FUZZ-001: Fuzzed tool arguments never crash the server or go unanswered

Severity	Applies to	Spec section
error	both eras, stdio, http	2026-07-28/basic/index#messages

Calls each tool with arguments generated from its inputSchema: valid values, and near-misses (a required field dropped, a wrong type, a value just past a bound, an extra property, null, deep nesting, a 100,000-character string, or odd Unicode such as a lone surrogate). All messages MUST follow JSON-RPC 2.0, which requires a response to every request, and servers MUST validate all tool inputs (https://modelcontextprotocol.io/specification/2026-07-28/server/tools#security-considerations), so no input may crash the server or leave its request unanswered. Fails on a crash, on a request dropped while the server keeps answering others, and (as a timeout) on a server that stops answering altogether. Each call carries a progressToken, and progress notifications keep a slow call alive up to three times --timeout (https://modelcontextprotocol.io/specification/2026-07-28/basic/patterns/cancellation#timeouts). A crash is shrunk to a minimal input (on stdio by restarting the server) and saved for mcpprobe replay; unanswered requests are not shrunk, since every attempt costs a full timeout. Runs --examples argument sets per tool (default 50) from the run seed. Tools are called only if marked readOnlyHint: true or destructiveHint: false (all tools with --allow-destructive), and never if openWorldHint: true unless --allow-open-world is passed. Generated URLs, emails and hostnames use the reserved .invalid TLD. Runs once per server, in the modern era if the server speaks it.

From the command line: mcpprobe checks explain FUZZ-001. To leave it out of a run, add "FUZZ-001" to [checks] disable in mcpprobe.toml; to change how much it counts, use severity_overrides.