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API Contract Tester
What it is
API Contract Tester is a web-based tool for checking whether API responses follow an expected contract. It compares TypeScript or OpenAPI schemas with actual responses and identifies missing or extra fields, type mismatches, and null violations. Validation also covers nested objects, arrays, enums, and optional fields, and the tool produces 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 command is `npx @clinetools/contract-test` and 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 allow an agent to validate contracts automatically. Contract checks can also run in a deployment pipeline to catch breaking backend changes before a frontend deploys. The free plan is free, billed forever, with 100 executions per month; premium modules are excluded. Individual Dev costs $19 a month and includes 5,000 executions, priority support, and early access to new tools. Pro AI Agent costs $49 a month, and Enterprise costs $499 a month.
For and against
For
- Checks missing fields, extra fields, type mismatches, and null violations.
- Validates nested objects, arrays, enums, and optional fields.
- Mismatch reports include fix suggestions.
- Free plan includes 100 executions per month.
Against
- CLI requires Node.js 18 or later.
- Free plan excludes premium modules.
- Individual Dev costs $19 a month.
The verdict
API Contract Tester offers schema checks through a web tool or local CLI, with options for agent and pipeline workflows. The free plan has a monthly execution limit, while listed paid plans add higher limits and other features.
Who it suits
It suits frontend and backend teams that want to check whether both sides follow an agreed API schema. Developers can use the web tool, local CLI, agent setup, or deployment pipeline checks.
Plans and prices
Free
100 executions/month · all 42+ core tools · premium modules excluded
Read 7 Oct 2026 · clinetools.com
Individual Dev
5,000 executions/month · priority support · early access to new tools
Read 7 Oct 2026 · clinetools.com
Pro AI Agent
50,000 executions/month · team analytics (5 members) · advanced premium modules
Read 7 Oct 2026 · clinetools.com
Enterprise
Unlimited executions · self-hosted deployment · private LLM integration · SLA & dedicated support
Read 7 Oct 2026 · clinetools.com
On the record
- Agent integrations
- The page gives MCP setup instructions for Cline and Claude Code so an agent can validate contracts automatically.clinetools.com · read 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 · read 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 · read 7 Oct 2026
- Configuration
- On first run, the CLI asks about contract strictness and ignored fields.clinetools.com · read 7 Oct 2026
- Mismatch detection
- It detects missing fields, extra fields, type mismatches, and null violations.clinetools.com · read 7 Oct 2026
- Purpose
- API Contract Tester verifies frontend-backend API contracts and detects schema mismatches, missing fields, and type incompatibilities.clinetools.com · read 7 Oct 2026
- Reports
- It generates detailed mismatch reports with fix suggestions.clinetools.com · read 7 Oct 2026
- Requirements
- The CLI install command is `npx @clinetools/contract-test` and it requires Node.js 18+.clinetools.com · read 7 Oct 2026
- Schema comparison
- It compares an expected TypeScript or OpenAPI schema against actual API responses.clinetools.com · read 7 Oct 2026
- Security review
- ClineTools says every tool undergoes a four-phase security audit.clinetools.com · read 7 Oct 2026
- Validation
- It validates nested objects, arrays, enums, and optional fields.clinetools.com · read 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 · read 7 Oct 2026
