fix: surface failed request forwards as JSON-RPC errors instead of hanging - #308
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SvenJuergens wants to merge 1 commit into
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fix: surface failed request forwards as JSON-RPC errors instead of hanging#308SvenJuergens wants to merge 1 commit into
SvenJuergens wants to merge 1 commit into
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When forwarding a client request to the remote server fails, the error was only logged. The local client keeps waiting for a response that never arrives and appears to hang forever — e.g. when the server expired the session and answers HTTP 404 (see geelen#106). Send a JSON-RPC error response for the affected request id back to the local client instead, so hosts fail fast and can reconnect. Notifications carry no id and are still not answered.
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Problem
Fixes the "hang" part of #106.
When
transportToServer.send()rejects while forwarding a client request, the error is only logged:The local client keeps waiting for a response to its request id that will never arrive — from the host's perspective (Claude Code, Cursor, …) the tool call hangs indefinitely. We debugged this in production against a TYPO3 MCP server: the server answered a spec-conform HTTP 404 for an expired session (
Mcp-Session-Idunknown), mcp-remote loggedError POSTing to endpoint (HTTP 404)— and the host waited 30 minutes into its idle timeout.Change
If forwarding a request (message with an
id) fails, send a JSON-RPC error response for that id back to the local client, mirroring the pattern already used for blocked tools ininterceptRequest. Hosts then fail fast and can reconnect. Failed notifications (noid) are still not answered, as JSON-RPC forbids responses to them.This intentionally does not implement the full spec behaviour for 404 ("client MUST start a new session by sending a new InitializeRequest") — that needs replaying the cached initialize params and is better done as a follow-up. Surfacing the error at least turns a silent forever-hang into an actionable failure.
Tests
Two new scenarios in
utils.test.ts(55/55 passing): a rejected request forward produces a JSON-RPC error response with the original id; a rejected notification forward produces no response.prettier --checkandtscare clean.