FUZZ-002: Invalid tool arguments are rejected cleanly, not with an internal error

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

Judged from the FUZZ-001 traffic. Arguments that fail the tool's own inputSchema are input validation errors, which the spec reports as tool execution errors (isError: true), and servers MUST validate all tool inputs (https://modelcontextprotocol.io/specification/2026-07-28/server/tools#security-considerations). Passes on isError: true, -32602 (Invalid params) or any other specific error. Fails on -32603 (Internal error), which means the input reached code that did not expect it, or on a reply that is not a JSON-RPC response at all (for example an HTML error page). The failing input is shrunk and saved for mcpprobe replay.

From the command line: mcpprobe checks explain FUZZ-002. To leave it out of a run, add "FUZZ-002" to [checks] disable in mcpprobe.toml; to change how much it counts, use severity_overrides.