fix: return HTTP 404 for METHOD_NOT_FOUND in stateless transport (#1072) - #1083
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fix: return HTTP 404 for METHOD_NOT_FOUND in stateless transport (#1072)#1083elang2 wants to merge 1 commit into
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…elcontextprotocol#1072) The HttpServletStatelessServerTransport was returning HTTP 200 for all JSON-RPC responses, including METHOD_NOT_FOUND errors. Per the MCP Streamable HTTP specification (2026-07-28), an unrecognized method must return HTTP 404 with JSON-RPC error code -32601, not HTTP 500 or 200. This caused OpenAI's hosted MCP client to receive HTTP 500 (or 200 with an error body it could not process) when sending server/discover requests, resulting in HTTP 424 external_connector_error from the Responses API. The fix maps METHOD_NOT_FOUND (-32601) responses to HTTP 404 at the transport layer. All other JSON-RPC errors continue to use HTTP 200 per standard JSON-RPC conventions.
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Summary
The 2026-07-28 Streamable HTTP spec requires that an unsupported RPC method return HTTP 404 with JSON-RPC error -32601, not HTTP 500. Right now
HttpServletStatelessServerTransportreturns 200 for everything, including unrecognized methods likeserver/discover. OpenAI's hosted MCP client hits this path and surfaces a 424external_connector_errorto users.This PR adds a
mapJsonRpcErrorToHttpStatus()helper that maps -32601 to 404. It's a small, isolated change that unblocks production traffic without waiting for full 2026-07-28 lifecycle support tracked in #1011.Test plan
testMissingHandlerReturnsMethodNotFoundErrorto verify HTTP 404 statustestUnknownMethodReturnsHttp404WithMethodNotFoundErrorforserver/discoverspecificallyFixes #1072