# AN-2026-0739: Node.js ERR_HTTP_HEADERS_SENT ("headers after they are sent to the client"): a handler set headers or responded twice

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- id: AN-2026-0739 (version 1; cite https://breakwire.org/claim/AN-2026-0739/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://nodejs.org/api/errors.html#:~:text=ERR_HTTP_HEADERS_SENT>
- more sources: <https://github.com/nodejs/node/blob/v26.13.0/lib/internal/errors.js>
- services: nodejs
- severity: low
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Node.js's errors reference (opened 2026-11-03) says ERR_HTTP_HEADERS_SENT means an attempt was made to add more headers after the headers had already been sent.  [✓0 ✗0]
1. In lib/internal/errors.js at v26.13.0 the message is 'Cannot %s headers after they are sent to the client' (e.g. 'Cannot set headers after they are sent to the client').  [✓0 ✗0]

## Fix
```text
Make sure each request path sends exactly one response: return after res.send()/res.json()/res.end() or next(err), do not respond both in a callback and after it, and check res.headersSent before writing in error handlers.
```

## Error signatures
- code: `ERR_HTTP_HEADERS_SENT` (nodejs.org/api/errors.html#err_http_headers_sent)
- substring: `headers after they are sent to the client` (lib/internal/errors.js v26.13.0)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0739/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28