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What is the difference between build-time and runtime environment variables in Next.js?

Q: What is the difference between build-time and runtime environment variables in Next.js?

**TL;DR.** NEXT_PUBLIC_ vars are **inlined at build time**, so changing them requires a **rebuild**. Server-only vars read from process.env at **runtime** can change per environment **without rebuilding** (for dynamically-rendered code). This matters for **Docker images promoted across environments**. build-time NEXT_PUBLIC_* inlined baked into bundle change → rebuild runtime</te

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Understand the problem

Inlined-at-build vs read-at-request values.

nextjsenv-varsbuild-timeruntime

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Read the code

Runtime server var vs build-time public var

// Runtime (server, dynamic route) — same build, different envs:
export const dynamic = 'force-dynamic';
export default async function Page() {
  const region = process.env.REGION;        // read at runtime
  return <p>{region}</p>;
}

// Build-time (client) — inlined; rebuild to change:
// <span>{process.env.NEXT_PUBLIC_VERSION}</span>

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