Type Alias PathsKey<D>

PathsKey: D extends { paths: infer P }
    ? P extends Record<string, unknown>
        ? string extends keyof P ? string : Extract<keyof P, string>
        : never
    : never

Literal paths keys, or never when the document does not declare a paths object. Runtime documents (whose keyof collapses to string) widen to string so callers retain prior freedom.