mcp-fuzz is a stdlib-only Python robustness harness for AAIF that throws 17 malformed and adversarial JSON-RPC cases at an MCP server over stdio and grades it A to F on crash resistance, responsivenes
A robustness and negative-test harness for Model Context Protocol servers. mcp-fuzz throws malformed and adversarial JSON-RPC at a server over stdio and grades how gracefully it degrades: does it stay alive, return well-formed JSON-RPC error objects, stay silent to notifications, and remain responsive afterwards.
It is a defensive quality tool. You point it at your own server before you ship, the same way you would run a linter or a fuzz test in CI. Zero dependencies, stdlib only, Python 3.9+.
Most MCP quality tooling checks a server on well-formed input (does a valid request return a valid result) or measures performance. Almost nothing checks what happens on the ragged edge: a truncated line, a batch array, a 200 KB argument, an unknown notification, an id that is an object. That edge is exactly where servers crash, hang, or leak malformed responses in production. mcp-fuzz fills that gap and slots in as a robustness dimension alongside conformance, surface-hygiene, and load testing.
There is nothing to install to try it, though. The package ships a sample server:
Fuzz your own server by giving the command that launches it over stdio:
Fuzz the bundled robust sample (scores A) or the bundled fragile server (crashes on the first bad line, to show detection):
Seventeen cases across seven categories. Each is followed by a liveness probe, so a crash or hang is caught wherever it happens.
From the project README.
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