FUZZ-004: Schema-valid tool arguments are not rejected as invalid params

Severity	Applies to	Spec section
warn	both eras, stdio, http	2026-07-28/server/tools#tool

Judged from the FUZZ-001 traffic. A tool's inputSchema is the JSON Schema defining its expected parameters, so arguments that validate against it should not be answered with -32602 (Invalid params): when they are, the schema promises more than the tool accepts, and a model that follows the schema will fail. Business-logic failures (isError: true) and server errors are not flagged. The failing input is shrunk and saved for mcpprobe replay.

From the command line: mcpprobe checks explain FUZZ-004. To leave it out of a run, add "FUZZ-004" to [checks] disable in mcpprobe.toml; to change how much it counts, use severity_overrides.