Vercel preset emits trailing-slash override paths

[JonathanXDR]

Environment

• Nitro: 2.13.4 (via Nuxt 4.4.8)
• Node: 24.18.0
• Preset: vercel

Reproduction

• Repo: https://github.com/JonathanXDR/repro-nuxt-vercel-trailing-slash-override
• Live demo: /slash/ (broken) vs /noslash (works)

nuxt.config.ts (no Nuxt modules needed):

export default defineNuxtConfig({
  nitro: { prerender: { crawlLinks: false, routes: ['/', '/slash/', '/noslash'] } },
})

NITRO_PRESET=vercel npm run build
node -e "console.log(require('./.vercel/output/config.json').overrides)"
npx vercel deploy --prebuilt   # then GET /slash/ and /noslash a few times

StackBlitz/WebContainers can't run the Vercel build/runtime, so use a real vercel build + deploy to observe it.

Describe the bug

A prerendered route with a trailing slash gets a Build Output overrides entry whose path keeps the slash, and Vercel does not serve the static file from such a path. The request falls through to the SSR function.

Generated overrides:

{
  "index.html":         { "path": "" },        // static ✓
  "noslash/index.html": { "path": "noslash" }, // static ✓
  "slash/index.html":   { "path": "slash/" }   // function ❌, trailing slash
}

Each page renders a server-side useState(new Date().toISOString()) (frozen if static, fresh per request if the function runs):

Route	override path	x-vercel-cache	timestamp over 3 requests	served by
/slash/	slash/	MISS	changes each request	function ❌
/noslash	noslash	HIT	frozen	static ✓

Changing only slash's override from slash to slash/ on the same build flips it from static to function, so the trailing slash is the sole cause.

Root cause

src/presets/vercel/utils.ts, generateBuildConfig():

{ path: route.replace(/^\//, '') }   // strips the leading slash, KEEPS the trailing slash

Prerender routes are stored verbatim, so a /slash/ route becomes { path: "slash/" }. Per the Build Output API v3 spec an override path is always slash-free (blog.html → { path: "blog" }). Trailing-slash behavior is expressed via 308 redirect routes, not the override path. So { path: "slash/" } is off-spec.

Suggested fix

Make the override path slash-free, either:

1. Omit the override for a <dir>/index.html (Vercel's directory-index already serves it at both /dir and /dir/), or
2. emit a clean path plus 308 trailing-slash redirect routes (e.g. @vercel/routing-utils getTransformedRoutes({ trailingSlash })).

Note: rewriting to { path: "slash" } alone would leave the canonical /slash/ unserved, since the preset emits no redirect routes. Likely also fixes #4242. Happy to open a PR.

Additional context

Real-world trigger: a Nuxt site using @nuxtjs/i18n + nuxt-site-config with trailingSlash: true produces { "de/index.html": { "path": "de/" } } and serves /de/ from the function.

Related:

• Prerendered index.html not served statically by Vercel preset — root URL goes through SSR function #4242 (open, root / variant)
• Vercel returns 404 pages when using links that end in a slash and the vercel-static preset #1651 / PR fix(vercel, azure): serve prerendered routes statically #500 (original "serve prerendered routes statically" fix)

Logs

# /slash/  (override "slash/")  -> timestamp changes = function
req1 x-vercel-cache=MISS renderedAt=2026-06-28T14:20:44.124Z
req2 x-vercel-cache=MISS renderedAt=2026-06-28T14:20:44.563Z
req3 x-vercel-cache=MISS renderedAt=2026-06-28T14:20:45.019Z

# /noslash (override "noslash") -> timestamp frozen = static
req1-3 x-vercel-cache=HIT renderedAt=2026-06-28T14:20:06.367Z

[pi0]

cc @RihanArfan