fix(core): avoid mutating runtimeEnv when emptyStringAsUndefined is true

[FrancoKaddour]

When emptyStringAsUndefined: true is set, the current implementation iterates over runtimeEnv and calls delete runtimeEnv[key] for every empty-string entry. If the caller passes process.env (the most common pattern), this permanently removes those keys from the global process.env object for the lifetime of the process.

Reproduction

process.env.DATABASE_URL = "";

createEnv({
  server: { DATABASE_URL: z.string().optional() },
  runtimeEnv: process.env,
  emptyStringAsUndefined: true,
});

console.log("DATABASE_URL" in process.env); // false — key was deleted

Any code that reads process.env.DATABASE_URL after createEnv (e.g. Prisma, a logger, another createEnv call) sees undefined instead of "". The effect is silent and hard to debug.

Fix

Build a filtered copy of the env object instead of mutating it in place:

// before
const runtimeEnv = opts.runtimeEnvStrict ?? opts.runtimeEnv ?? process.env;
if (emptyStringAsUndefined) {
  for (const [key, value] of Object.entries(runtimeEnv)) {
    if (value === "") {
      delete runtimeEnv[key];
    }
  }
}

// after
const rawRuntimeEnv = opts.runtimeEnvStrict ?? opts.runtimeEnv ?? process.env;
const runtimeEnv = emptyStringAsUndefined
  ? (Object.fromEntries(
      Object.entries(rawRuntimeEnv).filter(([, v]) => v !== ""),
    ) as typeof rawRuntimeEnv)
  : rawRuntimeEnv;

The original runtimeEnv / process.env reference is never touched.

Tests added (in packages/core/test/smoke-zod4.test.ts):

• Verifies the passed object is not mutated after createEnv
• Verifies empty strings are still treated as undefined in validation (required field throws)
• Verifies schema defaults are applied when the env var is an empty string

[vercel]

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