v0 environment variables undefined: API routes 500 in production but work in preview

v0 by Vercel Easy 10-20 minutes Last verified Jul 17, 2026

The fix Check the connected Vercel project, not just v0: Vercel dashboard > Project > Settings > Environment Variables, and confirm each variable exists for Production. Then redeploy, because Next.js injects env vars at build time and existing deployments never pick up new values. Browser code only sees NEXT_PUBLIC_ variables. If vars vanish from v0's settings on reload, that is a known platform bug.

Symptoms

The app works in the v0 preview, but the deployed version's API routes return 500
process.env.MY_KEY comes back undefined in production
NEXT_PUBLIC_ variables are undefined in client code
Env vars you add in v0 Project Settings vanish as soon as you reload the page

Why it happens

Confirmed v0 platform bugs

There have been incidents where environment variables disappeared from project settings for all users. The v0 team acknowledged and fixed one such incident. If your vars vanish on reload, this is the likely culprit, not your config.

Vars not on the connected Vercel project, or added after the last deploy

Variables set inside v0 are not guaranteed to exist on the Vercel project that actually serves your app. And because Next.js injects env vars at build time, a deployment built before you added a variable never sees it.

Missing NEXT_PUBLIC_ prefix for client code

Client-side code can only read variables prefixed NEXT_PUBLIC_, and those are inlined into the bundle at build time. Everything else is server-only by design.

Localhost or placeholder values carried into production

A variable that exists but still points at localhost or holds a placeholder value fails just as hard as a missing one, usually as a 500 from the first API route that uses it.

How to fix it

Verify the variables on the Vercel project itselfOpen the Vercel dashboard > Project > Settings > Environment Variables. Confirm each variable exists for the Production environment, not just inside v0's settings UI. The two are not guaranteed to be in sync.
Redeploy after any changeValues are injected at build time, so an already-built deployment keeps returning undefined until you rebuild. Trigger a fresh deploy after every variable change, no exceptions.
Prefix browser-read variables with NEXT_PUBLIC_Anything read in client code must use the NEXT_PUBLIC_ prefix, and those values are inlined at build time. Keep secrets un-prefixed and read them only in API routes and server components.
Read the runtime logs for the real errorVercel dashboard > Project > Logs shows the actual stack trace behind the 500. It is usually an undefined key crashing an SDK constructor on the first request.
Recognize the disappearing-vars platform bugIf variables vanish from v0's settings UI when you reload the page, it is a platform bug, not your config. Check the v0 community forum, wait for the fix, and re-add the variables afterwards.

How to prevent it

Manage env vars on the Vercel project directly; treat v0's settings UI as a convenience, not the source of truth.
Make redeploying a reflex after any environment variable change.
Use the NEXT_PUBLIC_ prefix only for values that are genuinely safe to be public, because they ship in the bundle.
Keep a .env.example listing every variable the app needs, so a missing one is obvious before you deploy.

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Frequently asked questions

Why do my v0 environment variables work in preview but not in production?

The v0 preview runs in v0's own environment, where the variables you entered are present. The deployed app runs on the connected Vercel project, which may not have those variables at all, or received them after the last build. Confirm each variable exists in the Vercel dashboard under Settings > Environment Variables for Production, then redeploy.

Do I need to redeploy after adding an environment variable in Vercel?

Yes. Next.js injects environment variables at build time, so a deployment built before you added the variable will keep returning undefined forever. Add or change the variable, then trigger a new deploy so the value gets baked into the build.

Why is my NEXT_PUBLIC_ variable undefined in the browser?

NEXT_PUBLIC_ variables are inlined into the JavaScript bundle at build time, not read at runtime. If the variable was added after the last build, or the prefix is missing or misspelled, client code sees undefined. Fix the name, confirm it exists on the Vercel project, and redeploy.

My environment variables keep disappearing from v0's settings. Is that me?

No. There have been confirmed v0 platform bugs where environment variables disappeared from project settings for all users, and the v0 team acknowledged and fixed one such incident. Check the v0 community forum for an active thread, wait for the fix, and re-add your variables afterwards.

Sources

The Vibe Oops briefing

One email when something ships to production that should not have. New incidents, new error guides, no filler.