Testing MCP Servers with MCP Inspector (chrisebert.net)

MCP Inspector is a free, browser-based tool (launch with npx @modelcontextprotocol/inspector) that makes testing and debugging MCP (Model Context Protocol) servers far easier by exposing raw MCP messages, server notifications, resources, prompts, and callable tools in a single UI (default http://localhost:6274). It supports servers installed in your project or spawned for you (works with npm/PyPI/uv/uvx and languages like Node, Python, Rust, Go), lets you choose transports (typically stdio), pass env vars, save/load server configs (servers.json), and connect to public or authenticated MCP endpoints. The inspector also validates server responses against declared schemas, captures history, and provides interactive run/copy inputs for tools—features that remove much of the friction when developing MCP integrations. This matters because MCP servers are usually invisible to developers who interact with them indirectly via LLMs; that opacity makes debugging schemas, tool contracts, and prompts difficult. MCP Inspector gives developers a reproducible, inspectable workflow—spawn a server (example: launching the AWS Documentation MCP Server with uvx and an AWS_DOCUMENTATION_PARTITION env var), inspect tools like search_documentation, run test calls, and see validated request/response pairs. For engineers building LLM toolchains, it accelerates development, surface schema mismatches early, and clarifies how agents use external capabilities.