fix(server): stop createMcpHandler chaining onclose on reused instances - #2787
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fix(server): stop createMcpHandler chaining onclose on reused instances#2787voidstackloop wants to merge 1 commit into
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`createMcpHandler` wraps `server.onclose` on every request to track the
instance in `inflight` until its exchange tears down:
const previousOnClose = server.onclose;
inflight.add(server);
server.onclose = () => {
inflight.delete(server);
previousOnClose?.();
};
If the factory returns the same `Server`/`McpServer` instance for every
session (a valid pattern), this installs a new wrapper on every request
instead of once per instance, chaining an unbounded closure onto the
previous `onclose`. The chain grows for the process's lifetime and,
when it eventually runs (session cleanup under load, or
`handler.close()`), recurses one stack frame per accumulated wrapper —
observed overflowing around 19-25k accumulated sessions with
`RangeError: Maximum call stack size exceeded`, surfacing as an
uncaught async error after `close()` has already resolved.
Track which instances already have the wrapper installed in a
`WeakSet` and only install once per instance, keeping the chain length
at 1 regardless of how many requests reuse it. The normal case (a
fresh instance per request, the common factory shape) is unaffected.
Added a regression test that reuses one `McpServer` across five
requests and asserts `onclose` is only ever assigned once.
Closes modelcontextprotocol#2607
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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@modelcontextprotocol/client
@modelcontextprotocol/codemod
@modelcontextprotocol/core
@modelcontextprotocol/server
@modelcontextprotocol/server-legacy
@modelcontextprotocol/express
@modelcontextprotocol/fastify
@modelcontextprotocol/hono
@modelcontextprotocol/node
commit: |
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createMcpHandlerwrapsserver.oncloseon every request to track the instance ininflightuntil its exchange tears down:If the factory returns the same
Server/McpServerinstance for every session (a valid pattern), this installs a new wrapper on every request instead of once per instance, chaining an unbounded closure onto the previousonclose. The chain grows for the process's lifetime and, when it eventually runs (session cleanup under load, orhandler.close()), recurses one stack frame per accumulated wrapper — observed overflowing around 19-25k accumulated sessions withRangeError: Maximum call stack size exceeded, surfacing as an uncaught async error afterclose()has already resolved.Track which instances already have the wrapper installed in a
WeakSetand only install once per instance, keeping the chain length at 1 regardless of how many requests reuse it. The normal case (a fresh instance per request, the common factory shape) is unaffected.Added a regression test that reuses one
McpServeracross five requests and assertsoncloseis only ever assigned once.Closes #2607
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