BaseSession.send_request fails when forwarding a request received via ServerSession (envelope-fields collision)

[Botschagow]

Repro

Any time a ServerSession.incoming_messages RequestResponder.request is forwarded to a ClientSession.send_request(...) — the canonical pattern for an MCP gateway/proxy — the SDK raises:

TypeError: mcp.types.JSONRPCRequest() got multiple values for keyword argument 'jsonrpc'

Root cause

1. In ServerSession._receive_loop, the incoming wire message is parsed via:

   self._receive_request_type.model_validate(
       message.message.root.model_dump(by_alias=True, mode="json", exclude_none=True)
   )

   The dump of a JSONRPCRequest includes jsonrpc and id.

2. CallToolRequest, ListToolsRequest, etc. are configured with model_config = {'extra': 'allow'}, so the envelope fields are preserved on the validated ClientRequest.

3. When that received ClientRequest is then forwarded via BaseSession.send_request, the SDK does:

   request_data = request.model_dump(by_alias=True, mode="json", exclude_none=True)
   # ...
   jsonrpc_request = JSONRPCRequest(jsonrpc="2.0", id=request_id, **request_data)

   request_data already contains jsonrpc and id → TypeError.

Workaround in consumer code

def _strip_envelope_fields(request: ClientRequest) -> ClientRequest:
    raw = request.model_dump(by_alias=True, mode="json", exclude_none=True)
    raw.pop("jsonrpc", None)
    raw.pop("id", None)
    return ClientRequest.model_validate(raw)

This is currently in production in MCP-IDS's ProxySession._handle_one.

Question

Is the extra='allow' on Request subtypes intentional? If yes, a non-breaking fix would be inside BaseSession.send_request:

request_data = request.model_dump(by_alias=True, mode="json", exclude_none=True)
request_data.pop("jsonrpc", None)
request_data.pop("id", None)
jsonrpc_request = JSONRPCRequest(jsonrpc="2.0", id=request_id, **request_data)

This preserves consumer expectations (extra fields tolerated on receive) and fixes proxy use cases without breaking anything.

Affected use cases

• MCP gateways / proxies / IDS (a gateway is by definition a ServerSession ↔ ClientSession bridge)
• Test harnesses that round-trip messages through both session types
• Anything relying on ClientRequest's wire-form survival across the parse/forward boundary

SDK version observed

mcp==1.27.0. Likely reproduces on later 1.x — happy to confirm if there's a specific version you'd like tested.