Requests with "id": null silently misclassified as notifications

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

When a JSON-RPC request arrives with "id": null, the SDK should reject it. Both JSON-RPC 2.0 and the MCP spec restrict request IDs to strings or integers. Instead, the request is silently reclassified as a JSONRPCNotification and the caller gets a 202 with no response.

This happens because of how JSONRPCMessage union resolution interacts with extra='allow':

1. RequestId correctly excludes None (Annotated[int, Field(strict=True)] | str).
2. JSONRPCRequest validation rejects id: null, working as intended.
3. Pydantic falls through to JSONRPCNotification, which absorbs "id": None as an extra field via extra='allow'.
4. The streamable HTTP transport sees "not a request" and returns 202.

The net effect is the caller gets no error and no response, which is hard to debug. Found via authprobe scanning.

I suspect the v2 migration to TypeAdapter and dropping extra='allow' on top-level types would resolve this, but wanted to flag it for the current release line too.

Example Code

from mcp.types import JSONRPCMessage, JSONRPCRequest

msg = {"jsonrpc": "2.0", "method": "initialize", "id": None}

# JSONRPCRequest correctly rejects null id
try:
    JSONRPCRequest.model_validate(msg)
except Exception:
    print("JSONRPCRequest rejects null id")  # Expected

# JSONRPCMessage falls through to JSONRPCNotification
parsed = JSONRPCMessage.model_validate(msg)
print(type(parsed.root).__name__)   # JSONRPCNotification (unexpected)
print(parsed.root.model_extra)      # {'id': None}

Python & MCP Python SDK

Python 3.13
mcp 1.14.1 (also reproduced on 1.26.0, latest at time of filing)

Start with JSONRPCMessage and JSONRPCRequest validation, then trace how the streamable HTTP transport handles a parsed message. Reproduce the provided null-id example and verify that it is rejected as an invalid request rather than classified as a JSONRPCNotification or answered with 202.