You test an MCP server by chatting with it. Ask for the open bugs, get the open bugs, ship it. The model never sends limit: "ten", an empty path or a 200 KB query while you’re watching, so those paths stay dark until a real session hits one. Say it’s the limit. The handler throws, the framework wraps the exception in a 40 KB stack trace, and the model reads all of it, adjusts, and retries with the same bug in a new shape.