SvelteKit ISR + Paraglide JS Localized Routes Return 404 on Vercel

[d1d4r]

Environment

• Framework: SvelteKit

• Adapter: @sveltejs/adapter-vercel

• i18n Library: Paraglide JS (@inlang/paraglide-sveltekit)

• Locales: en, ckb (path-based, e.g., /en/isr, /ckb/isr)

• Hosting: Vercel

Problem

ISR (Incremental Static Regeneration) works for the base route /isr, but all localized routes (/en/isr, /ckb/isr) return 404 Not Found on Vercel. Everything works correctly in local development.

Repo

https://github.com/d1d4r/isr-sveltekit

https://isr-sveltekit.vercel.app/ckb/isr

What Works

• /isr — works and caches correctly

• Local development — all localized routes work

What Doesn’t Work

• /en/isr — returns 404 on Vercel

• /ckb/isr — returns 404 on Vercel

Root Cause Analysis

Vercel’s ISR operates at the edge layer, which runs before SvelteKit’s reroute hook processes the request. When a request comes in for /en/isr:

1. Vercel’s edge looks for a cached ISR page at /en/isr

2. No cache exists (ISR config only exists for /isr)

3. Returns 404 before SvelteKit’s reroute can map /en/isr → /isr

Solutions Attempted

Attempt	Result
Changed expiration: false to expiration: 60	Still 404
Added vercel.json rewrites for locale paths	Routes work but locale detection breaks (always defaults to en)
Using [lang] dynamic route segment	Not compatible with Paraglide’s architecture

Expected Behavior

Localized routes (/en/isr, /ckb/isr) should work with ISR, respecting SvelteKit’s reroute hook to map them to the underlying /isr route while maintaining correct locale context.

Question

How can ISR be configured to work with Paraglide JS’s middleware-based rerouting approach on Vercel? Is there a way to make Vercel’s edge layer aware of the rerouted paths, or a recommended pattern for this use case?

[system]

There’s another community post with 404 debugging tips that might be helpful. Please give these solutions a try and let us know how it goes.

Debugging 404 Errors

Sometimes things don’t go as expected when deploying a website. If you see a 404 on your site, that means the page or resource couldn’t be found. There are many possible causes, and the absence of something can be tricky debug if you don’t know where to look. Here are some things you can do to find the cause and fix it. Debugging Tips Check the error code If you see a mostly white screen with 404: NOT_FOUND along with a Code and and ID then you can click the blue info box below the error deta…

A human should be around soon to offer more advice. But you can also get helpful information quickly by asking v0.