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MCP Tool Contract Testing Skill

Validate MCP server tools with contract-style tests to catch schema drift, unsafe behavior, and integration regressions early.

by JSONbored·added 2026-04-10·
Claude CodeCodexWindsurfGeminiCursorCLI
HarnessClaude CodeCodexWindsurfGeminiCursorCLI
Level:advancedType:generalVerified:draft
Review first — review before installing

Open the source and read safety notes before installing.

Prerequisites

  • MCP server implementation and tool catalog
  • Staging environment for test execution
  • Known expected outputs per critical tool

Schema details

Install type
package
Reading time
6 min
Difficulty score
77
Troubleshooting
Yes
Breaking changes
No
Package metadata
Package verified
Yes
SHA-256
7e8f76baf26241e849bdb0108bf57628864ffb91a056e3878de27247f1d7d352
Skill and platform metadata
Skill type
general
Skill level
advanced
Verification
draft
Verified at
2026-04-10
Retrieval sources
https://modelcontextprotocol.io/specification/2025-06-18/
Tested platforms
ClaudeCodexOpenClawCursorWindsurfGemini
PlatformSupportInstall path
claude-codeNative.claude/skills/<skill-name>/SKILL.md
codexNative.agents/skills/<skill-name>/SKILL.md
windsurfNative.windsurf/skills/<skill-name>/SKILL.md
geminiNative.gemini/skills/<skill-name>/SKILL.md or .agents/skills/<skill-name>/SKILL.md
cursorAdapter.cursor/rules/<skill-name>.mdc
cliManualAGENTS.md or tool-specific context file
Full copyable content
# Trigger
"Run the MCP tool contract testing skill on this server update."

# Required output
1) Tool contract matrix
2) Positive/negative test cases
3) Schema compliance report
4) Release go/no-go summary

About this resource

Overview

This skill helps AI agents enforce explicit behavior contracts for MCP tools. It prevents accidental schema drift, unsafe edge-case handling, and hidden regressions when tools evolve.

Compatibility

Native

  • Claude Code / Claude: native skill usage via SKILL.md.
  • Codex/OpenAI workflows: compatible with Agent Skills-style SKILL.md content as reusable workflow instructions.

Manual Adaptation

  • Gemini CLI: native skill usage via .gemini/skills/<skill-name>/SKILL.md or .agents/skills/<skill-name>/SKILL.md where supported.
  • Cursor: use the generated .cursor/rules/*.mdc adapter for project rules.
  • OpenClaw and similar agents: use the same skill content as a reusable prompt/workflow file when native skill import is unavailable.

Prerequisites

  • Contract definitions for high-value tools
  • Stable test inputs and expected outputs
  • Regression thresholds for release decisions

What This Skill Delivers

  • Contract catalog for each tool endpoint
  • Validation suite for payload and response guarantees
  • Safety/negative tests for malformed and adversarial inputs
  • Regression summary for release gating

How to Use This Skill

  1. Define mandatory response fields and allowed ranges.
  2. Build positive and negative test vectors.
  3. Execute against baseline and candidate builds.
  4. Compare behavior and schema diffs.
  5. Block release when contract-critical checks fail.

Troubleshooting

Issue: Tool output changes break downstream clients
Fix: Add strict backward-compatibility contract checks for public fields.

Issue: Tests pass but runtime still fails on edge cases
Fix: Add adversarial input suites and timeout/error propagation checks.

Issue: Frequent false positives in test suite
Fix: Split deterministic contract checks from heuristic quality checks.

Knowledge Freshness

Treat tooling details as time-sensitive. Re-validate APIs, limits, pricing, auth models, and deployment flags immediately before implementation. If docs conflict with prior memory, follow current official docs and release notes.

Retrieval Sources

Output Contract

  1. Return a concrete plan with implementation order.
  2. Provide production-ready commands/config/code snippets (not placeholders).
  3. Include explicit assumptions and unresolved risks.
  4. Include a verification checklist with pass/fail criteria.

Quality Gates

  • All commands are copy/paste ready.
  • Security-sensitive steps call out secret handling and least privilege.
  • Version-sensitive guidance cites current docs used.
  • Rollback path is included for risky changes.
  • Final output includes quick validation commands/tests.
Content outline
#mcp#testing#contracts#reliability#api

Source citations

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