Split a benefits API integration with unreliable docs

You are connecting your company's HR system, written in Java, to a benefits provider's API. The work covers auth, mapping employee data to the provider's format, a nightly sync and reconciling errors. The provider's API docs are incomplete and, in places, wrong. You want an agent to do as much as makes sense.

How do you break the work into agent-sized steps with checkpoints, and which step is least suited to an agent?