Fastify integration captures 4xx errors raised before the reply status is set (e.g. empty JSON body) #24926

Is there an existing issue for this?

• I have checked for existing issues https://github.com/getsentry/sentry-javascript/issues
• I have reviewed the documentation https://docs.sentry.io/
• I am using the latest SDK release https://github.com/getsentry/sentry-javascript/releases

How do you use Sentry?

Sentry Saas (sentry.io)

Which SDK are you using?

@sentry/nestjs

SDK Version

11.1.0

Framework Version

NestJS 12, @nestjs/platform-fastify 12, Fastify 5.12.5

Link to Sentry event

https://kpler.sentry.io/issues/7763870286/events/77524398057d4e309eeb9a3f443f4af0/?project=4511819824889856

Reproduction Example/SDK Setup

Sentry.init({
  integrations: defaults => [
    ...defaults.filter(integration => integration.name !== 'ProcessSession'),
    Sentry.httpIntegration({ sessions: false }),
  ],
});

No tracesSampleRate, no setupFastifyErrorHandler, no custom shouldHandleError.

Steps to Reproduce

1. Run a NestJS app on FastifyAdapter (or a plain Fastify 5 app) with the SDK set up as above.
2. Send a request with a JSON content type and an empty body: curl -X POST http://localhost:3000/ -H 'Content-Type: application/json'

Expected Result

The client gets a 400 and nothing is sent to Sentry, since the default shouldHandleError skips 4xx.

Actual Result

The client gets a 400, but Sentry receives an unhandled error:

FastifyError: Body cannot be empty when content-type is set to 'application/json'
mechanism: auto.function.fastify, handled: false

The same happens for an invalid JSON body (FST_ERR_CTP_INVALID_JSON_BODY).

Additional Context

Fastify runs onError hooks before it applies the error's status to the reply. For errors raised outside the route handler, such as body parsing, defaultShouldHandleError still reads reply.statusCode === 200, which it treats as "capture". #18418 fixed the same symptom upstream in Fastify 5.7.0, but only for route handler errors on the diagnostics channel path.

This started for us with v11, where fastifyIntegration registers the onError hook automatically (#23460). Before that, the hook was only added by setupFastifyErrorHandler, which NestJS apps don't call.

A fix is up in #24924: when reply.statusCode is still the default 200, the default shouldHandleError resolves the status the same way Fastify's setErrorStatusCode does.

Priority

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