INCIDENT RUNBOOK

Fix: Env Variable Undefined At Build in Next.js on Vercel

Comprehensive architectural post-mortem and verified configuration patch for next.js deployed in vercel production environments.

Automated Remediation ProtocolUpdated for 2026 Production

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● Observed Terminal Error / Stack Trace

Error: `process.env.MY_VARIABLE` is not defined
    at /vercel/path/to/nextjs-app/pages/_app.js:10:24
    at Module._compile (internal/modules/cjs/loader.js:1085:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1114:10)
    at Module.load (internal/modules/cjs/loader.js:950:32)

Root Cause Analysis

The error occurs due to a mismatch in the lifecycle of environment variable availability during the build process on Vercel. Next.js applications on Vercel are built using the `vercel build` command, which may not have access to environment variables defined in the Vercel dashboard or in a `.env` file. This is because Vercel's build process happens before the runtime environment is fully set up, leading to undefined environment variables during build time.

● Configuration & Code Resolution

To fix this issue, you can define environment variables in a `vercel.json` file or use Vercel's built-in environment variable configuration. For example, add the following to your `vercel.json`:
json
{
  "version": 2,
  "env": {
    "MY_VARIABLE": "@my-variable"
  }
}
Alternatively, you can also define the variable directly in your Next.js code using a default value or by checking if the variable is defined before using it:
--- a/pages/_app.js
+++ b/pages/_app.js
@@ -7,7 +7,11 @@
 import Head from 'next/head';
 
 function MyApp({ Component, pageProps }) {
-  const myVariable = process.env.MY_VARIABLE;
+  const myVariable = process.env.MY_VARIABLE || 'default-value';
   //...
 }
Make sure to replace `MY_VARIABLE` with your actual environment variable name.

Verification & Build Command

Run `vercel build` and `vercel dev` to verify that the environment variable is correctly set and accessible during the build and runtime. You can also check the Vercel dashboard logs for any errors related to environment variables.