Loading options — .env paths, variable expansion, validation schema.

Forwarded to the loader and carried into the application's own config arm, so one options object governs both.

The environment to read configuration from, instead of the platform's.

Required on Cloudflare Workers, and the only way this factory works there: Workers bindings arrive as the env argument of the fetch handler, never as a process-wide global, so runtime services built outside a request report an EMPTY environment. Without this the application composes from no configuration at all — and a resolver calling getOrThrow then fails on the first request and, because the boot promise is memoised, on every request after it.

Omit it on Node, Deno, and Bun: those expose the environment process-wide, and the detected runtime already reads it.

Non-string values are ignored, so a Workers env carrying KV, D1, or R2 bindings alongside its string variables can be passed verbatim.

Explicitly | undefined under exactOptionalPropertyTypes: callers forward an optional binding straight through ({ env } where env may be undefined), which would not otherwise type-check against an optional property. The generated Workers entry does exactly that on all four targets.