fix(shared): strip envelope fields when forwarding JSONRPCRequest

src/mcp/shared/session.py

@@ -282,6 +282,12 @@ async def send_request(
                 meta: dict[str, Any] = request_data.setdefault("params", {}).setdefault("_meta", {})
                 inject_trace_context(meta)
 
+                # Strip envelope fields that would collide with the explicit kwargs below.
+                # Happens when request_data comes from model_dump() on a JSONRPCRequest
+                # (e.g. forwarding/proxy scenarios). See issue #2548.
+                request_data.pop("jsonrpc", None)
+                request_data.pop("id", None)
+
                 jsonrpc_request = JSONRPCRequest(jsonrpc="2.0", id=request_id, **request_data)
                 await self._write_stream.send(SessionMessage(message=jsonrpc_request, metadata=metadata))
 

tests/shared/test_session.py

@@ -416,3 +416,56 @@ async def make_request(client_session: ClientSession):
     # Pending request completed successfully
     assert len(result_holder) == 1
     assert isinstance(result_holder[0], EmptyResult)
+
+
+@pytest.mark.anyio
+async def test_send_request_strips_envelope_fields_when_forwarding():
+    """Test that send_request does not raise TypeError when request_data contains
+    jsonrpc and id fields (e.g. from model_dump() on a JSONRPCRequest).
+
+    This is the forwarding/proxy scenario: a session receives an incoming
+    JSONRPCRequest and re-emits it via another session's send_request.
+    model_dump() on a JSONRPCRequest preserves the envelope fields jsonrpc and
+    id, which used to collide with the explicit kwargs at the construction site.
+    Regression test for issue #2548.
+    """
+    ev_response_received = anyio.Event()
+    result_holder: list[EmptyResult] = []
+
+    async with create_client_server_memory_streams() as (client_streams, server_streams):
+        client_read, client_write = client_streams
+        server_read, server_write = server_streams
+
+        async def mock_server() -> None:
+            """Receive the forwarded request and send back an empty response."""
+            message = await server_read.receive()
+            assert isinstance(message, SessionMessage)
+            assert isinstance(message.message, JSONRPCRequest)
+            request_id = message.message.id
+            await server_write.send(SessionMessage(message=JSONRPCResponse(jsonrpc="2.0", id=request_id, result={})))
+
+        async def forward_request(client_session: ClientSession) -> None:
+            """Simulate a proxy forwarding an incoming JSONRPCRequest."""
+            # Build a JSONRPCRequest as a proxy would receive it from an upstream
+            # server. model_dump() on this object includes jsonrpc and id, which
+            # previously caused TypeError: got multiple values for keyword argument.
+            incoming = JSONRPCRequest(jsonrpc="2.0", id=99, method="ping", params=None)
+            result = await client_session.send_request(
+                incoming,  # type: ignore[arg-type]
+                EmptyResult,
+            )
+            result_holder.append(result)
+            ev_response_received.set()
+
+        async with (
+            anyio.create_task_group() as tg,
+            ClientSession(read_stream=client_read, write_stream=client_write) as client_session,
+        ):
+            tg.start_soon(mock_server)
+            tg.start_soon(forward_request, client_session)
+
+            with anyio.fail_after(2):  # pragma: no branch
+                await ev_response_received.wait()
+
+    assert len(result_holder) == 1
+    assert isinstance(result_holder[0], EmptyResult)