API Diff

API Diff is a web-based tool that lets you compare two versions of an API specification. You upload your files—or provide URLs to them—and it shows you exactly what changed between versions. It supports OpenAPI and AsyncAPI formats, which are the standard ways to describe RESTful and event-driven APIs. The tool highlights modifications in endpoints, parameters, schemas, and more, clearly marking which changes are 'breaking' and could break existing client integrations. This makes it a straightforward utility for anyone managing API evolution.

It works by parsing the structured API definition files you provide. You can drag and drop files, paste URLs, or use the provided command-line tool for local development workflows. For teams, the integrated GitHub Action can automatically run a diff whenever a pull request is opened and post the results as a comment. This automation helps catch problematic changes early in the development cycle. The interface is minimal and focused on the diff output, presenting changes in a clear, side-by-side format.

This tool is most useful for developers and technical writers who need to track and communicate API changes. A common use case is reviewing a pull request that updates an OpenAPI file; running a diff shows the impact at a glance. Product managers can use the output to understand the scope of a new API version. By integrating the diff check into CI/CD pipelines, teams can enforce rules against introducing breaking changes without proper review, improving API stability for consumers.