API Contract Tester
API Contract Tester checks whether API responses match an expected contract, helping frontend and backend teams find schema differences.
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 flags missing or extra fields, type mismatches, and null violations. Validation also covers nested objects, arrays, enums, and optional fields, and results include detailed mismatch reports with fix suggestions. Users can paste a schema and response into the online tool or run it locally as an MCP server. The CLI install command is `npx @clinetools/contract-test` and requires Node.js 18 or later; its first run asks for contract strictness and fields to ignore. Setup instructions are provided for Cline and Claude Code to enable automatic contract validation by an agent. The tool can also run contract checks in a deployment pipeline to catch breaking backend changes before a frontend deploys. The free plan costs nothing and allows 100 executions per month, excluding premium modules. Individual Dev is $19 a month for 5,000 executions, while higher plans are also listed.
Good
- Checks TypeScript and OpenAPI schemas against responses.
- Detects field, type, and null mismatches.
- Validates nested objects, arrays, enums, and optional fields.
- Reports include suggested fixes.
- Free plan allows 100 executions per month.
Watch out
- CLI requires Node.js 18 or later.
- Free plan excludes premium modules.
- Free usage is limited to 100 executions per month.
Who it suits
It suits frontend and backend teams that want to verify responses against agreed schemas. Developers can use the web tool, a local MCP server, or pipeline checks.
Verdict
API Contract Tester provides schema checks through a web interface, local MCP server, and deployment-pipeline use. The free plan offers a way to start, while paid plans begin at $19/mo.
Plans and prices
From API Contract Tester's own pricing page, checked 7 Oct 2026.
| Plan | Price | What it gives you |
|---|---|---|
| Free | Free | 100 executions/month · all 42+ core tools · premium modules excluded · forever |
| Individual Dev | $19 a month | 5,000 executions/month · priority support · early access to new tools |
| Pro AI Agent | $49 a month | 50,000 executions/month · team analytics (5 members) · advanced premium modules |
| Enterprise | $499 a month | Unlimited executions · self-hosted deployment · private LLM integration · SLA & dedicated support |
The facts
Each one read from API Contract Tester's own pages, with the date it was checked.
What it does
- Agent integrations
- The page gives MCP setup instructions for Cline and Claude Code so an agent can validate contracts automatically.checked 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.checked 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.checked 7 Oct 2026
- Configuration
- On first run, the CLI asks about contract strictness and ignored fields.checked 7 Oct 2026
- Mismatch detection
- It detects missing fields, extra fields, type mismatches, and null violations.checked 7 Oct 2026
- Purpose
- API Contract Tester verifies frontend-backend API contracts and detects schema mismatches, missing fields, and type incompatibilities.checked 7 Oct 2026
- Reports
- It generates detailed mismatch reports with fix suggestions.checked 7 Oct 2026
- Requirements
- The CLI install command is `npx @clinetools/contract-test` and it requires Node.js 18+.checked 7 Oct 2026
- Schema comparison
- It compares an expected TypeScript or OpenAPI schema against actual API responses.checked 7 Oct 2026
- Security review
- ClineTools says every tool undergoes a four-phase security audit.checked 7 Oct 2026
- Validation
- It validates nested objects, arrays, enums, and optional fields.checked 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.checked 7 Oct 2026
Alternatives to API Contract Tester
- 1
Karate
Free planOpen sourceAPIExtensionLinuxmacOS8.2From $100 a year - 2
PactFlow
Free planWebAPILinuxSelf-hosted7.6From $115.42 a month - 3
Pact
Free planAPILinuxmacOSSelf-hosted7.5From $115.42 a month - 4
Microcks
Free planWebAPIExtensionLinux7.2Free plan - 5
oasdiff
Free planWebAPILinuxmacOS7.2From $100 a month - 6
Confluent Cloud
Free planWebAPILinux7.1From $385 a month