OpenAPI (formerly Swagger) is a language-agnostic, machine-readable specification standard for describing RESTful HTTP APIs. The specification allows both humans and computers to understand API capabilities without accessing source code or documentation through network inspection. OpenAPI descriptions can be written in YAML or JSON format, with YAML being more popular for manual authoring due to readability and comment support, while JSON offers stricter syntax and faster parsing for automation. OpenAPI 3.1 achieved full compatibility with JSON Schema Draft 2020-12, making it the most expressive version yet. A robust OpenAPI document serves as a single source of truth for API design, documentation generation, mock server creation, client SDK generation, server stub generation, and automated validation — streamlining the entire API development lifecycle.

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