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1. Is this request related to a challenge you're experiencing? Tell me about your story.

def test_sse_message_id_coercion():
        """Test that string message IDs that look like integers are parsed as integers.
    
        See <https://github.com/modelcontextprotocol/python-sdk/pull/851> for more details.
        """
        json_message = '{"jsonrpc": "2.0", "id": "123", "method": "ping", "params": null}'
        msg = types.JSONRPCMessage.model_validate_json(json_message)
        expected = types.JSONRPCMessage(root=types.JSONRPCRequest(method="ping", jsonrpc="2.0", id=123))
    
        # Check if both are JSONRPCRequest instances
>       assert isinstance(msg.root, types.JSONRPCRequest)
E       AssertionError: assert False
E        +  where False = isinstance(JSONRPCNotification(method='ping', params=None, jsonrpc='2.0', id='123'), <class 'core.mcp.types.JSONRPCRequest'>)
E        +    where JSONRPCNotification(method='ping', params=None, jsonrpc='2.0', id='123') = JSONRPCMessage(root=JSONRPCNotification(method='ping', params=None, jsonrpc='2.0', id='123')).root
E        +    and   <class 'core.mcp.types.JSONRPCRequest'> = types.JSONRPCRequest

it can be basically determined that this is an issue arising from changes in Pydantic v2's union type matching behavior. But more precisely, it's the result of these factors (stacking together):

• JSONRPCMessage is a plain union: JSONRPCRequest | JSONRPCNotification | JSONRPCResponse | JSONRPCError
• JSONRPCNotification has extra="allow" enabled, so although it doesn't declare an id field, it can still accept input like {"id": "123", "method": "ping"}.
• id="123" in JSONRPCRequest goes through RequestId = Annotated[int | str, Field(union_mode="left_to_right")] and gets coerced into 123.
• Newer versions of Pydantic prefer the union branch that is more precise / involves less coercion, so it may ultimately classify the input as JSONRPCNotification instead of JSONRPCRequest.

2. Additional context or comments

No response

3. Can you help us with this feature?

• I am interested in contributing to this feature.