Middleware does not run for 404s in dev

[delucis]

Astro Info

Astro                    v4.15.8
Node                     v18.20.3
System                   Linux (x64)
Package Manager          npm
Output                   static
Adapter                  none
Integrations             none

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

n/a — Astro dev server issue

Describe the Bug

In dev mode, middleware is not run for URLs that 404. We hit this with Starlight’s new locals.t() function as it causes Starlight code to error when trying to call t() which is undefined.

In the minimal repro the following set-up is in place:

1. src/middleware.ts attaches a simple function to context.locals.
2. src/pages/index.astro and src/pages/404.astro both use this function to render some text.
3. Visiting / works as expected — the Astro.locals.t() call renders some text.
4. Visiting other routes, which trigger the custom 404, cause a Astro2.locals.t is not a function error to be thrown.

What's the expected result?

Locals set by middleware should be available in a custom 404 as they are in a regular page.

Link to Minimal Reproducible Example

https://stackblitz.com/edit/github-dzgmby?file=src%2Fpages%2F404.astro,src%2Fmiddleware.ts,src%2Fpages%2Findex.astro

Participation

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