How do you add custom response headers or set cookies inside an exception filter in NestJS?
Access the raw response object via host.switchToHttp().getResponse() and call setHeader() or clearCookie() before calling res.status().json(). For Fastify use res.header() instead of setHeader(). To stay platform-agnostic use HttpAdapterHost instead of casting to a framework-specific type.
Setting headers and cookies inside an exception filter
Platform-specific header API differences:
Express — res.setHeader('key', 'value') and res.clearCookie('name', options).
Fastify — res.header('key', 'value') — setHeader() does not exist on Fastify response.
To stay platform-agnostic inject HttpAdapterHost and use adapter.setHeader(res, key, value).
Always set headers before calling res.status().json() — headers cannot be set after the response starts.
Custom error headers like X-Error-Code or X-Request-Id help clients correlate errors with logs.