Creates detailed use case specification documents with actors, preconditions, main success scenarios, alternative flows, postconditions, and business rules. Use when the user asks to "write a use case", "specify a use case", "document system behavior", "define scenarios", "write a functional spec", or mentions use case specification, acceptance criteria, or user scenarios. Also trigger whenever the task is to write use case specification documents for the use cases in a use case diagram (e.g. docs/use_cases.puml) — including phrasings like "detailed use case specifications before writing any code", "one file per use case", or a request to cover the happy path, alternative flows, postconditions, and business rules for each use case.