Set HTTP response headers from anywhere during server-side rendering - loaders, components, or middleware. Headers are accumulated and merged onto the final response.

import { setResponseHeaders } from 'one'

await setResponseHeaders((headers) => {
  headers.set('X-Custom-Header', 'value')
})

The headers parameter is a standard Headers object.

Caching and ISR

Use setResponseHeaders in loaders to enable CDN caching and Incremental Static Regeneration:

app/blog/[slug]+ssr.tsx

import { setResponseHeaders, useLoader } from 'one'

export async function loader({ params }) {
  await setResponseHeaders((headers) => {
    headers.set('Cache-Control', 'public, s-maxage=3600, stale-while-revalidate=86400')
  })

  const post = await fetchPost(params.slug)
  return { post }
}

export default function BlogPost() {
  const { post } = useLoader(loader)
  return <article>{post.content}</article>
}

Common cache header patterns:

s-maxage=3600 - CDN caches for 1 hour
stale-while-revalidate=86400 - Serve stale content while revalidating in background (up to 1 day)
max-age=0, must-revalidate - Always revalidate with origin
private, no-store - Never cache (for user-specific data)

Note: stale-while-revalidate requires CDN support (Vercel, CloudFront, Fastly). Cloudflare does not currently support it.

Cookies

Set cookies using the standard Set-Cookie header:

app/auth/login+api.ts

import { setResponseHeaders } from 'one'

export async function POST(request: Request) {
  const { token } = await authenticateUser(request)

  await setResponseHeaders((headers) => {
    headers.append('Set-Cookie', `session=${token}; HttpOnly; Secure; SameSite=Strict; Path=/`)
  })

  return Response.json({ success: true })
}

In Middleware

app/_middleware.ts

import { createMiddleware, setResponseHeaders } from 'one'

export default createMiddleware(({ request }) => {
  setResponseHeaders((headers) => {
    headers.set('X-Request-Id', crypto.randomUUID())
  })
})

Important Notes

Only works server-side (SSR routes and loaders)
Can be called multiple times - headers are merged
Must be called during the request lifecycle (not in event handlers or timeouts)

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