MCP Test Harness

Build production-ready, fully automated integration test suites for MCP servers using STDIO transport. Tests run against live servers and validate real data.

Quick Start Decision

Core Philosophy

These are integration tests, not unit tests:

• •Tests run against the actual MCP server (not mocks)
• •Tests validate real data returned from tools
• •Tests use STDIO transport (the standard for local MCP servers)
• •Tests should pass before any deployment or version bump

Workflow

Step 1: Gather Requirements

Before writing tests, ask the user for:

1. •Server location: Path to the MCP server entry point
2. •Server language: Python (FastMCP) or TypeScript
3. •Tool inventory: List all tools the server exposes (or discover via Inspector)
4. •For each tool to test:
   • •Example valid inputs
   • •Expected output values or patterns to validate
   • •Edge cases and known error conditions
   • •External dependencies (APIs, databases, files)
5. •Environment requirements: API keys, config files, setup steps

Step 2: Choose Testing Approach

Python/FastMCP (Recommended for Python servers):

• •In-memory testing via Client(server) - no subprocess needed
• •Fastest, most reliable approach
• •Full protocol compliance
• •See references/python-fastmcp.md

TypeScript/Vitest (Recommended for TS servers):

• •Subprocess-based via StdioClientTransport
• •Native TypeScript, excellent DX
• •See references/typescript-vitest.md

MCP Inspector CLI (Universal fallback):

• •Works with any language
• •Good for quick validation or CI pipelines
• •See references/inspector-cli.md

Step 3: Implement Tests

Follow the test patterns in references/test-patterns.md:

1. •Discovery Tests: Verify all expected tools appear in tools/list
2. •Execution Tests: Call each tool with valid inputs, validate response data
3. •Validation Tests: Confirm response structure and content matches expectations
4. •Error Tests: Verify graceful handling of invalid inputs
5. •Concurrency Tests: (Optional) Test parallel tool invocations

Step 4: Run and Iterate

# Python
pytest tests/ -v --tb=short

# TypeScript  
npx vitest run

# Inspector CLI (any language)
npx @modelcontextprotocol/inspector --cli <server-command> --method tools/list

Windows 11 / Claude Code Notes

When running commands in Claude Code on Windows:

For npx and npm commands, use them directly - they work cross-platform.

Python virtual environments on Windows:

# Create venv
python -m venv .venv

# Activate (PowerShell)
.venv\Scripts\Activate.ps1

# Activate (cmd)
.venv\Scripts\activate.bat

# Activate (Git Bash)
source .venv/Scripts/activate

Test Checklist

Before considering tests complete:

• • All tools from tools/list have corresponding tests
• • Each tool tested with at least one valid input
• • Response data validated against expected values (not just "truthy")
• • Error cases tested for each tool
• • Tests run successfully in isolation
• • Tests are deterministic (same result every run)
• • No hardcoded secrets in test files (use environment variables)

Reference Files

Load these as needed during implementation:

• •Python/FastMCP Testing Guide - Complete pytest setup and patterns
• •TypeScript/Vitest Testing Guide - Complete Vitest setup and patterns
• •MCP Inspector CLI Guide - Universal CLI-based testing
• •Universal Test Patterns - Language-agnostic testing patterns

Asset Templates

Copy and customize these starter files:

• •assets/python/ - pyproject.toml snippet, conftest.py, test template
• •assets/typescript/ - vitest.config.ts, package.json snippet, test template