Validate env in next.js https://github.com/t3-oss/create-t3-app/tree/next/cli/template/base https://create.t3.gg/en/usage/env-variables

next.config.mjs

// @ts-check
/**
 * Run `build` or `dev` with `SKIP_ENV_VALIDATION` to skip env validation.
 * This is especially useful for Docker builds.
 */
!process.env.SKIP_ENV_VALIDATION && (await import("./src/env/server.mjs"));

/** @type {import("next").NextConfig} */
const config = {
  reactStrictMode: true,
  swcMinify: true,
  i18n: {
    locales: ["en"],
    defaultLocale: "en",
  },
};
export default config;

other-files.md

https://github.com/t3-oss/create-t3-app/tree/next/cli/template/base/src/env

server.mjs

// @ts-check
/**
 * This file is included in `/next.config.mjs` which ensures the app isn't built with invalid env vars.
 * It has to be a `.mjs`-file to be imported there.
 */
import { serverSchema } from "./schema.mjs";
import { env as clientEnv, formatErrors } from "./client.mjs";

const _serverEnv = serverSchema.safeParse(process.env);

if (!_serverEnv.success) {
  console.error(
    "❌ Invalid environment variables:\n",
    ...formatErrors(_serverEnv.error.format()),
  );
  throw new Error("Invalid environment variables");
}

for (let key of Object.keys(_serverEnv.data)) {
  if (key.startsWith("NEXT_PUBLIC_")) {
    console.warn("❌ You are exposing a server-side env-variable:", key);

    throw new Error("You are exposing a server-side env-variable");
  }
}

export const env = { ..._serverEnv.data, ...clientEnv };