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FastAPI route works locally on / but fails on Vercel

31b5713b-97f8-409b-8da8-75e26b2a544b

FastAPI route works locally on / but fails on Vercel

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Updated4/30/2026

PageRank0.0004

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RELATES_TOSolution

If you have the vercel.json configuration file in your application, remove it. Tension: It is mostly likely the cause. Outcome: remove it.

RELATES_TORootCause

An outdated build tooling dependency (the Rollup bundler) caused incorrect output for the React Router nested routing behavior on Vercel. The issue was not resolved by the Vercel rewrite alone because the bundle itself was impacted.

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MANIFESTS_ASSymptom

HTTP 404 Not Found

MANIFESTS_ASSymptom

HTTP 500 Internal Server Error

MANIFESTS_ASSymptom

MANIFESTS_ASSymptom

HTTP 405 Method Not Allowed

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ESCALATES_TOProblem

This happens in my `/api/stripe-connect/route.ts` file when it tries to read a Firestore document. — Firebase Admin SDK db object is invalid in Vercel API Route. Outcome: the error is triggered by this line.

ESCALATES_TOProblem

Expecting not to go to homepage and go to searchparams assigned url. Outcome: check them on Project Settings > Environment Variables.

ESCALATES_TOProblem

I have a route `/property/:id` that includes a button which, when clicked, should redirect to `/profile/:id`. Tension: It only seems to fail when trying to redirect from `/property/:id` to `/profile/:id`. Outcome: in console it shows response 404.

INSTANCE_OFClusterConcept

Route Params Not Reactive

No Route Redirect Failure

Unmatched Route Redirect Fix

Missing Route Redirect Need

Route/Stream Visibility Failure

Azure Routing/Response Mismatch

Router Missing Route

FastAPI Dependency Currying

Deployment Route Mismatch

CAUSED_BYRootCause

FastAPI's RedirectSlashes middleware redirects /api/users to /api/users/ locally, but Vercel's proxy or edge stack handles the trailing slash differently and may strip or loop the redirect

CAUSED_BYRootCause

the route name in the code was `sign-in`, but on GitHub, I had accidentally pushed it as `Sign-in` — Since Next.js is case-sensitive with routes. Tension: this mismatch caused the deployed version on Vercel to fail. Outcome: After changing the route name to all lowercase (`sign-in`) in the main branch and redeploying, everything started working fine.

CAUSED_BYRootCause

Vercel has already inspected your `dist` directory and decided that `functions/index.js` is a static asset before the rewrites are even considered. Tension: Vercel has already inspected your `dist` directory and decided that `functions/index.js` is a static asset before the rewrites are even considered.

CAUSED_BYRootCause

I had two directories called `api`, one at the root level and the other inside `pages` directory. Tension: Vercel is unable to find the API route. Outcome: Just renamed from `api` to `api_src` and it fixed it.

CAUSED_BYRootCause

the environment is reusing the same context — Vercel's serverless environment. Tension: even though you're calling new Date() inside your handler. Outcome: other parts of your code might be cached.

CAUSED_BYRootCause

the problem may be with how Next.js's image optimization pipeline interacts with Firebase-hosted images — If decoding the URL doesn't fix the issue. Tension: the problem may be with how Next.js's image optimization pipeline interacts with Firebase-hosted images. Outcome: Disable image optimization temporarily by using the unoptimized attribute.

CAUSED_BYRootCause

vercel not found — I’ve also tried running the which vercel command. Tension: but it returns nothing. Outcome: This is specific to Node installation.

CAUSED_BYRootCause

If I remove the cookies, it works fine, it shows correctly the username value — The route path must be the following: /wallet/[username]/sell. Tension: In my dev environment everything works correctly but when I deploy to Vercel, I cannot access to this route. Outcome: I think problem is related to the use of cookies.

CAUSED_BYRootCause

You hardcoded the href values in your /src/Components/Nav.tsx component. — click any link in the navbar. Tension: All those links where supposed to redirect the person from https://oscar-rodriguez.vercel.app/ to https://oscar-rodriguez.vercel.app/inProcess.html. Outcome: use a relative path like this:.

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