Unable to find source file for page middleware with extensions

[siamahnaf]

Hello After updating my project to nextjs 16 and deploying to vercel always showing an warning-

WARNING: Unable to find source file for page middleware with extensions: tsx, ts, jsx, js, this can cause functions config from `vercel.json` to not be applied

But I do not have any vercel.json in my project. As project is real project I can't share directly but I can send some screenshots

Here is my folder structure-

Anyone getting same warning or if anyone has any idea, please help me. From where actually this warning coming and also vercel.json is coming.

###Update
In create a small application, I am seeing vercel is only showing warning when I add proxy.ts.

https://github.com/siamahnaf/nextjs-vercel-warning

[icyJoseph]

Hi, I found this PR in the Vercel repository, vercel/vercel#14140 - your issue should be fixed when that lands.

[mic34]

cosmetic Vercel warning caused by Next 16’s new proxy/middleware model or an unexpected file-location/name, not something magically creating a vercel.json. Vercel is warning that it couldn’t map a built “page middleware / proxy” output back to a source file with an expected extension, which means any vercel.json function mappings (if you had one) wouldn’t get applied. In practice people are seeing this when they add proxy.ts (the Next-16 replacement for middleware.ts) or when the proxy/middleware isn’t located where the build expects.

What’s actually happening

• During Vercel’s Next.js build step the platform tries to match compiled serverless function output to the original source file (so it can apply function config). If it cannot find the matching source file (by searching common extensions: tsx, ts, jsx, js) it emits that warning. It’s not evidence that a vercel.json exists — the message just says that if you had function config there, it might not be applied.
• Next 16 renamed/deprecated middleware.ts → proxy.ts and changed some expectations about location/exports. If proxy.ts is not in the expected place or has an unexpected export shape, the builder can fail to match it. People have seen related warnings/behavior (and sometimes a workaround is renaming back to middleware.ts).

Concrete things to try (ordered by easiest → more invasive)

1. Make sure proxy.ts is in the right place

   • Put the file at the project root (or src/ root) — same level as your app/ (or pages/) folder. Next expects a single proxy/middleware file at root/src. If it lives deep inside another folder it may not be discovered/matched.

2. Check the export shape

   • In Next 16, export the proxy function (recommended):

     export function proxy(request: Request) { ... }

     or export default function proxy(...) {}. If you still export middleware or use an unusual default, try renaming to proxy.

3. Try the middleware.ts fallback (diagnostic)

   • As a diagnostic: rename proxy.ts → middleware.ts and redeploy. If the warning disappears, it’s linked to the proxy rename logic/identification in the build path (some platforms/builders still have edge cases). Several users reported this reproduces or temporarily avoids issues.

4. Check pageExtensions or custom config

   • If you set custom pageExtensions in next.config.js, ensure the build still treats the proxy/middleware as a top-level special file — custom extensions can confuse file matching in some edge cases. There are historical issues where custom extensions affected middleware matching.

5. Clean caches + redeploy

   • Locally: stop dev server, delete .next, delete node_modules, reinstall, build locally: next build.
   • On Vercel: trigger a “clear build cache” deploy (the Vercel dashboard has the option). Many build-mismatch warnings go away after a fresh cache because old compiled artifacts can confuse the mapping. (Community recommendations echo this.)

6. If you need to avoid the warning for now

   • If nothing else helps and you can’t find the root cause, you can keep the proxy.ts logic but also add a middleware.ts wrapper that re-exports/forwards to the same implementation (temporary shim) so the older builder logic finds a source file it recognizes.

When this is a real problem vs cosmetic

• If you don’t have a vercel.json with function configuration, this warning is usually cosmetic — your app will still run and the proxy should execute, but Vercel is cautioning you that it couldn’t apply vercel.json function config because it couldn’t match the source file. If you do rely on function config mapped by source paths, investigate immediately.

Evidence of this being a current/known area

• Next 16’s migration notes (rename middleware → proxy) and recent community threads and Vercel posts show multiple people hitting related issues after upgrading to v16 / Turbopack. There are ongoing bug reports/threads; if the steps above don’t help it’s reasonable to open an issue (or link your small repro) with the Next/Vercel teams.