fix(web): silence MaxListenersExceededWarning on proxied rewrites

[brendan-kellam]

Problem

The web server logs this on every request that goes through an external rewrite (/ingest/* → PostHog, /monitoring → Sentry tunnel):

(node:172) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 close listeners added to [ServerResponse]. MaxListeners is 10. Use emitter.setMaxListeners() to increase limit

In prod this is ~100–200k log lines/day; staging shows the same pattern.

It's a false positive. Since Next 16.3 (which we adopted in #1594), the external rewrite proxy attaches 9 close listeners to each ServerResponse, from router-utils/proxy-request.js, its bundled httpxy and the compression stream cleanup. Sentry adds 2 more, from server-subscription.js (isolation scope) and record-request-session.js. That's 11, one over Node's default limit. All of them belong to the one response and are released with it, so nothing accumulates.

Upstream issue: vercel/next.js#97757 (open; the fix PR vercel/next.js#97824 was closed unmerged).

Fix

In instrumentation.ts, subscribe to Node's http.server.request.start diagnostics channel and call response.setMaxListeners(20). The channel fires before Next handles the request, so the limit is in place before any listener is attached. It's scoped to server responses rather than raising EventEmitter.defaultMaxListeners, so the warning stays meaningful for every other emitter. The comment links the upstream issue so this can be removed once Next trims its listeners.

Testing

• Reproduced locally with a minimal Next 16.3.3 app with one external rewrite: without Sentry, 9 listeners and no warning; with Sentry.init (no tracing, matching staging and prod), 11 listeners and one warning per proxied request.
• With this change applied to that repro, warnings went from 3 → 0 over 3 proxied requests while the same 11 listeners were still attached.
• On staging, bursts to /ingest/* and /monitoring each produced warnings; /api/health, the polling APIs, /metrics and pages produced none.

🤖 Generated with Claude Code

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Note

Low Risk
Startup-only instrumentation that raises per-response listener limits; no auth, data, or request-handling logic changes.

Overview
Stops false-positive MaxListenersExceededWarning spam on proxied external rewrites (PostHog /ingest/*, Sentry /monitoring), which was flooding production logs (~100–200k lines/day) after Next 16.3’s rewrite proxy plus Sentry attach 11 close listeners per ServerResponse.

In instrumentation.ts, on the Node runtime it subscribes to http.server.request.start and calls response.setMaxListeners(20) before Next/Sentry attach listeners—scoped per response instead of raising the global default. The changelog Unreleased section documents the fix.

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Summary by CodeRabbit

• Bug Fixes
  • Silenced a false-positive MaxListenersExceededWarning that appeared on requests proxied through external rewrites.

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• CHANGELOG.md
• packages/web/src/instrumentation.ts

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Walkthrough

On Node.js, instrumentation sets the maximum listener limit to 20 for responses included in HTTP request-start events. The Unreleased changelog records that the false-positive warning was silenced.

Changes

Response listener limit

Layer / File(s)	Summary
Set response listener limit packages/web/src/instrumentation.ts, CHANGELOG.md	Node.js instrumentation listens for HTTP request-start events and sets the maximum listener limit to 20 when a response is present. The changelog records the warning fix.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~8 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: msukkari

Merge Risk: ⚪ Minimal · up to 3a614

The response-specific limit is applied before the affected listener burst, and no other material issue is established. The change appears ready to merge.

🚥 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 1 functions across 1 files. (1 skipped: 1 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: silencing the MaxListenersExceededWarning for proxied rewrites.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

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 1 functions across 1 files. (1 skipped: 1 unsupported.)

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