Fix: Environment Variable 'Undefined' on Vercel (Next.js Guide)

url: "https://www.flowql.com" datePublished: "2025-12-28" dateModified: "2025-12-28" mainEntity: type: "FAQPage" questions:

• question: "Why is my environment variable undefined on Vercel?" answer: "Variables are usually undefined because they are missing the NEXT_PUBLIC_ prefix (required for client-side access), they haven't been added to the Vercel Dashboard, or the project hasn't been redeployed after the variables were added."
• question: "Do I need to redeploy after changing Vercel env vars?" answer: "Yes. Environment variables are injected into your application during the build process (for client-side vars) or at the start of the deployment (for server-side vars). Changing them in the dashboard does not update existing deployments; you must trigger a new build."

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It works perfectly on your machine. You check your .env.local file, and your API keys are all there. But the moment you deploy to Vercel, everything breaks. Your logs are filled with "TypeError: Cannot read properties of undefined" or simple "Variable not found" errors.

This is the "Secret Gap"—the mismatch between your local development environment and the Vercel production environment. For "Vibe Coders" who rely on AI to generate their deployment scripts and configuration, this is one of the most common causes of a "failed vibe."

The issue is rarely that Vercel is "broken." It's usually a misunderstanding of how and when environment variables are injected into your application. In Next.js specifically, the distinction between Build-Time and Runtime variables is the difference between a successful launch and a broken production site.

In this guide, we’ll show you how to audit your environment variables, master the NEXT_PUBLIC_ prefix, and use the Vercel CLI to ensure your secrets are perfectly synced between your computer and the cloud. If your build is failing entirely, check out our guide on Vercel build error exit code 1.

Build-Time vs. Runtime: When Variables are Injected

To fix "Undefined" variables, you first have to understand the Next.js build pipeline.

Build-Time Variables (The "Static" Trap)

Next.js is a framework that prioritizes speed. To achieve this, it "pre-renders" as much as possible during the Build Phase on Vercel.

If your code uses an environment variable inside a getStaticProps function or a Server Component that is statically generated, Next.js reads that variable while it is building your app on Vercel's servers. It then bakes that value directly into the static HTML.

The Problem: If you add the variable to the Vercel Dashboard after the build has already finished, the static HTML is already "baked" with the old (or undefined) value.

Runtime Variables (Serverless Functions)

Variables used in API routes or getServerSideProps are "Runtime" variables. These are injected every time a Vercel Serverless Function "wakes up" to handle a request. These are more flexible, but they still require a fresh deployment to be recognized by Vercel's infrastructure.

NEXT_PUBLIC_ Prefix: The Client-Side Gatekeeper

By default, environment variables in Next.js are Private. They are only accessible in the Node.js environment (the server). This is a security feature to prevent your sensitive API keys from being leaked to the browser.

If you try to access process.env.MY_SECRET inside a Client Component (anything with 'use client'), Next.js will return undefined to protect you.

The Fix: Public Variables

If you need a variable to be available in the browser (like a public Firebase key or a Posthog ID), you must prefix it with NEXT_PUBLIC_.

# ❌ Invisible to the browser
ANALYTICS_ID=12345

# ✅ Visible to the browser
NEXT_PUBLIC_ANALYTICS_ID=12345

Warning: Never put sensitive secrets (like database passwords or Stripe Secret Keys) into a NEXT_PUBLIC_ variable. Anyone who visits your site can see these values in the JavaScript source code.

Common Mistake 1: Forgetting to Trigger a Redeploy

This is the most common "Facepalm" moment in Vercel deployments.

1. You realize a variable is missing.
2. You go to the Vercel Dashboard -> Settings -> Environment Variables.
3. You add the variable and hit "Save."
4. You refresh your production site... and it's still undefined.

Why? Vercel does not automatically update existing deployments when you change settings. You must Redeploy the project to inject the new values into the build.

The Fix: Go to the "Deployments" tab in Vercel, click the three dots next to your latest deployment, and select "Redeploy."

Common Mistake 2: Scoping Variables to the Wrong Environment

Vercel allows you to set different variables for different environments:

• Production: Your live site.
• Preview: Your pull request branches.
• Development: Your local machine (via Vercel CLI).

If you add a variable but only check the "Development" box, it will be missing when your code merges to the main branch.

The Fix: Ensure all three boxes (Production, Preview, Development) are checked unless you specifically want different values for each.

Debugging UNDEFINED: Logging Safely in Production

When a variable is undefined, don't just guess. Add a "Diagnostic Log" to your server-side code (e.g., in a middleware.ts or a Server Component).

// ❌ DANGEROUS: Logging secrets can leak them to logs
console.log("My Key:", process.env.MY_SECRET);

// ✅ SAFER: Just check if it exists
console.log("Is My Key defined?", !!process.env.MY_SECRET);

Check the Logs tab in your Vercel project dashboard to see these outputs. If the log says false, the variable truly isn't being injected.

Using the Vercel CLI to Pull and Push Secrets

The best way to avoid the "Secret Gap" is to use the Vercel CLI to keep your local .env.local in sync with the cloud.

1. Link your project

vercel link

2. Pull the latest variables

vercel env pull .env.local This command downloads the variables you set in the Vercel dashboard and writes them to your local file. This is much safer than copy-pasting values manually.

3. Push local changes (optional)

vercel env add Use this to add a new variable directly from your terminal to the Vercel dashboard.

FlowQL: Bridging the Gap Between Local Dev and Production Reality

Environment variables are the "plumbing" of your application. When they leak or get blocked, the whole system grinds to a halt. AI assistants are great at telling you how to use a variable in code, but they are blind to the Vercel Dashboard settings and the build-time injection logic.

At FlowQL, we help developers solve these environment-specific bugs. When your "works on local" app is failing in production and you've already checked your NEXT_PUBLIC_ prefixes ten times, we provide the expert human-in-the-loop debugging to find the specific configuration mismatch in your Vercel setup.

Conclusion

"Undefined" variables on Vercel are rarely a mystery—they are almost always a result of missing prefixes, missing redeploys, or incorrect environment scoping. By mastering the Vercel CLI and understanding the Next.js build lifecycle, you can ensure your app has the "fuel" it needs to run in the cloud.

Don't let environment bugs block your launch. [Book a session with FlowQL] and let’s get your production secrets synced today.

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FAQ: Vercel Environment Variables

Q: Why is my environment variable undefined on Vercel? A: Usually because it's missing the NEXT_PUBLIC_ prefix (if used in client code), it hasn't been added to the dashboard, or you haven't redeployed since adding it.

Q: Can I change environment variables without redeploying? A: No. Environment variables are injected at build time or at the start of a deployment's lifecycle. You must trigger a new deployment to apply changes.

Q: Is it safe to use NEXT_PUBLIC_ for API keys? A: Only if the API key is intended to be public (like a Mapbox token or a Posthog ID). If the key allows sensitive actions (like deleting data or spending money), never use the NEXT_PUBLIC_ prefix.