API Contract Tester vs oasdiff

API Contract Tester is a web tool for checking whether frontend and backend APIs follow the same contract. It compares an expected TypeScript, OpenAPI, or JSON schema with actual API responses and identifies missing fields, extra fields, incompatible types, and null violations. Validation covers nested objects, arrays, enums, and optional fields, with mismatch reports that include suggested fixes. You can paste a schema and response into the online interface or run the tool locally as an MCP server. Installation uses `npx @clinetools/contract-test` and requires Node.js 18 or newer. The first CLI run asks about contract strictness and fields to ignore. Setup instructions support Cline and Claude Code, and contract checks can run in deployment pipelines before a frontend release. The free plan costs nothing, billed forever, and includes 100 executions per month; premium modules are excluded. Individual Dev costs $19 a month.

oasdiff compares OpenAPI specifications, produces changelogs, and identifies breaking changes before release. It supports OpenAPI 3.0, 3.1, and 3.2, and detects 755 distinct breaking and non-breaking API changes. The CLI is available through Homebrew, a shell installer, Docker, and pre-built Linux, macOS, and Windows binaries. Output formats include text, JSON, YAML, HTML, Markdown, and GitHub Actions annotations. Its GitHub Action compares pull-request and base-branch specifications, adding inline annotations for breaking changes. A browser diff tool compares OpenAPI or Swagger specifications without installation or sign-up. Specs and changelogs are encrypted client-side before upload, and servers retain ciphertext they cannot read. The open-source engine, CLI, and GitHub Action use the Apache 2.0 license. A free plan includes CLI diffing, changelogs, GitHub Action annotations, side-by-side review, and encrypted spec storage. Pro costs $100/month and adds approval workflow, audit trail, merge gate, and up to five reviewers across five repositories. Free review links expire after seven days; Enterprise can load specs from GitHub without uploading or storing them.