D1279 — A tool argument the schema does not declare is JSON-RPC -32602, checked after authorization

Decision. A tools/call carrying a top-level argument that is not in the tool's inputSchema answers JSON-RPC -32602 (invalid params) naming the unknown key(s). The check runs after the unknown-tool answer, the argument-shape check and authorization, and the refusal is audited as a denial before it is answered. Only the top level is checked; the routed kb and the arguments a tool lists in Tool.HiddenArguments (accepted, kept out of tools/list, such as map_create's map alias and the removed search arguments that have their own message) are accepted. Messages the HTTP transport cannot take as a request answer JSON-RPC bodies (-32700, -32600, -32601), like a refused Origin.

Why. A typo such as if_mach was ignored, so the optimistic-concurrency check it asked for silently did not happen. Doing the check after authorization keeps the denial the answer a caller who may not use the tool gets, leaves a record of the refusal (D119, D132) and does not let a caller without access probe a tool's schema. It costs a Go error out of the SDK handler on this one path, the only one: every other outcome stays a result with isError.

Alternatives rejected. - Ignoring unknown keys (the previous behaviour): a typo drops the check silently. - A warning in the result: clients do not read it, and the write has already happened. - Checking before authorization: the denial would go unaudited and an unauthorized caller could map the schema. - Declaring hidden aliases in the schema: every tools/list pays for them (D285).

Consequences. A handler that reads a key its schema does not declare must list it in HiddenArguments; TestHandlerArgumentsAreDeclaredOrHidden fails otherwise. A client that sent undeclared keys now gets an error: this is a behaviour change, noted in the PR.