Astro Info

Astro                    v5.18.0
  Vite                     v6.4.1
  Node                     v22.20.0
  System                   macOS (x64)
  Package Manager          npm
  Output                   server
  Adapter                  @astrojs/node (v9.5.4)

If this issue only occurs in one browser, which browser is a problem?

No response

Describe the Bug

Bug description:
When output: 'server' is used with i18n fallback configured, the fallback middleware intercepts 302 redirect responses returned by page handlers on non-default locale routes, and incorrectly replaces them with a redirect to the default-locale equivalent of the current page. The fallback should only trigger on 404 responses.

What's the expected result?

Expected result:
POST /en/example returns 302 Location: /en/success — the redirect target set by Astro.redirect() in the page handler.

Actual result:
POST /en/example returns 302 Location: /example — the German (default locale) equivalent of the current page. GET /en/example correctly returns 200; only requests where the handler itself produces a 3xx are affected.

Link to Minimal Reproducible Example

https://github.com/gabplanalp/astro-i18n-fallback-redirect-bug

Participation

• I am willing to submit a pull request for this issue.