Login redirect crashes with ERR_HTTP_HEADERS_SENT after middleware fetch failure

Error

Error: Cannot append headers after they are sent to the client

Stack trace

ServerResponse.appendHeader (node:_http_outgoing:746)
  ← handleResponse (next/dist/esm/build/templates/app-page.js [_not-found/page]:1119)
  ← handler (next/dist/esm/build/templates/app-page.js [_not-found/page]:1326)
  ← NextNodeServer.renderToResponseWithComponentsImpl (next/dist/server/base-server.js:1462)
  ← NextNodeServer.renderErrorToResponseImpl (next/dist/server/base-server.js:1808)
  ← pipe.req.req (next/dist/server/base-server.js:1660)
  ← NextNodeServer.pipeImpl (next/dist/server/base-server.js:1044)
  ← invokeRender (next/dist/server/lib/router-server.js:266)
  ← handleRequest (next/dist/server/lib/router-server.js:505)

Source (node_modules/next/dist/esm/build/templates/app-page.js:1114-1124, page=/_not-found/page)

                        }
                    } else if (typeof value === 'number') {
                        value = value.toString();
                        res.appendHeader(key, value);
                    } else {
                        res.appendHeader(key, value);
                    }
                }
            }
            // Add the cache tags header to the response if it exists and we're in
            // minimal mode while rendering a static page.

Request context

• URL: https://app.devguard.org/self-service/login/browser
• Culprit route: GET /_not-found/page
• A middleware fetch failed (TypeError, via NextNodeServer.runMiddleware) occurred ~15ms before this error, which appears to have triggered Next's not-found/error rendering path on a response whose headers were already sent, causing the ERR_HTTP_HEADERS_SENT crash.

GlitchTip

• Issue: https://error-tracking.devguard.org/devguard/issues/3418
• Event ID: 019fc7498fa27d4a8ef21afba17ad173
• First seen: 2026-08-03T11:01:54.602Z

Start by tracing the GET /self-service/login/browser request through NextNodeServer.runMiddleware and the not-found rendering path in next/dist/esm/build/templates/app-page.js:1114-1124. Reproduce the middleware fetch failure and inspect when response headers are sent before handleResponse calls appendHeader. Done means the request no longer crashes with ERR_HTTP_HEADERS_SENT after that failure.