Why

loadConfig is async (it uses dynamic import(), which is irreducibly asynchronous). That's fine in app code, but it breaks in config files that tooling loads synchronously — files pulled in via require() or bundled to CJS, where a top-level await is rejected (ERR_REQUIRE_ASYNC_MODULE, or the esbuild "top-level await is not supported with the cjs output format"). There's no runtime-agnostic way to make loadConfig sync (a sync module load means require(), which only exists on CJS-capable runtimes and can't load .ts without a transpile hook).

What

selectConfig(configs) — a synchronous, runtime-agnostic counterpart. Pair it with static imports so the runtime/bundler resolves and transpiles each config at parse time; it picks the entry matching envName() and throws when there's none (the map is explicit, so a miss is almost always a typo).

import { defineEnv, selectConfig } from "@vlandoss/env";
import development from "./config/development.ts";
import production from "./config/production.ts";

export const env = defineEnv({
  schema: Env,
  config: selectConfig({ development, production }),
});

Changes

• core: selectConfig (package/src/lib/select-config.ts) + export from the main entry.
• tests: runtime tests (select-config.test.ts) + type tests in lib.test-d.ts (return type is T, never T | undefined; pipes into defineEnv).
• example: examples/config-cjs/ (pinned to Node 22.22.3) — a node:test that require()s both a selectConfig config (loads ✓) and a loadConfig + top-level await config (throws ERR_REQUIRE_ASYNC_MODULE ✓). Registered in the root config_roots.
• docs: new section in the loadConfig guide + a row in the core API reference + example table.
• changeset: minor.

Node pin rationale: require(esm) is default from 22.12 and native TS stripping from 22.18, so 22.22.3 is the lowest LTS line where the reproduction (and fix) behave as described. Node 20 would throw ERR_REQUIRE_ESM — a different failure.

Verification

• Package suite: 87 tests pass (node + browser projects).
• examples/config-cjs test:e2e: both assertions pass.
• rr check (package + docsite) + example check: clean.

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