Optic

Optic treats your OpenAPI specification like source code that deserves version control. Its CLI diffs two versions of a spec with behavior-aware comparison, so it can tell you whether a change actually breaks consumers rather than just flagging textual differences. It lints specs against your team's style guide, generates a visual API changelog for pull requests, and can capture real API traffic to keep a spec in sync with the implementation. The pitch is 'git for APIs': undocumented endpoints are treated like untracked files until you document them.

The project is open source under MIT with about 1,500 stars, and version 1.0.0 removed the old cloud-connected features, making it a fully local CLI. The honest caveats: it is OpenAPI-centric, so teams without spec-first workflows get less value, and the website was unreachable from our network at research time, so verify docs availability yourself. For teams that already maintain OpenAPI specs, though, breaking-change detection in CI is a genuinely useful safety net.

1. Install the CLI: npm install -g @useoptic/optic.
2. Capture real traffic to bootstrap a spec: optic capture openapi.yaml https://your-api.
3. In CI, diff your spec against the base branch: optic diff openapi.yaml --base main --check.
4. Add style-guide rules in optic.yml and generate visual changelogs with --web for pull requests.