A great API is invisible; it does exactly what the consumer expects without them having to constantly reference the documentation. The foundation of excellent API design is strict consistency. Whether you are using REST or GraphQL, naming conventions, pagination strategies, and response structures must remain uniform across all endpoints.
Error handling is where many APIs fail their consumers. Returning a generic 500 Internal Server Error is useless. A pragmatic API returns appropriate HTTP status codes accompanied by a structured JSON payload containing a machine-readable error code, a human-readable message, and ideally, a link to the relevant documentation to resolve the issue.
Versioning strategy should be decided before the first endpoint goes live. URL versioning (e.g., /v1/users) is the most straightforward and cache-friendly approach. The cardinal rule of API development is to never introduce a breaking change to an existing version. If a change modifies the contract, it requires a new version, allowing consumers to migrate at their own pace.