A model choosing a tool has never run it. All it sees is metadata — a server name, a tool name, a description, and the shape of the parameters. Across 367,174 published tool definitions, 169 records named send_email expose 78 different parameter sets; swap one piece of metadata at a time across five models and five tasks, with candidate order rotated, and the parameter schema is the strongest effect in 19 of 30 conditions, about twice the size of the server name. Models want separate top-level fields, and they read your field names closely enough that renaming one halves the odds.