3. Exhaustive Checks with `never`

Assign the switch default to a never-typed variable. If a new union member is added, the compiler raises an error here.

File: packages/ai/src/providers/google-shared.ts L307-310

default: {
  const _exhaustive: never = reason;
  throw new Error(`Unhandled stop reason: ${_exhaustive}`);
}

File: packages/coding-agent/src/core/messages.ts L188-191

default:
  const _exhaustiveCheck: never = m;
  return undefined;

This pattern guarantees at compile time that every branch of a discriminated union is handled.