enu: Skill for Claude Code

.claude/skills/mutacion/SKILL.md

mutacion is a skill for Claude Code from dbareagimeno/enu. It costs 79 tokens per session (1,207 once invoked), scanned A, original, Apache-2.0.

A procedure for mutation testing in Go using Gremlins. Mutation testing makes small artificial code changes, such as reversing a condition, and checks whether the test suite detects them.

In plain words
What is it for?
Use it to test files changed in a session or selected packages, review surviving mutants, and strengthen edge-case tests before closing work; do not use it on the excluded areas or as a CI gate.
Why use it?
Normal code coverage shows which lines run, but not whether tests catch incorrect behavior. This procedure finds specific changes that existing tests would allow to survive.

Skill for Claude Code

Written for Claude Code: installed under .claude/.

This is dbareagimeno/enu's own configuration. It tells Claude Code how to work on enu itself, so it is not a mod to install elsewhere. Copy it as a starting point and replace the rules that are about this project. Everything enu configures →

Reuse

Borrowing it

Nothing to install: this file belongs to dbareagimeno/enu. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file
curl -O https://raw.githubusercontent.com/dbareagimeno/enu/develop/.claude/skills/mutacion/SKILL.md
Clone the repo
git clone --depth 1 https://github.com/dbareagimeno/enu

Made for: Claude Code.

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.

agentmods badge for mutacion

README.md
[![agentmods](https://agentmods.dev/badge/skills/dbareagimeno/enu/mutacion/github.svg)](https://agentmods.dev/skills/dbareagimeno/enu/mutacion)
Your own site
<a href="https://agentmods.dev/skills/dbareagimeno/enu/mutacion"><img src="https://agentmods.dev/badge/skills/dbareagimeno/enu/mutacion/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.

agentmods 80×15 button for mutacion

Your own site · 80×15
<a href="https://agentmods.dev/skills/dbareagimeno/enu/mutacion"><img src="https://agentmods.dev/badge/skills/dbareagimeno/enu/mutacion.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 79 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 1,207 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 original No closer match found 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.

ModelPer sessionOnce invoked
Fable 5.1 $0.00079 $0.01207
Opus 5 $0.00039 $0.00603
Sonnet 5 $0.00016 $0.00241
Haiku 4.5 $0.00008 $0.00121

Measured 12d ago against content hash 6ca68376abd8, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-12, from the pricing page.

Security

Grade A, and why

mutacion 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.

.claude/skills/mutacion/SKILL.md · 88 lines

How it starts

The opening of the file, as written. The whole thing — 88 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Mutation testing (gremlins)

Convierte "creo que a la suite le falta un caso límite" en "este mutante concreto sobrevivió": gremlins introduce fallos artificiales (invierte una condición, cambia un + por -, un < por <=) y ejecuta los tests; si ninguno falla, esos tests no muerden. Complementa el inventario 🔒 — la cobertura mide qué líneas se ejecutan; la mutación mide si los tests detectan que están rotas.

Instalación (pineada, fuera del proyecto)

go install github.com/go-gremlins/gremlins/cmd/[email protected]

Nunca en go.mod (cero dependency hell, ADR-001): es un binario externo de desarrollo. Versión pineada, no @latest — gremlins es 0.x sin garantía de compatibilidad; si se sube de versión, se prueba primero y se actualiza este fichero.

Ejecución

Siempre acotada: internal/runtime es un único paquete Go enorme, y el módulo entero tiene >1000 mutantes cubiertos a ~20 s de suite por mutante — horas. El alcance se recorta con -E (regexp de exclusión) hasta dejar solo los ficheros de la sesión, o con -D <rama> para mutar solo el diff.

# Modo sesión: solo los ficheros tocados (ejemplo: diff.go)
gremlins unleash \
  -E 'web/.*' -E 'internal/vmwasm/.*' -E 'cmd/enu/main\.go' \
  -E 'internal/runtime/[a-ce-z].*' -E 'internal/runtime/d(efault_config|river).*' \
  --timeout-coefficient 300 \
  --output <scratchpad>/gremlins.json \
  .

# Modo diff: solo las líneas que cambia la rama actual frente a main
gremlins unleash -D main --timeout-coefficient 300 -E 'internal/vmwasm/.*' .

Reglas operativas:

  • --dry-run primero, siempre: cuenta los mutantes RUNNABLE y permite estimar el coste (mutantes × duración de suite / workers) antes de lanzar.
  • --timeout-coefficient 300 es imprescindible aquí: gremlins calcula el timeout por mutante a partir de la duración base de go test, y la caché de Go la hace parecer de milisegundos → todos los mutantes saldrían TIMED OUT falsamente (pasó en el piloto de 2026-07).
  • Exclusión dura de internal/vmwasm: cada mutante recompila y re-ejecuta la suite; con wazero es prohibitivo (misma razón por la que CI lo separa).
  • Ejecuta desde la raíz del módulo.

Read the full file on GitHub · 88 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.

  1. 12d ago First seen · 88 lines · 79 tokens per session scan A 6ca68376abd8

Subscribe to this mod's changes

mutacion is a skill published in the GitHub repository dbareagimeno/enu (2 stars, last pushed 1mo ago), licensed Apache-2.0. It adds 79 tokens to every session and 1,207 once invoked, about $0.0004 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

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