How to build an API: A complete step-by-step guide for April 2026
Blog post from Fern
Creating an API requires careful planning and design, treating it as an essential product that others will depend on. Before writing code, developers should engage in spec-first planning to define resources and operations, ensuring consistency and predictability in endpoint naming and versioning to prevent integration issues. REST remains a dominant protocol, with API keys being the most practical authentication method for automated clients like AI agents. Comprehensive testing is crucial, involving unit, integration, and contract tests to maintain API reliability and prevent breaking changes. Documentation should be automatically generated from the API spec to avoid drift and ensure clarity for both human developers and AI agents. Fern offers a solution by generating type-safe SDKs and interactive documentation from a single API definition, streamlining production and maintenance processes while improving the scalability and reliability of APIs.
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