Docs: Clarify status codes w/ streaming (#54374)

Clarify what status code is returned when streaming, and in relation to `redirect` and `not-found`. Relates to: #54361 Co-authored-by: Lee Robinson <[email protected]>
vercel · Aug 22, 2023 · 9ca82f6 · 9ca82f6
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diff --git a/.../02-app/01-building-your-application/01-routing/06-loading-ui-and-streaming.mdx b/.../02-app/01-building-your-application/01-routing/06-loading-ui-and-streaming.mdx
@@ -172,3 +172,9 @@ For more Suspense examples and use cases, please see the [React Documentation](h
 
 - Next.js will wait for data fetching inside [`generateMetadata`](/docs/app/api-reference/functions/generate-metadata) to complete before streaming UI to the client. This guarantees the first part of a streamed response includes `<head>` tags.
 - Since streaming is server-rendered, it does not impact SEO. You can use the [Mobile Friendly Test](https://search.google.com/test/mobile-friendly) tool from Google to see how your page appears to Google's web crawlers and view the serialized HTML ([source](https://web.dev/rendering-on-the-web/#seo-considerations)).
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+### Status Codes
+
+When streaming, a `200` status code will be returned to signal that the request was successful.
+
+The server can still communicate errors or issues to the client within the streamed content itself, for example, when using [`redirect`](/docs/app/api-reference/functions/redirect) or [`notFound`](/docs/app/api-reference/functions/not-found). Since the response headers have already been sent to the client, the status code of the response cannot be updated. This does not affect SEO.
diff --git a/docs/02-app/02-api-reference/02-file-conventions/not-found.mdx b/docs/02-app/02-api-reference/02-file-conventions/not-found.mdx
@@ -3,7 +3,7 @@ title: not-found.js
 description: API reference for the not-found.js file.
 ---
 
-The **not-found** file is used to render UI when the [`notFound`](/docs/app/api-reference/functions/not-found) function is thrown within a route segment. Along with serving a custom UI, Next.js will also return a `404` HTTP status code.
+The **not-found** file is used to render UI when the [`notFound`](/docs/app/api-reference/functions/not-found) function is thrown within a route segment. Along with serving a custom UI, Next.js will return a `200` HTTP status code for streamed responses, and `404` for non-streamed responses.
 
 ```tsx filename="app/blog/not-found.tsx" switcher
 import Link from 'next/link'

diff --git a/docs/02-app/02-api-reference/04-functions/redirect.mdx b/docs/02-app/02-api-reference/04-functions/redirect.mdx
@@ -5,7 +5,7 @@ description: API Reference for the redirect function.
 
 The `redirect` function allows you to redirect the user to another URL. `redirect` can be used in Server Components, Client Components, [Route Handlers](/docs/app/building-your-application/routing/route-handlers), and [Server Actions](/docs/app/building-your-application/data-fetching/server-actions).
 
-If you need to redirect to a 404, use the [`notFound` function](/docs/app/api-reference/functions/not-found) instead.
+If a resource doesn't exist, you can use the [`notFound` function](/docs/app/api-reference/functions/not-found) instead.
 
 ## Parameters