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fix: handle HEAD on SSE endpoint to prevent connection leaks - #3744

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Jihaochang1 wants to merge 1 commit into
modelcontextprotocol:mainfrom
Jihaochang1:fix/sse-head-request-leak
Closed

Jihaochang1 wants to merge 1 commit into
modelcontextprotocol:mainfrom
Jihaochang1:fix/sse-head-request-leak

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@Jihaochang1

@Jihaochang1 Jihaochang1 commented Oct 23, 2026 •

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Summary

Problem

When a HEAD request is sent to the SSE endpoint, the connect_sse() function never completes, causing connection leaks and blocking keep-alive connections.

Root Cause

The SSE endpoint expects an event stream, but HEAD requests only request headers without a body. The current implementation doesn't handle this case explicitly.

Fix

Add an early return in handle_sse() for HEAD requests before calling connect_sse(), preventing the connection leak.

Changes

  • src/fastmcp/server/sse.py: Added HEAD request guard
  • src/fastmcp/server/http.py: Added HEAD request guard

Closes #3736

Type of Change

  • Bug fix (non-breaking change which fixes an issue)

Checklist

  • My code follows the style guidelines of this project

@github-actions github-actions Bot added the missing-issue-link Auto-closed: PR needs a linked issue assigned to its author (see CONTRIBUTING.md) label Oct 23, 2026
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This PR has been closed automatically. This repo only keeps pull requests open when they come from a maintainer, or from a contributor a maintainer has assigned to the linked issue, and you aren't currently assigned to #3736.

If a maintainer assigns you to #3736, this PR reopens on its own and there's nothing more you need to do here. Assignment is a maintainer call based on capacity; comments that only ask to be assigned don't factor in. What does help is engaging on the issue itself by confirming the repro, explaining why it matters for your use case, or describing the approach you'd take.

You're welcome to keep pushing commits here (just avoid force-pushing, since GitHub can't reopen a rewritten branch), but that on its own won't get the PR reviewed or the issue assigned, and realistically most auto-closed PRs stay closed. There's no need to open a new PR either way.

CONTRIBUTING.md has the full reasoning, but in short:

  • We're a small team with very little capacity to review community PRs right now.
  • Many recent PRs are AI-generated with little human review, and reviewing one carefully still costs a maintainer as much time as it ever did. A well-described issue is usually more useful to us than the code.

Maintainers: reopen, remove missing-issue-link, or add bypass-issue-check to override.

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SSE transport: HEAD on the SSE endpoint never completes, hanging keep-alive connections and leaking the handler

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