization `code` flow
+        # with PKCE
+        if "code" not in client_metadata.response_types:
+            return PydanticJSONResponse(
+                content=RegistrationErrorResponse(
+                    error="invalid_client_metadata",
+                    error_description="response_types must include 'code' for authorization_code grant",
+                ),
+                status_code=400,
+            )
+
         client_id_issued_at = int(time.time())
         client_secret_expires_at = (
             client_id_issued_at + self.options.client_secret_expiry_seconds

src/mcp/shared/auth.py

@@ -51,8 +51,9 @@ class OAuthClientMetadata(BaseModel):
         "authorization_code",
         "refresh_token",
     ]
-    # this implementation only supports code; ie: it does not support implicit grants
-    response_types: list[Literal["code"]] = ["code"]
+    # The MCP spec requires the "code" response type, but OAuth
+    # servers may also return additional types they support
+    response_types: list[str] = ["code"]
     scope: str | None = None
 
     # these fields are currently unused, but we support & store them for potential

tests/server/fastmcp/auth/test_auth_integration.py

@@ -942,6 +942,62 @@ async def test_client_registration_invalid_grant_type(self, test_client: httpx.A
         assert error_data["error"] == "invalid_client_metadata"
         assert error_data["error_description"] == "grant_types must be authorization_code and refresh_token"
 
+    @pytest.mark.anyio
+    async def test_client_registration_with_additional_response_types(
+        self, test_client: httpx.AsyncClient, mock_oauth_provider: MockOAuthProvider
+    ):
+        """Test that registration accepts additional response_types values alongside 'code'."""
+        client_metadata = {
+            "redirect_uris": ["https://client.example.com/callback"],
+            "client_name": "Test Client",
+            "grant_types": ["authorization_code", "refresh_token"],
+            "response_types": ["code", "none"],  # Keycloak-style response with additional value
+        }
+
+        response = await test_client.post("/register", json=client_metadata)
+        assert response.status_code == 201
+        data = response.json()
+
+        client = await mock_oauth_provider.get_client(data["client_id"])
+        assert client is not None
+        assert "code" in client.response_types
+
+    @pytest.mark.anyio
+    async def test_client_registration_response_types_without_code(self, test_client: httpx.AsyncClient):
+        """Test that registration rejects response_types that don't include 'code'."""
+        client_metadata = {
+            "redirect_uris": ["https://client.example.com/callback"],
+            "client_name": "Test Client",
+            "grant_types": ["authorization_code", "refresh_token"],
+            "response_types": ["token", "none", "nonsense-string"],
+        }
+
+        response = await test_client.post("/register", json=client_metadata)
+        assert response.status_code == 400
+        error_data = response.json()
+        assert "error" in error_data
+        assert error_data["error"] == "invalid_client_metadata"
+        assert "response_types must include 'code'" in error_data["error_description"]
+
+    @pytest.mark.anyio
+    async def test_client_registration_default_response_types(
+        self, test_client: httpx.AsyncClient, mock_oauth_provider: MockOAuthProvider
+    ):
+        """Test that registration uses default response_types of ['code'] when not specified."""
+        client_metadata = {
+            "redirect_uris": ["https://client.example.com/callback"],
+            "client_name": "Test Client",
+            "grant_types": ["authorization_code", "refresh_token"],
+            # response_types not specified, should default to ["code"]
+        }
+
+        response = await test_client.post("/register", json=client_metadata)
+        assert response.status_code == 201
+        data = response.json()
+
+        assert "response_types" in data
+        assert data["response_types"] == ["code"]
+
 
 class TestAuthorizeEndpointErrors:
     """Test error handling in the OAuth authorization endpoint."""