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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"BaseSession.send_request fails when forwarding a request received via ServerSession (envelope-fields collision)","articleBody":"## Repro\n\nAny 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:\n\n```\nTypeError: mcp.types.JSONRPCRequest() got multiple values for keyword argument 'jsonrpc'\n```\n\n## Root cause\n\n1. In `ServerSession._receive_loop`, the incoming wire message is parsed via:\n   ```python\n   self._receive_request_type.model_validate(\n       message.message.root.model_dump(by_alias=True, mode=\"json\", exclude_none=True)\n   )\n   ```\n   The dump of a `JSONRPCRequest` includes `jsonrpc` and `id`.\n\n2. `CallToolRequest`, `ListToolsRequest`, etc. are configured with `model_config = {'extra': 'allow'}`, so the envelope fields are **preserved** on the validated `ClientRequest`.\n\n3. When that received `ClientRequest` is then forwarded via `BaseSession.send_request`, the SDK does:\n   ```python\n   request_data = request.model_dump(by_alias=True, mode=\"json\", exclude_none=True)\n   # ...\n   jsonrpc_request = JSONRPCRequest(jsonrpc=\"2.0\", id=request_id, **request_data)\n   ```\n   `request_data` already contains `jsonrpc` and `id` → `TypeError`.\n\n## Workaround in consumer code\n\n```python\ndef _strip_envelope_fields(request: ClientRequest) -\u003e ClientRequest:\n    raw = request.model_dump(by_alias=True, mode=\"json\", exclude_none=True)\n    raw.pop(\"jsonrpc\", None)\n    raw.pop(\"id\", None)\n    return ClientRequest.model_validate(raw)\n```\n\nThis is currently in production in MCP-IDS's [`ProxySession._handle_one`](https://github.com/Botschagow/MCP-IDS/blob/main/src/mcp_ids/gateway/proxy/session.py).\n\n## Question\n\nIs the `extra='allow'` on Request subtypes intentional? If yes, a non-breaking fix would be inside `BaseSession.send_request`:\n\n```python\nrequest_data = request.model_dump(by_alias=True, mode=\"json\", exclude_none=True)\nrequest_data.pop(\"jsonrpc\", None)\nrequest_data.pop(\"id\", None)\njsonrpc_request = JSONRPCRequest(jsonrpc=\"2.0\", id=request_id, **request_data)\n```\n\nThis preserves consumer expectations (extra fields tolerated on receive) and fixes proxy use cases without breaking anything.\n\n## Affected use cases\n\n- MCP gateways / proxies / IDS (a gateway is by definition a `ServerSession` ↔ `ClientSession` bridge)\n- Test harnesses that round-trip messages through both session types\n- Anything relying on `ClientRequest`'s wire-form survival across the parse/forward boundary\n\n## SDK version observed\n\n`mcp==1.27.0`. Likely reproduces on later 1.x — happy to confirm if there's a specific version you'd like tested.\n","author":{"url":"https://github.com/Botschagow","@type":"Person","name":"Botschagow"},"datePublished":"2026-05-06T20:19:01.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/2548/python-sdk/issues/2548"}