APIs are the contracts that hold a system together. The best ones survive decades of change because they got a few fundamentals right, not because they chased the latest style.
Be Predictable Before Being Clever
Consistent naming, consistent paginations, consistent error shapes. A developer who has read one of your endpoints should be able to guess the rest.
Make Versioning a First-Class Citizen
You will need to evolve. Decide your versioning strategy on day one, even if your first version never needs it.
Errors Are Part of the API
A bad error is a bug. Return a stable code, a human-readable message, and enough context for the caller to recover or escalate.
Idempotency Is a Gift
Network failures are the rule, not the exception. Idempotent write endpoints turn retries from a debate into a checkbox.
Conclusion
Good APIs feel inevitable in hindsight. Get the boring parts right and the rest of the system inherits that calm.