Reject JSON-RPC requests with null id instead of misclassifying as notifications by BabyChrist666 · Pull Request #2075 · modelcontextprotocol/python-sdk

BabyChrist666 · 2026-02-17T14:47:17Z

Summary

Fixes #2057 — prevents "id": null JSON-RPC requests from being silently misclassified as notifications.

Root cause

When Pydantic validates a JSONRPCMessage union with {"jsonrpc": "2.0", "method": "initialize", "id": null}:

JSONRPCRequest correctly rejects it (RequestId = int | str, no None)
JSONRPCNotification absorbs it because "id": None becomes an extra field
The transport layer sees "notification" → returns 202 with no response
The caller gets no error and no response — a silent failure

This violates the MCP spec (§4.2.1) which mandates that request IDs must be strings or integers, and requests with id: null must be rejected.

Fix

Add a model_validator(mode="before") on JSONRPCNotification that rejects any input containing an "id" field. Per JSON-RPC 2.0, a notification is defined as a request without an id member — if "id" is present (regardless of value), it's a request, not a notification.

Changes

src/mcp/types/jsonrpc.py: Add _reject_id_field model validator to JSONRPCNotification
tests/issues/test_2057_null_id_rejected.py: 12 test cases covering:
- Null id rejection at JSONRPCRequest, JSONRPCNotification, and JSONRPCMessage union levels
- Both validate_python and validate_json paths (matching transport usage)
- Rejection of any id value on notifications (null, int, str)
- Valid requests and notifications still parse correctly

Test plan

JSONRPCRequest still rejects id: null (existing behavior preserved)
JSONRPCNotification now rejects any input with "id" field
jsonrpc_message_adapter.validate_python({"id": null, ...}) raises ValidationError
jsonrpc_message_adapter.validate_json(...) raises ValidationError
Valid notifications (no id field) still work
Valid requests (string/int id) still work
ruff check passes
All existing tests pass

🤖 Generated with Claude Code

…tifications When a JSON-RPC message contains "id": null, Pydantic's union resolution would silently reclassify it as a JSONRPCNotification (since the Request type correctly rejects null ids, but the Notification type absorbed it as an extra field). This violates the MCP spec which requires request ids to be strings or integers only. Add a model_validator on JSONRPCNotification that rejects any input containing an "id" field, ensuring that malformed requests with null ids produce a proper validation error instead of being silently swallowed. Fixes modelcontextprotocol#2057 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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

With dict[str, Any] type annotation, isinstance(data, dict) is redundant. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

clouatre

The model_validator approach is correct. I verified that ConfigDict(extra="forbid") would also reject unknown fields like {"foo": "bar"} on notifications, which is a broader contract change. The targeted validator is the right minimal fix.

Two things to tighten up:

Docstrings. Every other model in jsonrpc.py uses a single-line docstring. The expanded class docstring (6 lines) and validator docstring (4 lines) break that consistency. Also, :issue:\2057`is Sphinx cross-reference syntax, but this project has no Sphinx issue role configured, so it renders as literal text. Use#2057` or the full URL instead.

Test count. 12 tests for one behavioral change. 8 of them (test_request_rejects_null_id, test_valid_notification_still_works, test_valid_notification_with_params, test_valid_request_with_string_id, test_valid_request_with_int_id, test_message_adapter_parses_valid_request, test_message_adapter_parses_valid_notification, test_message_adapter_parses_notification_json) verify pre-existing behavior already covered by tests/test_types.py. The convention in tests/issues/ is 1-4 focused tests per issue, as module-level functions.

Suggested test set (4 tests, module-level functions):

test_notification_rejects_id_field (core fix)
test_notification_rejects_any_id_value (parameterized edge cases)
test_message_adapter_rejects_null_id (the actual bug path)
test_message_adapter_rejects_null_id_json (transport path)

clouatre · 2026-02-17T15:41:05Z

src/mcp/types/jsonrpc.py

 class JSONRPCNotification(BaseModel):
-    """A JSON-RPC notification which does not expect a response."""
+    """A JSON-RPC notification which does not expect a response.
+


Every other model in this file uses a single-line docstring (JSONRPCRequest, JSONRPCResponse, JSONRPCError). Consider trimming back to:

"""A JSON-RPC notification which does not expect a response."""

The spec rationale fits better as a short comment above the validator. Also, :issue:\2057`won't render as a link (no Sphinx issue role configured). Use#2057` or the full GitHub URL.

clouatre · 2026-02-17T15:41:05Z

src/mcp/types/jsonrpc.py

+    @classmethod
+    def _reject_id_field(cls, data: dict[str, Any]) -> dict[str, Any]:
+        """Reject messages that contain an ``id`` field.
+


The validator logic is clean and correct. The docstring could be a one-liner to match the file's style:

"""Reject payloads containing an 'id' field (notifications must not have one)."""

clouatre · 2026-02-17T15:41:05Z

tests/issues/test_2057_null_id_rejected.py

+
+
+class TestNullIdRejection:
+    """Verify that ``"id": null`` is never silently absorbed."""


All 18 other files in tests/issues/ use module-level test functions. This is the only one using a test class. Switching to module-level functions would match the convention.

clouatre · 2026-02-17T15:41:05Z

tests/issues/test_2057_null_id_rejected.py

+    """Verify that ``"id": null`` is never silently absorbed."""
+
+    def test_request_rejects_null_id(self) -> None:
+        """JSONRPCRequest correctly rejects null id."""


This test and the 7 others below it (test_valid_notification_still_works, test_valid_notification_with_params, test_valid_request_with_string_id, test_valid_request_with_int_id, test_message_adapter_parses_valid_request, test_message_adapter_parses_valid_notification, test_message_adapter_parses_notification_json) verify pre-existing behavior already covered by tests/test_types.py::test_jsonrpc_request and the broader suite.

The tests/issues/ convention is to test the specific bug and its fix. I'd keep 4 tests:

test_notification_rejects_id_field (core fix)

test_notification_rejects_any_id_value (parameterized edge cases)

test_message_adapter_rejects_null_id (actual bug path)

test_message_adapter_rejects_null_id_json (transport path)

BabyChrist666 mentioned this pull request Feb 17, 2026

Requests with "id": null silently misclassified as notifications #2057

Open

2 tasks

BabyChrist666 and others added 3 commits February 17, 2026 09:48

Fix pre-commit formatting: inline short dict arguments

4f9bc5a

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

Fix pyright: use concrete dict type for model_validator parameter

2a0d65d

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

Fix pyright: remove unnecessary isinstance check

a841b3c

With dict[str, Any] type annotation, isinstance(data, dict) is redundant. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

clouatre reviewed Feb 17, 2026

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Reject JSON-RPC requests with null id instead of misclassifying as notifications#2075

Reject JSON-RPC requests with null id instead of misclassifying as notifications#2075
BabyChrist666 wants to merge 4 commits intomodelcontextprotocol:mainfrom
BabyChrist666:fix/reject-null-id-jsonrpc-2057

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		class TestNullIdRejection:
		"""Verify that ``"id": null`` is never silently absorbed."""

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BabyChrist666 commented Feb 17, 2026

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