There should be enough detail in the spec that it's pretty obvious. And unlike - many - human devs, agents don't shy away from writing tests. Add a coverage guardrail and tell them to use red/green TDD and you're already a good way there. The main thing is to ensure shared understanding, so they also know business expectations and write appropriate tests. And... nothing here is really specific to an agent, as a human dev also needs sufficient guidance (tailored to their experience level).