INCIDENT RUNBOOK

Fix: Env Variable Undefined At Build in Express on Vercel

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

Automated Remediation ProtocolUpdated for 2026 Production

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● Observed Terminal Error / Stack Trace

Error: ENV_VARIABLE is not defined
    at Module._compile (internal/modules/cjs/loader.js:1158:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1178:10)
    at Module.load (internal/modules/cjs/loader.js:1002:32)
    at Function.Module._load (internal/modules/cjs/loader.js:928:14)
    at Module.require (internal/modules/cjs/loader.js:1040:19)
    at require (internal/modules/cjs/helpers.js:77:18)
    at Object.<anonymous> (/vercel/path0/server.js:10:20)
    at Module._compile (internal/modules/cjs/loader.js:1158:30)

Root Cause Analysis

The error occurs because Vercel builds the application at build time, and Express tries to access environment variables that are not yet defined. This is due to the fact that Vercel's build process does not have access to the environment variables defined in the Vercel dashboard. As a result, when the application is built, the environment variables are undefined, causing the error.

● Configuration & Code Resolution

To fix this issue, you need to define the environment variables in the `vercel.json` file. You can do this by adding the following configuration:
{
  "version": 2,
  "builds": [
    {
      "src": "server.js",
      "use": "@vercel/static-build"
    }
  ],
  "env": {
    "ENV_VARIABLE": "@env(ENV_VARIABLE)"
  }
}
You also need to define the environment variable in the Vercel dashboard.

Verification & Build Command

vercel build && vercel env add ENV_VARIABLE production