fix: drain accepted JSON-RPC requests after read EOF

[matthewchen94]

Fixes #2678.

This changes the JSON-RPC dispatcher shutdown path so read EOF does not immediately cancel request handlers that were already accepted. The dispatcher now:

• marks the connection closed and wakes outbound waiters after read EOF,
• keeps the write stream open briefly for accepted inbound requests to finish their response writes,
• bounds that drain window to avoid hanging shutdown,
• tracks active inbound requests through the response write window, not just while they are present in _in_flight.

This targets stdio servers driven with redirected stdin, where EOF can arrive immediately after the last JSON-RPC request is read while an async tool call is still mid-await. In that case the response should be flushed to stdout before shutdown.

Validation:

• python3 -m py_compile src/mcp/shared/jsonrpc_dispatcher.py tests/shared/test_jsonrpc_dispatcher.py passed on the patched files.
• Added a dispatcher-level regression test that accepts a request, closes the read side, and asserts the response is still written.

[cubic-dev-ai]

1 issue found across 2 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/mcp/shared/jsonrpc_dispatcher.py">

<violation number="1" location="src/mcp/shared/jsonrpc_dispatcher.py:556">
P2: Track builder-rejection response writes in the EOF drain too. Otherwise a request whose `transport_builder` raises just before EOF can have its spawned INTERNAL_ERROR response cancelled before it reaches the peer.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Fix all with cubic | Re-trigger cubic</sub>

[cubic-dev-ai]

P2: Track builder-rejection response writes in the EOF drain too. Otherwise a request whose transport_builder raises just before EOF can have its spawned INTERNAL_ERROR response cancelled before it reaches the peer.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/mcp/shared/jsonrpc_dispatcher.py, line 556:

<comment>Track builder-rejection response writes in the EOF drain too. Otherwise a request whose `transport_builder` raises just before EOF can have its spawned INTERNAL_ERROR response cancelled before it reaches the peer.</comment>

<file context>
@@ -545,6 +553,7 @@ async def _dispatch_request(
             _progress_token=progress_token,
         )
         scope = anyio.CancelScope()
+        self._active_inbound_requests += 1
         # TODO(maxisbey): duplicate ids blind-overwrite (v1/TS parity); revisit
         # rejecting with INVALID_REQUEST. Key coerced so a stringified
</file context>