Once integrations become business-critical, the work is no longer just calling APIs. Reliable integration systems need boundaries, state, failure handling, idempotency, observability, operations, and auditability.
Bidirectional sync turns integration into an authority problem. Reliable systems need field ownership, conflict detection, event history, reconciliation, and explainable state.
External systems do not fail only during outages. They drift, evolve, and quietly violate the assumptions your product has turned into invariants.
API documentation gets you to the first request. Real integration engineering starts when your system has to own the boundary between external behavior and internal meaning.