fix(stdio): serve requests off the read loop by fesiqueira · Pull Request #977 · mark3labs/mcp-go

fesiqueira · 2026-09-11T19:05:19Z

Description

Fixes #976

On stdio, subscriptions/listen permanently blocks the server. processMessage
dispatched every method except tools/call inline on the read loop
(stdio.go:609, // Handle other messages synchronously), and
handleSubscriptionsListen blocks on <-ctx.Done() for the life of the
subscription (subscriptions.go:143). The reader never reaches readNextLine
again, so every later request goes unread and unanswered. Cancelling does not
help: notifications/cancelled arrives on the same stdin the blocked handler
stops the server reading, so the block sustains itself.

This serves anything carrying a JSON-RPC id on its own goroutine and keeps
notifications inline, so a cancellation is never queued behind the request it
cancels. Writes remain serialised by writeMu; the tools/call worker pool is
untouched.

Nothing else was needed — per-request cancellable contexts and the
notifications/cancelled handler already exist (request_handler.go:111-121,
server.go:2477); they were simply unreachable behind the blocked reader.

Type of Change

Bug fix (non-breaking change that fixes an issue)
New feature (non-breaking change that adds functionality)
MCP spec compatibility implementation
Breaking change (fix or feature that would cause existing functionality to not work as expected)
Documentation update
Code refactoring (no functional changes)
Performance improvement
Tests only (no functional changes)
Other (please describe):

Checklist

My code follows the code style of this project
I have performed a self-review of my own code
I have added tests that prove my fix is effective or that my feature works
I have updated the documentation accordingly

Additional Information

TestStdioServeRequestsDuringSubscriptionsListen opens a subscription stream and
then calls tools/list. It times out on main and passes here.
go test ./... -race -count=1 is clean across all packages, e2e included.

Two things reviewers should weigh:

Requests on stdio are now served concurrently. tools/call already was,
via the worker pool, so handlers had to be concurrency-safe for the common
case already — but a server relying on strictly serial handling of other
methods over stdio would see a behaviour change. I marked this non-breaking on
that basis; say the word if you read it differently.
Goroutine per request, unbounded. This matches what streamable HTTP gets
from net/http, and concurrency here is bounded by what one client can push
through one pipe. A pool would bound it, but a slot held by a long-lived
subscriptions/listen would starve the pool at its default size of 5
(stdio.go:377), so the subscription stream would need its own goroutine
regardless. Happy to switch if you prefer a bound.

With this in place, stdio can serve 2026-07-28 legitimately, so no change to
what server/discover advertises is needed.

Summary by CodeRabbit

Bug Fixes
- Internal error responses now remain associated with the originating request when request handling encounters a panic.
- In-flight request handlers are canceled and allowed to finish cleanly when the input connection reaches EOF.
- The server now waits for active request handlers to exit before completing shutdown.
Tests
- Added coverage for request-handler cleanup and shutdown behavior when input ends.

processMessage dispatched every method except tools/call inline, so a handler that blocks blocked the reader. subscriptions/listen blocks on <-ctx.Done() for the life of the subscription, leaving every later request unread. The block could not be lifted either: the notifications/cancelled that would cancel it arrives on the same stdin the blocked handler prevents the server from reading. Serve anything carrying a JSON-RPC id on its own goroutine, and keep notifications inline so a cancellation is never queued behind the request it cancels. Writes stay serialised by writeMu and the tools/call worker pool is unchanged.

mark-iii-labs-huly · 2026-09-11T19:05:27Z

Connected to Huly®: MCP_G-538

coderabbitai · 2026-09-11T19:05:32Z

Walkthrough

The stdio server now handles identified requests in separate goroutines and tracks them until shutdown. Notifications remain synchronous. Shutdown cancels and joins active handlers. Panic responses preserve request IDs. Tests cover request joining on EOF.

Changes

Stdio request concurrency

Layer / File(s)	Summary
Request lifecycle and shutdown `server/stdio.go`, `server/stdio_test.go`	`Listen` cancels the request context after input processing and waits for active handlers before session cleanup. The EOF test verifies handler cancellation and completion.
Request classification and dispatch `server/stdio.go`	`processMessage` stores parse results, reads request IDs, and dispatches identified requests asynchronously. Notifications continue through synchronous handling.
Request recovery and validation `server/stdio.go`, `server/stdio_test.go`	Panic recovery preserves request IDs for tool-call and regular request errors. The EOF test uses `require` assertions and validates successful completion.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to 68220

A client that stops reading responses during shutdown can prevent the stdio server from exiting. Make output writes interruptible or unblock the writer before waiting for handlers.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main change: serving stdio requests outside the read loop to prevent blocking. It is concise and specific.
Description check	✅ Passed	The description explains the deadlock, implementation, testing, compatibility considerations, and checklist status. It includes the required description and change-type sections. The omitted MCP Spec …
Linked Issues check	✅ Passed	The changes satisfy issue `#976`. `server/stdio.go` dispatches JSON-RPC requests with IDs in separate goroutines and keeps notifications inline. This allows later requests and `notifications/cancelled` …
Out of Scope Changes check	✅ Passed	The changes remain within issue `#976`. The worker request-ID propagation supports correct error correlation for the new concurrent request handling. The EOF and handler-lifecycle test covers shutdown b…

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Actionable comments posted: 3

🧹 Nitpick comments (1)

server/stdio_test.go (1)
564-568: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Use testify/require for the new response assertions.

This test file must follow the repository’s testify/assert and testify/require convention. Add github.com/stretchr/testify/require, then replace the branches with require.Nil(t, listed["error"]) and require.NotNil(t, listed["result"]).
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In `@server/stdio_test.go` around lines 564 - 568, Update the tools/list response
assertions in the test to use the repository’s testify/require convention: add
the require import, replace the manual error branch with require.Nil, and
replace the result nil check with require.NotNil.

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Inline comments:
In `@server/stdio.go`:
- Line 614: Update Listen and the processInputStream request path to derive a
cancellable child context, track every handleRequest goroutine with a wait
group, cancel the context when input processing ends, and wait for all request
handlers before unregistering the session or returning.
- Line 614: Update the request dispatch in the stdio read loop around
s.handleRequest to bound concurrent non-tool requests with an admission limit,
release capacity when each handler finishes, and reserve capacity for ordinary
requests so long-lived subscriptions/listen handlers cannot exhaust all slots.
Keep tool request handling behavior unchanged.
- Line 637: Update the HandleMessage recovery path to pass baseMessage.ID into
handleRequest and use that request ID when calling createErrorResponse for
recovered panics, preserving null only for genuinely unidentified requests.

---

Nitpick comments:
In `@server/stdio_test.go`:
- Around line 564-568: Update the tools/list response assertions in the test to
use the repository’s testify/require convention: add the require import, replace
the manual error branch with require.Nil, and replace the result nil check with
require.NotNil.

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Listen returned on EOF while a blocked handler - subscriptions/listen waits for cancellation - was still running, holding a session that the deferred UnregisterSession had already torn down. EOF does not cancel the caller's context, so the goroutine leaked for the life of the host. Derive a cancellable child context, cancel it once input processing ends, and join the handlers through a new requestWg before returning. Cancel has to precede the wait, or Listen deadlocks on the handler it is waiting for. Also carry the request's JSON-RPC id into handleRequest, so a recovered panic returns an error the client can correlate rather than id null. Both new tests use testify/require, per the repository convention.

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Actionable comments posted: 2

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Inline comments:
In `@server/stdio_test.go`:
- Line 592: Update the test around Listen to use io.Discard directly for stdout
output instead of creating an io.Copy goroutine with stdoutReader, since the
test does not inspect output and should not leave a blocked goroutine running.
- Around line 585-586: Update the StdioServer.Listen cancellation test so the
handler signals cancellation but remains blocked until the test verifies Listen
is still blocked after EOF; only then release the handler and allow it to
return. Ensure the test’s join assertion requires Listen to wait for requestWg
rather than relying on finished.Store(true) before the handler exits.

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fesiqueira · 2026-09-12T02:53:26Z

I'll address the comments tomorrow

toolCallWork now carries the JSON-RPC id, so the worker's panic recovery returns an error the client can match to its pending request. This is what handleRequest already does; both request paths now behave the same. TestStdioJoinsRequestHandlersOnEOF passed with Listen's requestWg.Wait removed, because the handler recorded completion as soon as the context was cancelled. It covered cancellation, not the join it is named for. The handler now stays in the call past cancellation so the test can assert Listen has not returned, and releases it only afterwards. The test also writes to io.Discard instead of a pipe it never reads, which left a copy goroutine running past the test, and takes its context from t.Context. Listen dispatches through WaitGroup.Go. Both keep the modernize analyzer quiet.

fesiqueira · 2026-09-15T13:31:02Z

Pushed 68220ad. This also fixes the two modernize findings that were failing the lint job (WaitGroup.Go in processMessage, t.Context in the new test).

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Actionable comments posted: 1

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Inline comments:
In `@server/stdio.go`:
- Around line 625-627: Update the request shutdown flow around Listen and
handleRequest so blocked writeResponse calls are unblocked before
requestWg.Wait() runs. Make response writes cancellation-aware or close the
appropriate writer before waiting, while preserving normal response delivery and
cleanup behavior.

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fesiqueira · 2026-09-15T13:50:40Z

@ezynda3 I believe all automated review comments were addressed. Could you look into it, please? Thank you!

mcp-go v1.0.0 (pulled in by grafana#1147) introduced the 2026-07-28 protocol's subscriptions/listen method, but StdioServer handled every request synchronously on its single read loop. A client that opens subscriptions/listen (a long-lived request with no response until cancelled) permanently blocks that loop: every later request, including tools/list, times out and is never answered. Any MCP client that negotiates the 2026-07-28 protocol over stdio and opens a subscriptions/listen subscription hits this on first connection - mcp-grafana starts, negotiates 2026-07-28, and then never answers tools/list. Confirmed with GitHub Copilot CLI 1.0.86-1.0.88. The deadlock is tracked at mark3labs/mcp-go#976 and was fixed by mark3labs/mcp-go#977 (serving requests off the read loop), released in mcp-go v1.1.1. Verified locally: - go build ./... succeeds with the bump, no code changes needed - go test -tags unit ./... passes unchanged - Manual stdio repro (server/discover -> subscriptions/listen -> tools/list) hangs on mcp-go v1.0.0 (mcp-grafana 1.4.0-1.5.1) and returns tools immediately after this bump

mcp-go v1.0.0 (pulled in by #1147) introduced the 2026-07-28 protocol's subscriptions/listen method, but StdioServer handled every request synchronously on its single read loop. A client that opens subscriptions/listen (a long-lived request with no response until cancelled) permanently blocks that loop: every later request, including tools/list, times out and is never answered. Any MCP client that negotiates the 2026-07-28 protocol over stdio and opens a subscriptions/listen subscription hits this on first connection - mcp-grafana starts, negotiates 2026-07-28, and then never answers tools/list. Confirmed with GitHub Copilot CLI 1.0.86-1.0.88. The deadlock is tracked at mark3labs/mcp-go#976 and was fixed by mark3labs/mcp-go#977 (serving requests off the read loop), released in mcp-go v1.1.1. Verified locally: - go build ./... succeeds with the bump, no code changes needed - go test -tags unit ./... passes unchanged - Manual stdio repro (server/discover -> subscriptions/listen -> tools/list) hangs on mcp-go v1.0.0 (mcp-grafana 1.4.0-1.5.1) and returns tools immediately after this bump

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ezynda3 merged commit 94d6b83 into mark3labs:main Sep 23, 2026
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dgotbaum mentioned this pull request Sep 23, 2026

fix(deps): bump mark3labs/mcp-go to v1.1.1 to fix stdio subscriptions/listen deadlock grafana/mcp-grafana#1236

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cpruijsen mentioned this pull request Sep 23, 2026

fix(server): keep stdio serving requests during subscriptions/listen (#976) #979

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github-actions Bot mentioned this pull request Sep 28, 2026

Update module github.com/mark3labs/mcp-go to v1.2.1 claytono/go-unifi-mcp#322

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fix(stdio): serve requests off the read loop - #977

fix(stdio): serve requests off the read loop#977
ezynda3 merged 3 commits into
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fesiqueira:fix/stdio-dispatch-requests-off-read-loop

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Description

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