This is @t3-oss/env-core but if you don't want to send zod to client bundle (be careful, it's for Astro, not Next.js) https://twitter.com/JLarky/status/1653595995744395264

01_t3Env.ts

// src/t3Env.ts
import { createEnv, LooseOptions, StrictOptions } from '@t3-oss/env-core';
import { z, ZodType, ZodString } from 'zod';

function validateEnv<
	TPrefix extends string,
	TServer extends Record<string, ZodType> = NonNullable<unknown>,
	TClient extends Record<string, ZodType> = NonNullable<unknown>
>(opts: LooseOptions<TPrefix, TServer, TClient> | StrictOptions<TPrefix, TServer, TClient>) {
	return opts;
}

const x = validateEnv({
	clientPrefix: 'PUBLIC_',
	server: {
		DATABASE_URL: z.string().url(),
		OPEN_AI_API_KEY: z.string().min(1),
	},
	client: {
		// PUBLIC_AUTH_URL_WRONG: z.number().min(1),
		PUBLIC_AUTH_URL: z.string().min(1),
	},
	runtimeEnv: import.meta.env,
	skipValidation:
		!!import.meta.env.SKIP_ENV_VALIDATION &&
		import.meta.env.SKIP_ENV_VALIDATION !== 'false' &&
		import.meta.env.SKIP_ENV_VALIDATION !== '0',
});

// this function would be dead code eliminated
const client = () => {
	// add check that values are strings
	const v = (_: ZodString[]) => {};
	// this line will error if the client env is not a string
	v(Object.values(x.client));
	// generate type just for the client
	return z.object(x.client);
};
export type ClientEnv = z.infer<ReturnType<typeof client>>;

// export env for server code
export const serverEnv = createEnv(x);

02_config.ts

// src/config.ts
import type { ClientEnv } from './t3Env';

// use clientEnv for client-only code so that you don't ship zod dependency to the client
export const env = import.meta.env as unknown as ClientEnv;

03_client.tsx

// src/components/example.tsx
import { env } from '../config';

console.log(env.PUBLIC_AUTH_URL);

04_server.ts

// src/pages/example.ts
import { serverEnv } from '../t3Env';

console.log(serverEnv.OPEN_AI_API_KEY);