Reeve makes it safe for a Rails application to expose MCP (Model Context Protocol) tools to AI agents. Authentication says who is at the door; Reeve decides what they may touch and remembers what they touched: declarative per-record authorization scoped to the human the agent acts for, an append-only audit ledger of every call, and a testing kit that turns both guarantees into CI assertions. Works alongside fast-mcp, ActionMCP, and the official mcp gem. Developed under the working name mcp-guardrails.