fix: reject JSON-RPC requests with id: null instead of misclassifying as notification

When a JSON-RPC message arrives with "id": null, it should be
rejected.  Both JSON-RPC 2.0 and the MCP spec restrict request IDs
to strings or integers.

Previously, JSONRPCRequest correctly rejected null IDs, but Pydantic
fell through to JSONRPCNotification which silently absorbed the extra
"id" field via implicit extra='allow'.  The caller got a 202 with
no response and no error.

Set extra='forbid' on JSONRPCNotification so that any unrecognised
field (including "id": null) causes a validation error.

Fixes #2057

Author: g97iulio1609

src/mcp/types/jsonrpc.py

@@ -4,7 +4,7 @@
 
 from typing import Annotated, Any, Literal
 
-from pydantic import BaseModel, Field, TypeAdapter
+from pydantic import BaseModel, ConfigDict, Field, TypeAdapter
 
 RequestId = Annotated[int, Field(strict=True)] | str
 """The ID of a JSON-RPC request."""
@@ -22,6 +22,8 @@ class JSONRPCRequest(BaseModel):
 class JSONRPCNotification(BaseModel):
     """A JSON-RPC notification which does not expect a response."""
 
+    model_config = ConfigDict(extra="forbid")
+
     jsonrpc: Literal["2.0"]
     method: str
     params: dict[str, Any] | None = None

tests/issues/test_2057_null_id.py

@@ -0,0 +1,52 @@
+"""Test for issue #2057: Requests with "id": null silently misclassified as notifications."""
+
+import pytest
+from pydantic import ValidationError
+
+from mcp.types import JSONRPCMessage, JSONRPCNotification, JSONRPCRequest, jsonrpc_message_adapter
+
+
+class TestNullIdRejection:
+    """Verify that JSON-RPC messages with id: null are rejected."""
+
+    def test_request_rejects_null_id(self):
+        """JSONRPCRequest should reject id: null."""
+        with pytest.raises(ValidationError):
+            JSONRPCRequest.model_validate(
+                {"jsonrpc": "2.0", "method": "initialize", "id": None}
+            )
+
+    def test_notification_rejects_extra_id_field(self):
+        """JSONRPCNotification should not absorb an extra 'id' field."""
+        with pytest.raises(ValidationError):
+            JSONRPCNotification.model_validate(
+                {"jsonrpc": "2.0", "method": "initialize", "id": None}
+            )
+
+    def test_message_adapter_rejects_null_id(self):
+        """The union adapter should reject messages with id: null entirely."""
+        with pytest.raises(ValidationError):
+            jsonrpc_message_adapter.validate_python(
+                {"jsonrpc": "2.0", "method": "initialize", "id": None}
+            )
+
+    def test_valid_notification_without_id(self):
+        """A proper notification (no id field) should still validate."""
+        msg = jsonrpc_message_adapter.validate_python(
+            {"jsonrpc": "2.0", "method": "notifications/initialized"}
+        )
+        assert isinstance(msg, JSONRPCNotification)
+
+    def test_valid_request_with_int_id(self):
+        """A proper request with an integer id should still validate."""
+        msg = jsonrpc_message_adapter.validate_python(
+            {"jsonrpc": "2.0", "method": "initialize", "id": 1}
+        )
+        assert isinstance(msg, JSONRPCRequest)
+
+    def test_valid_request_with_string_id(self):
+        """A proper request with a string id should still validate."""
+        msg = jsonrpc_message_adapter.validate_python(
+            {"jsonrpc": "2.0", "method": "initialize", "id": "abc-123"}
+        )
+        assert isinstance(msg, JSONRPCRequest)