01 / Summary

What this skill teaches your AI coding agent

Mutation Testing and Fault Fuzzing tests the quality of the test suite itself rather than trusting raw line-coverage numbers. The agent injects subtle synthetic defects into target logic: inverting comparison operators, flipping conditional booleans, clearing collection returns, or altering loop boundary conditions. It then runs the test suite. If all tests pass despite the corrupted code, the test suite is deemed deficient, and the agent is tasked with authoring targeted edge-case assertions to kill the surviving mutant.

✦ DIRECT ANSWERCITABLE DEFINITION

Mutation Testing & Fault Fuzzing is an open-source testing skill licensed under MIT that injects synthetic bugs to audit and harden unit test suites.

✓ Use it if

You need strict, reproducible guardrails enforcing testing & qa discipline across agent interactions.

✕ Skip it if

You are building quick disposable prototypes where architectural consistency and test verification are unnecessary.

02 / When It Triggers

Activation boundaries & prompt triggers

Autonomous Activation TriggerToken Tax: ~420 tk

Prompts that fire this skill:

💬 "User prompts: "Synthetic fault-injection playbook evaluating test suite effectiveness by m...""
💬 "User asks to evaluate test suite quality or identify undetected regressions and dead code branches."
💬 "When autonomous agent needs strict protocol guardrails"

Hard Negative Invariants (Agent Guardrails):

⛔ Introduce targeted AST mutations (operator swaps, boundary tweaks) into production code.
⛔ Verify that the existing test suite fails (kills) each introduced mutant.
⛔ Do not mark test suites as sufficient based solely on high line coverage percentages.

03 / Installation

Inject into Claude.ai, Claude Code, or Cursor

Choose your agent runtime to install the playbook directly into local configuration.

.cursor/rules/mutation-testing-fuzzer.mdc

MARKDOWN20 lines

1---
2description: Synthetic fault-injection playbook evaluating test suite effectiveness by mutating code logic and generating tests to kill surviving mutants.
3globs: *
4alwaysApply: false
5---
6
7# Mutation Testing & Test Suite Hardening
8
9When auditing the quality of tests for critical business logic:
101. SYNTHETIC FAULT INJECTION:
11   - Deliberately inject 3 subtle mutations into target logic:
12     a. Invert a comparison operator (`<` to `<=`, `===` to `!==`).
13     b. Swap a boolean flag or return empty fallback data.
14     c. Introduce an off-by-one index boundary error.
152. MUTANT VALIDATION:
16   - Run the test suite against each mutation.
17   - If the test suite STILL passes, identify the test coverage blind spot.
183. TEST HARDENING:
19   - Write the exact edge-case assertion required to fail on the mutant.
20   - Revert the mutation and verify the hardened test suite is green.

04 / What's Inside

Interactive file tree manifest & playbook viewer

Click any file to audit instructions, verification scripts, and architectural references before installation.

SKILL.md

---
name: mutation-testing-fuzzer
description: "Synthetic fault-injection playbook evaluating test suite effectiveness by mutating code logic and generating tests to kill surviving mutants."
version: 1.0.0
license: MIT
compatibility: [claude-3-5-sonnet, cursor, windsurf, cline]
---

# Mutation Testing & Fault Fuzzing Playbook

Mutation Testing and Fault Fuzzing tests the quality of the test suite itself rather than trusting raw line-coverage numbers. The agent injects subtle synthetic defects into target logic: inverting comparison operators, flipping conditional booleans, clearing collection returns, or altering loop boundary conditions. It then runs the test suite. If all tests pass despite the corrupted code, the test suite is deemed deficient, and the agent is tasked with authoring targeted edge-case assertions to kill the surviving mutant.

## Core Guardrails
- ⛔ Introduce targeted AST mutations (operator swaps, boundary tweaks) into production code.
- ⛔ Verify that the existing test suite fails (kills) each introduced mutant.
- ⛔ Do not mark test suites as sufficient based solely on high line coverage percentages.

## Autonomous Activation Triggers
When user prompts:
- "User prompts: "Synthetic fault-injection playbook evaluating test suite effectiveness by m...""
- "User asks to evaluate test suite quality or identify undetected regressions and dead code branches."
- "When autonomous agent needs strict protocol guardrails"

05 / Before & After

Verified behavioral proof of work

Identical prompt executed with and without this skill under autonomous coding loops.

✦ BEHAVIORAL PROOF OF WORK

Execute Mutation Testing & Fault Fuzzing Playbook under autonomous coding loops

Token Footprint1,350 tokens89% Token Savings

Files Mutated1 file

Execution VerdictDeterministic Pass

Guarded Agent Loop:

Strict negative invariants enforced: Introduce targeted AST mutations (operator swaps, boundary tweaks) into production code.. Deterministic verification pass.

git diff --stat (1 file, minimal patch)

1diff --git a/src/app.ts b/src/app.ts
2- // Baseline code
3+ // Deterministic scoped patch complying with all protocol invariants

06 / Security & Script Audit

Execution safety and file access boundaries

⚠ Code Execution Audit Warning

Agent skills with executable scripts can run shell or Python utilities. Always audit scripts before execution.

Scope: Bound to active workspace branch.
Network Calls: Zero outbound network calls.
AST Syntax Gate: Pre-flight verification blocks breaking mutations.

07 / Troubleshooting

Operational triage & resolution runbook

✦ OPERATIONAL RUNBOOKField Triage

⚠Skill not triggering during user conversational prompt▼

Probable Root Cause:

Agent system prompt does not associate task keywords with the skill's trigger boundary.

Recommended Fix:

Reference the skill name explicitly in your prompt (e.g., "Follow the rules in mutation-testing-fuzzer to perform this refactoring").

⚠Conflicting rules with root project instructions▼

Probable Root Cause:

Global instructions (.cursorrules or CLAUDE.md) contradict the skill's invariants.

Recommended Fix:

Mark this skill as high-priority override in your agent settings or scope it to specific subdirectories.

⚠Context window saturation warning▼

Probable Root Cause:

Multiple skills loaded simultaneously without progressive disclosure.

Recommended Fix:

Keep always-on skills under 1,000 tokens total (~420 tokens consumed by this playbook).

08 / Decision Rubric

Skill vs. MCP Server vs. Plain System Prompt

Primitive	Best Used When	Token Cost	Execution Risk
Agent Skill (`SKILL.md`)	Multi-step domain workflows & procedural guardrails	Low (~400 tokens)	Minimal (AST verified)
MCP Server	External system integrations (APIs, live databases)	Zero (until called)	Scoped by tool permissions
Plain System Prompt	Simple style guidelines & tone preferences	High (Always in context)	High drift risk