Your MCP unit tests pass, but did the agent call the tool? M3 runs your server through real Claude Code, Codex, OpenCode or Pi and asserts on the calls they actually made, in plain pytest. Direct tests need no API key. m3 ui shows every trace and tool call; m3 ci test --upload gates PRs and keeps runs on app.m3.sineframe.com. Open source, Apache-2.0.
