You are shipping a commitment
Once someone integrates, your API is infrastructure to them, and changing it becomes their emergency. That is the fundamental difference between an internal interface and a published one, and it should change how much care goes into the first version.
The practical consequence: design for the version you will still be supporting in three years, not the one that ships fastest this quarter.
Model the domain, not your database
APIs that mirror internal table structure leak decisions consumers should never have known about and become impossible to refactor behind. Expose the concepts your business actually has — orders, accounts, bookings — and keep the storage layout a private detail you can change.
Make errors part of the contract
Consumers spend far more time handling failure than success. Stable machine-readable error codes, a clear distinction between retryable and permanent failures, and enough detail to fix the request are documentation-grade features, not an afterthought.
The quality of an API is decided by how it behaves when things go wrong.
Version before you need to
Add the version to the path or header on day one, even with a single version, because retrofitting versioning to a live integration is a coordinated migration with every consumer you have. Additive changes should never require a bump; anything that removes or reshapes a field always should.
Design pagination and limits early
Collections grow, and an endpoint that returns everything works fine until the customer with fifty thousand records arrives. Cursor pagination, explicit rate limits, and documented maximums are far easier to introduce before anyone depends on their absence.
Write the client first
Build a real integration against your own API before publishing it. Every awkward sequence, every field that has to be fetched twice, and every ambiguous status shows up within an hour of actually using it — and can still be changed for free at that point.
This is also the fastest way to produce honest documentation, because the examples come from something that genuinely ran.