vercel adapter: redirects never match when trailingSlash: 'always'

Astro Info

Astro                    v7.3.1
Node                     v24.12.0
System                   macOS (arm64)
Package Manager          pnpm
Output                   static
Adapter                  @astrojs/vercel v11.0.10
Integrations             none

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

No response

Describe the Bug

With trailingSlash: 'always', redirects declared via redirects in astro.config.mjs never fire on Vercel. Requests to the redirect source return 404.

Config used in the repro:

export default defineConfig({
  output: 'static',
  adapter: vercel(),
  trailingSlash: 'always',
  redirects: { '/old': '/new/' },
});

@astrojs/vercel generates .vercel/output/config.json where:

1. the generic trailing-slash normalization route is emitted before the explicit redirect routes, and
2. each redirect is compiled to a strict unslashed regex (^/old$), even when the configured source has a trailing slash.

Request flow on Vercel:

GET /old   -> 308 /old/   (trailing-slash route matches first)
GET /old/  -> ^/old$ does not match
           -> filesystem miss -> 404

So the configured redirect is unreachable for both /old and /old/.

Generated routes (trimmed from the committed config.json):

{ "src": "^/((?:[^/]+/)*[^/\\.]+)$", "headers": { "Location": "/$1/" }, "status": 308 },
{ "src": "^/old$", "headers": { "Location": "/new/" }, "status": 301 }

This cannot be worked around from astro.config.mjs: writing the source as '/old/' produces the identical ^/old$ regex (the trailing slash is dropped when the source is normalized to route segments), and there is no syntax for a slash-optional redirect source.

Reproducing locally: in the linked repo, pnpm install && pnpm build && pnpm repro inspects the generated .vercel/output/config.json and shows that ^/old$ cannot match the post-normalization path /old/. The generated config.json is committed in the repo.

This is the same class of bug as #16030 (redirects 404 under trailingSlash on the Cloudflare adapter), which was fixed in #16034 by making @astrojs/underscore-redirects generate patterns that respect trailingSlash. @astrojs/vercel does not use underscore-redirects — it builds Vercel routes from RouteData itself — so it never received the fix.

What's the expected result?

The adapter emits a redirect pattern that matches the normalized request path, e.g. ^/old/$ under trailingSlash: 'always' (and the equivalent for 'never'/'ignore'), matching the behavior implemented in #16034. With the config above, GET /old should 308 to /old/ and then 301 to /new/, and GET /old/ should 301 to /new/ directly.

Link to Minimal Reproducible Example

https://github.com/mkusaka/astro-vercel-redirect-trailing-slash-repro

Participation

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