API design best practices

An API is a promise. Every client that integrates with it depends on that promise holding, which is why a well-designed API feels boring — predictable, consistent, and free of surprises. The APIs that cause pain are the ones that were designed one endpoint at a time, each with its own conventions. Here are the principles that keep an API pleasant to use and cheap to maintain for years.

Be relentlessly consistent

Consistency beats cleverness. Pick conventions once and apply them everywhere:

• Use nouns for resources, not verbs: GET /invoices/42\, not GET /getInvoice?id=42\.
• Use HTTP methods for their meaning — GET\ reads, POST\ creates, PATCH\ partially updates, DELETE\ removes.
• Pick one casing (snake_case\ or camelCase\) for JSON fields and never mix.
• Return the same shape for the same concept across every endpoint.

A developer who learns one of your endpoints should be able to guess the rest. That's the whole game.

Design errors as a first-class feature

Most APIs nail the happy path and neglect failures — which is exactly where integrators spend their debugging time. Return correct HTTP status codes (400\ for bad input, 401\ vs 403\ for auth, 404\, 409\ for conflicts, 422\ for validation) and a structured, machine-readable error body:

\
{ "error": { "code": "invalid_email", "message": "Email address is not valid", "field": "email" } }
\\

A stable code\ clients can branch on matters more than a pretty message. Never leak stack traces to callers.

Paginate and filter from day one

Any endpoint that returns a list will eventually return too many. Build pagination in before that happens. Cursor-based pagination is more robust than offset-based for large or frequently changing datasets, since it doesn't skip or duplicate rows when data shifts. Offer sensible filtering and sorting as query parameters, and always cap page size so one caller can't ask for everything at once.

Version deliberately

You will need to make breaking changes. Plan for it with a clear versioning strategy — a URL prefix like /v1/\ is simple and explicit. The rule that keeps clients calm: never break an existing version. Add fields, don't remove or repurpose them. When you must break, ship a new version and give integrators a real deprecation window.

Secure and rate-limit by default

Authenticate every non-public endpoint with a standard mechanism — OAuth or signed tokens — and never invent your own crypto. Return 429\ with a Retry-After\ header when clients exceed limits, so they can back off gracefully instead of hammering you. Validate and sanitize every input; treat all client data as hostile until proven otherwise.

Document as you build

An undocumented API doesn't really exist. Maintain an OpenAPI spec alongside the code so documentation, client SDKs, and request validation can all be generated from one source of truth. Include real request and response examples — developers copy examples far more than they read prose.

The payoff

Do these consistently and your API becomes an asset: partners integrate quickly, your support load drops, and you can evolve the product without breaking everyone downstream. Cut corners and every new integration re-teaches the same lessons at your customers' expense.

If you're designing an API you'll have to live with for years, talk to us.

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Originally published on the Doktouri Agency blog. We build web, mobile, SaaS, and AI products — let's talk.