[cursor]

Problem

parse_response only copied JSON-RPC id when the member was a number. JSON-RPC 2.0 allows String | Number | Null, and JS MCP stacks commonly echo the integer we sent as "id":"1".

Client::rpc matches resp.id == req.id (req.id starts at 1). A string id left Response::id at 0 — the same sentinel used for notifications — so a valid reply was skipped. The loop then sat until init_timeout / call_timeout and killed the MCP subprocess. Playwright and other stdio MCP servers that stringify ids hung on every initialize / tools/call.

Fix

Parse decimal-string ids with stoll (full-string match, base 10) into the same int64_t we already store for numeric ids. Null, empty, non-numeric, and fractional ("1.0") ids stay 0 so notifications and unmatched traffic still skip. Numeric ids are unchanged.

Tests

unit_mcp — string success/error ids, non-numeric / empty / null / "1.0" skip, omitted id (notification). 4 parse_response* cases locally; suite 48/48 with the pre-existing Subprocess: terminate SIGTERM flake excluded. ASan/UBSan driver over parse_response passed.

Independently mergeable against main (0456350). Code hunks (mcp/types.{h,cpp}, test_mcp.cpp inserted before the malformed-envelope case) auto-merge with #337 (null members, later in parse_response). CHANGELOG [Unreleased] will need a rebase after whichever sibling merges first.

  

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Note

Low Risk
Localized MCP JSON-RPC parsing change with tests; no auth or data-path impact, though incorrect id handling could still affect MCP subprocess RPC behavior.

Overview
Fixes MCP stdio hangs when JavaScript servers echo JSON-RPC request ids as strings ("id":"3") instead of numbers.

parse_response now normalizes decimal-string ids into the same int64_t used for numeric ids via a new parse_jsonrpc_id helper (stoll, full-string match). That lets Client::rpc match replies again instead of treating them as notifications (id == 0) and waiting until init/call timeout kills the subprocess. Empty, null, non-numeric, and fractional string ids still map to 0 so the notification skip path is unchanged. Unit tests cover success/error string ids and skip cases; the unreleased changelog documents the fix.

^{Reviewed by Cursor Bugbot for commit edc6427. Bugbot is set up for automated code reviews on this repo. Configure here.}