APIs.guru vs Kroki

APIs.guru maintains the largest searchable directory of OpenAPI/Swagger definitions for public APIs, normalizing specs pulled from thousands of providers into one browsable catalogue. Its own machine-readable endpoint (api.apis.guru/v2) needs no key or account and returns the full listing plus individual OpenAPI documents as JSON or YAML. The project is open-source and entirely free, run as a community effort rather than a paid product.

Kroki puts a single unified HTTP API in front of two dozen open-source diagram-as-code renderers, including PlantUML, Mermaid, GraphViz, D2, BPMN, Excalidraw, and WaveDrom, so callers send diagram source text and get back an image with no local toolchain to install. A GET request embeds the (deflate+base64-encoded) diagram directly in the URL, making it trivial to drop live diagrams straight into Markdown or HTML. The project and its public instance at kroki.io are free and open source, with self-hosting via Docker documented for anyone who needs a private instance.