Testing MCP Servers with MCP Inspector

The Model Context Protocol (MCP), introduced by Anthropic, standardizes how LLM applications connect to external tools, data sources, and prompt templates. MCP Inspector is an interactive developer tool for testing and debugging MCP servers. Its current documentation describes Web, CLI, and TUI clients that can connect to local stdio processes or remote HTTP transports; a CLI mode can probe methods directly. The Streamable HTTP transport may return JSON or use server-sent events, while the older HTTP+SSE transport remains a legacy compatibility path. Once connected, Inspector lists everything the server declares - its tools, resources, and prompts - and lets a developer invoke any tool by filling in its parameters directly, without a language model in the loop at all. This matters because many integration bugs are not about model behavior; they are plain protocol or schema errors, such as a tool that returns the wrong content type, a resource URI that the server cannot actually resolve, or a parameter marked required in the schema but never checked in the handler. Inspector exposes server responses, tool results, metadata, and logs so developers can isolate protocol behavior before integrating a model. The MCP revision matters: 2025-era clients use an initialize/initialized exchange, while the 2026-07-28 revision removes that handshake and requires modern servers to support server/discover, which clients may call optionally. Check the configured revision as well as the transport when a connection or method behaves differently. A common misconception is that passing manual tests in Inspector guarantees a model will use the tools well in practice; it only confirms the server behaves correctly when called - whether a model chooses to call a tool, and with sensible arguments, still depends heavily on how clearly the tool and its parameters are described. Another misconception is that Inspector requires a live LLM API key; it does not, since it acts as its own MCP client.