Infinite Redirect Loop with @astrojs/[email protected] When Using trailingSlash: 'always'

[faiz9068]

Astro Info

Astro                    v5.1.1
Node                     v20.14.0
System                   Windows (x64)
Package Manager          npm
Output                   server
Adapter                  @astrojs/vercel
Integrations             @astrojs/tailwind
                         astro-lottie
                         @astrojs/mdx
                         @astrojs/svelte
                         @astrojs/react
                         @astrojs/sitemap
                         sitemap-copier

Describe the Bug

When deploying to Vercel with trailingSlash: "always", routes enter an infinite redirect loop. The issue occurs even when directly accessing URLs with trailing slashes. This appears to be a regression in the Vercel adapter's handling of trailing slashes.

Reproduction

The Vercel adapter generates overlapping redirect rules in config.json:

{
  "src": "/guides",
  "headers": {
    "Location": "/guides/"
  },
  "status": 308
},
{
  "src": "/guides(?:\\/(.*?))?",
  "headers": {
    "Location": "/guides/$1/"
  },
  "status": 308
}

The second pattern matches /guides/ and attempts to redirect to /guides//, creating an infinite loop.

Steps to Reproduce

1. Create an Astro project with configurations:

   // astro.config.mjs
   export default defineConfig({
     trailingSlash: "always",
     output: "server",
     adapter: vercel()
   });

2. Add a route at /guides/ (either static or dynamic)
3. add /guides/[...slug] rest route
4. Deploy to Vercel
5. Visit /guides/

Expected Behavior

• /guides should redirect to /guides/ (once)
• /guides/ should render the page without redirecting

Actual Behavior

• Accessing /guides/ results in "Too Many Redirects" error
• Browser shows infinite loop between /guides/ and /guides//

Configuration

// astro.config.mjs
import { defineConfig } from "astro/config";
import tailwind from "@astrojs/tailwind";
import lottie from "astro-integration-lottie";
import mdx from "@astrojs/mdx";
import vercel from "@astrojs/vercel";
import svelte from "@astrojs/svelte";
import sitemap from "@astrojs/sitemap";
import react from "@astrojs/react";

export default defineConfig({
  trailingSlash: "always",
  integrations: [
    tailwind(),
    lottie(),
    mdx(),
    svelte(),
    react(),
    sitemap({...})
  ],
  output: "server",
  adapter: vercel()
});

Related Issues

• This appears similar to issue Routing too many redirects when using the config.json file generated by the Vercel adapter #418 but with the latest adapter version
• The problem seems to be a regression in how the adapter handles trailing slashes with dynamic routes

Environment

• Platform: Vercel
• Browser: All browsers (Chrome, Firefox, Safari)
• Operating System: All (not OS-specific)