Designing APIs That Developers Love

APIs Are User Interfaces

An API is a user interface for developers. The same principles that make a good GUI — consistency, discoverability, clear feedback — apply to API design.

Be Consistent Above All

Consistency trumps cleverness. If your API uses camelCase for one resource and snake_case for another, developers will waste time checking documentation for every field. Pick conventions and stick to them:

• Naming: consistent casing for fields, endpoints, and query parameters.
• Pagination: the same mechanism across all list endpoints.
• Error format: the same structure for every error response.
• Authentication: one scheme for the entire API.

Use HTTP Semantics Correctly

HTTP has well-defined methods, status codes, and headers. Use them:

• GET for reading. Never use GET for mutations.
• POST for creating new resources.
• PUT for full replacement, PATCH for partial updates.
• 204 for successful operations with no response body.
• 404 when a resource does not exist, 409 for conflicts, 422 for validation errors.

Returning 200 for every response and encoding the real status in the body defeats the purpose of HTTP.

Error Responses Should Help

A good error response tells the developer exactly what went wrong and how to fix it:

{
  "error": "validation_error",
  "message": "The 'email' field must be a valid email address.",
  "field": "email"
}

A bad error response says "Something went wrong" and leaves the developer guessing.

Version From Day One

You will need to make breaking changes eventually. Design for it from the start. URL versioning (/v1/) or header-based versioning both work — just be consistent and communicate deprecation timelines clearly.

Document With Examples

The most useful API documentation includes complete request and response examples for every endpoint. Developers copy-paste first and read prose second. Make the happy path obvious.