Freedom251 › Contract Testing Tools
API Contract Tester
What API Contract Tester is
API Contract Tester checks whether API responses match an expected contract, helping frontend and backend teams find schema differences. It compares TypeScript or OpenAPI schemas with actual responses and detects missing or extra fields, type mismatches, and null violations. Validation also covers nested objects, arrays, enums, and optional fields, with mismatch reports that include fix suggestions. Users can paste a schema and response into the online tool or run the CLI locally as an MCP server. The installation command is `npx @clinetools/contract-test`, and the CLI requires Node.js 18 or later. On first run, it asks about contract strictness and fields to ignore. Setup instructions for Cline and Claude Code let an agent validate contracts automatically. Tests can also run in a deployment pipeline to catch breaking backend changes before frontend deployment. The free plan allows 100 executions per month and excludes premium modules. Individual Dev costs $19 a month and includes 5,000 executions, priority support, and early access to new tools.
Who it suits
It suits frontend and backend teams that want to check responses against an agreed schema. Developers can use the web tool, run the CLI locally, or add contract checks to a deployment pipeline.
What it does well
- Checks missing and extra fields, types, and null violations.
- Validates nested objects, arrays, enums, and optional fields.
- Reports mismatches with fix suggestions.
- Supports local CLI use as an MCP server.
- Free plan includes 100 monthly executions.
What to know first
- CLI requires Node.js 18 or later.
- Free plan excludes premium modules.
- Individual Dev costs $19 a month.
Limits
The Free plan costs nothing, is billed forever, and allows 100 executions per month; premium modules are excluded. Individual Dev costs $19 a month and allows 5,000 executions per month.
Plans
| Plan | Billed and limits | Price |
|---|---|---|
| Free | forever · 100 executions/month · all 42+ core tools · premium modules excluded | Free |
| Individual Dev | 5,000 executions/month · priority support · early access to new tools | $19 a month |
| Pro AI Agent | 50,000 executions/month · team analytics (5 members) · advanced premium modules | $49 a month |
| Enterprise | Unlimited executions · self-hosted deployment · private LLM integration · SLA & dedicated support | $499 a month |
As clinetools.com lists them, read 7 Oct 2026. The maker's own page has the final word.
Getting started
- Open https://clinetools.com/tool-contract-test.
- For web use, paste an expected schema and an actual response.
- For local use, install with `npx @clinetools/contract-test` on a system with Node.js 18 or later.
- On first CLI run, set contract strictness and fields to ignore.
- Follow the MCP setup instructions for Cline or Claude Code if using an agent.
- Add contract tests to a deployment pipeline if you want to check backend changes before frontend deployment.
- Choose the Free plan or a paid plan based on execution needs.
Verdict
API Contract Tester offers schema checks through a web tool or local CLI, with options for agent integration and deployment pipelines. The free plan is limited to 100 executions per month and excludes premium modules.
Questions
Is API Contract Tester free?
Yes. The Free plan costs nothing, is billed forever, and includes 100 executions per month; premium modules are excluded.
What do paid plans cost?
Individual Dev is $19 a month, Pro AI Agent is $49 a month, and Enterprise is $499 a month.
Which platforms are available?
The listed platform is web. The tool can also be run locally as an MCP server.
Which contract formats does it support?
The listed formats are TypeScript, OpenAPI, and JSON.
Can it run in an agent or deployment pipeline?
The page provides MCP setup instructions for Cline and Claude Code. It also says contract tests can run in a deployment pipeline.
Does it offer self-hosted deployment?
Yes. The Enterprise plan includes self-hosted deployment.
Facts from API Contract Tester's own pages
- Agent integrations
- The page gives MCP setup instructions for Cline and Claude Code so an agent can validate contracts automatically. · clinetools.com, 7 Oct 2026
- Audience
- The page describes contract tests as a way for frontend and backend teams to verify that both sides honor an agreed schema. · clinetools.com, 7 Oct 2026
- CI/CD use
- The page says contract tests can run in a deployment pipeline to catch breaking backend changes before the frontend deploys. · clinetools.com, 7 Oct 2026
- Configuration
- On first run, the CLI asks about contract strictness and ignored fields. · clinetools.com, 7 Oct 2026
- Mismatch detection
- It detects missing fields, extra fields, type mismatches, and null violations. · clinetools.com, 7 Oct 2026
- Purpose
- API Contract Tester verifies frontend-backend API contracts and detects schema mismatches, missing fields, and type incompatibilities. · clinetools.com, 7 Oct 2026
- Reports
- It generates detailed mismatch reports with fix suggestions. · clinetools.com, 7 Oct 2026
- Requirements
- The CLI install command is `npx @clinetools/contract-test` and it requires Node.js 18+. · clinetools.com, 7 Oct 2026
- Schema comparison
- It compares an expected TypeScript or OpenAPI schema against actual API responses. · clinetools.com, 7 Oct 2026
- Security review
- ClineTools says every tool undergoes a four-phase security audit. · clinetools.com, 7 Oct 2026
- Validation
- It validates nested objects, arrays, enums, and optional fields. · clinetools.com, 7 Oct 2026
- Ways to use
- Users can try it online by pasting an expected schema and actual response, or run it locally as an MCP server. · clinetools.com, 7 Oct 2026
Where API Contract Tester stands on Freedom251
- Contract Testing Tools#5 of 22