A coverage-guided fuzzer for Ruby code and Ruby extensions written in C. Fuzzing repeatedly tests varied inputs to find crashes and other unexpected behaviour.

It is for testing Ruby applications, libraries, gems with native extensions, and Ruby C extensions. It supports Linux systems and can be used on other systems through the provided Docker or development environment.

It helps uncover memory-safety problems and undefined behaviour that ordinary examples or tests may not trigger.

ruzzy — Skill for Claude Code by marduk191

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent

npx skills add marduk191/qwen3_mcp --skill ruzzy

Clone the repo

git clone --depth 1 https://github.com/marduk191/qwen3_mcp

Made for: Claude Code.

Or install testing-handbook-skills, the plugin that ships this one along with the rest of its 15 skills.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

README.md

[![agentmods](https://agentmods.dev/badge/skills/marduk191/qwen3_mcp/ruzzy/github.svg)](https://agentmods.dev/skills/marduk191/qwen3_mcp/ruzzy)

Your own site

<a href="https://agentmods.dev/skills/marduk191/qwen3_mcp/ruzzy"><img src="https://agentmods.dev/badge/skills/marduk191/qwen3_mcp/ruzzy/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

Your own site · 80×15

<a href="https://agentmods.dev/skills/marduk191/qwen3_mcp/ruzzy"><img src="https://agentmods.dev/badge/skills/marduk191/qwen3_mcp/ruzzy.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 30 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 3,138 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin 98% copy Near-identical to another mod in the catalogue.

Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

Model	Per session	Once invoked
Fable 5.1	$0.00030	$0.03138
Opus 5	$0.00015	$0.01569
Sonnet 5	$0.00006	$0.00628
Haiku 4.5	$0.00003	$0.00314

Measured 12d ago against content hash 603cde0d4592, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-12, from the pricing page.

Security

Grade A, and why

ruzzy scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 12d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

This is a copy

98% identical to ruzzy — 7 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.

skills/testing-handbook-skills/skills/ruzzy/SKILL.md · 444 lines

How it starts

The opening of the file, as written. The whole thing — 444 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Ruzzy

Ruzzy is a coverage-guided fuzzer for Ruby built on libFuzzer. It enables fuzzing both pure Ruby code and Ruby C extensions with sanitizer support for detecting memory corruption and undefined behavior.

When to Use

Ruzzy is currently the only production-ready coverage-guided fuzzer for Ruby.

Choose Ruzzy when:

Fuzzing Ruby applications or libraries
Testing Ruby C extensions for memory safety issues
You need coverage-guided fuzzing for Ruby code
Working with Ruby gems that have native extensions

Quick Start

Set up environment:

export ASAN_OPTIONS="allocator_may_return_null=1:detect_leaks=0:use_sigaltstack=0"

Test with the included toy example:

LD_PRELOAD=$(ruby -e 'require "ruzzy"; print Ruzzy::ASAN_PATH') \
    ruby -e 'require "ruzzy"; Ruzzy.dummy'

This should quickly find a crash demonstrating that Ruzzy is working correctly.

Installation

Platform Support

Ruzzy supports Linux x86-64 and AArch64/ARM64. For macOS or Windows, use the Dockerfile or development environment.

Prerequisites

Linux x86-64 or AArch64/ARM64
Recent version of clang (tested back to 14.0.0, latest release recommended)
Ruby with gem installed

Installation Command

Install Ruzzy with clang compiler flags:

MAKE="make --environment-overrides V=1" \
CC="/path/to/clang" \
CXX="/path/to/clang++" \
LDSHARED="/path/to/clang -shared" \
LDSHAREDXX="/path/to/clang++ -shared" \
    gem install ruzzy

Environment variables explained:

MAKE: Overrides make to respect subsequent environment variables
CC, CXX, LDSHARED, LDSHAREDXX: Ensure proper clang binaries are used for latest features

Troubleshooting Installation

If installation fails, enable debug output:

RUZZY_DEBUG=1 gem install --verbose ruzzy

Verification

Verify installation by running the toy example (see Quick Start section).

Read the full file on GitHub · 444 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

12d ago First seen · 444 lines · 30 tokens per session scan A 603cde0d4592

Subscribe to this mod's changes

ruzzy is a skill published in the GitHub repository marduk191/qwen3_mcp (13 stars, last pushed 7mo ago), licensed MIT. It adds 30 tokens to every session and 3,138 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 0 findings. It is 98% identical to ruzzy, differing in 7 lines, and is treated as a copy.

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ruzzy