harness-writing

An agent skill by trailofbits, from trailofbits/skills. Tags: code-quality, developer-tools, fuzzing, testing.

What it does

Designs and improves fuzzing harnesses for C/C++ and Rust. Covers mapping raw bytes onto a target API, generating structured inputs, avoiding non-determinism and false crashes, and deciding what to fuzz together. Use when writing a first LLVMFuzzerTestOneInput or fuzz_target! harness, when a campaign finds nothing or reports crashes that will not reproduce, or when the target API needs structured rather than raw input.

Install

With the skills CLI, which installs into Claude Code, Codex, Cursor and other agents:

npx skills add trailofbits/skills --skill harness-writing

Or copy the skill folder into Claude Code's skills directory by hand (~/.claude/skills for every project, or .claude/skills inside one):

git clone --depth 1 https://github.com/trailofbits/skills
cp -r skills/plugins/testing-handbook-skills/skills/harness-writing ~/.claude/skills/harness-writing

Safety box score

Not rated yet. A safety box score grades what a skill and its scripts can reach on the machine of whoever installs it, across eight categories from shell execution to secrets access. Anyone can request one from this page; it is saved for everyone. How the score works.

Source

Repository

trailofbits/skills (all skills from this repository)

Path

plugins/testing-handbook-skills/skills/harness-writing/SKILL.md

Branch

main

Collection

testing-handbook-skills

Updated

2026-09-19

Related skills

• aflpp — Sets up and runs AFL++ for multi-core fuzzing of C/C++ projects built with afl-clang-fast or afl-gcc-fast.
• libfuzzer — Sets up and runs libFuzzer, the coverage-guided fuzzer built into LLVM, on C/C++ code that compiles with Clang.
• genotoxic — Graph-informed mutation testing triage.
• atheris — Sets up and runs Atheris, the coverage-guided Python fuzzer built on libFuzzer.
• cargo-fuzz — Sets up and runs cargo-fuzz, the standard fuzzing tool for Cargo-based Rust projects. Covers cargo fuzz init, the nightly toolchain requirement, fuzz_target!
• fuzzing-dictionary — Builds and applies fuzzing dictionaries so a fuzzer can produce the keywords, magic bytes, and tokens a target expects.