A tool is what an agent calls when it needs to do something the model alone cannot do — read a file, query a service, invoke a workflow. Tool selection is part of the agent's intrinsic definition or attached at an composition node. The mcp-catalog is the runtime-injectable index; the mcp-gateway brokers the actual transport with authentication and audit. Like every other primitive, tools are owned at a space — an admissible subset of the one SpaceKind vocabulary, collapsed by the declared SPACE_KIND_RANK ladder so the most specific owner wins — and may be shipped by biomes via contribution.