Astro Server Island returns 404 with i18n prefixDefaultLocale: true

[dallyh]

Astro Info

Astro                    v5.1.1
Node                     v22.12.0
System                   Windows (x64)
Package Manager          pnpm
Output                   static
Adapter                  @astrojs/node
Integrations             none

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

No response

Describe the Bug

When there is a deferred server island in an Astro page, then the island's endpoint returns status of 404, but it also returns the rendered HTML.

Setting prefixDefaultLocale to false fixes this issue, the island returns 200.

The reproducible example is set up to have a i18n routing middleware and manual routing, however it is also reproducible without it, that is having the configuration in the config file.

i18n: {
        locales: ["cs", "en"],
        defaultLocale: "cs",
        routing: {
            prefixDefaultLocale: true,
            redirectToDefaultLocale: false
        }
    },

In the example just navigate to either /en or /cs and check the terminal or browser network activity for an 404.

Edit:
Forgot to say that this is reproducible in the dev server and also after build.

What's the expected result?

Reponse code of 200 with the specified config.

Link to Minimal Reproducible Example

https://github.com/dallyh/astro-server-island-404

Participation

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

[sasoria]

Thank you for the issue, this looks similar to #12817.

[ematipico]

Duplicate of #12817