Designing APIs that survive contact with real integration partners

APIs are usually designed and tested against a well-behaved internal client. The first real integration partner rarely behaves that way, and that gap is where most integration problems show up.

Versioning is the most common gap. If there's no explicit versioning strategy from the first release, any change to a field's meaning or a response's shape becomes a breaking change for whoever is already depending on it.

Error responses matter more than they're usually given credit for. A partner's engineering team needs enough information in an error response to know whether to retry, fix their request, or escalate, not just a generic 500 status.

Rate limiting and pagination decisions made for internal use often don't hold up under a partner's actual usage pattern, particularly batch jobs that call an API far more aggressively than a human-driven client would.

The practical fix is to treat the API contract as a product in its own right: documented, versioned, and tested against the kind of client behavior you can't fully control, not just the behavior you expect.