mewt and muton Mutation Testing

What it does

When your agent uses it

• Setting up and scoping a mutation testing campaign with mewt or muton
• Making a slow campaign run faster
• Analyzing surviving mutants and spotting equivalent ones
• Using escaped mutants to find bugs in untested code

Example prompts

• “Set up a mewt campaign for the src directory and trim it so it finishes sooner.”
• “Our muton run finished. Which surviving mutants are equivalent and which hide real bugs?”
• “Write a formal report from the mewt results, with severity ratings.”

Requirements

• mewt or muton installed
• Pre-approved tools (allowed-tools): Read, Write, Bash, Grep

What it can do on your machine

Read from SKILL.md and the folder at commit 82fe822. It shows what the files ask for, not the result of running them.

• Tool permissions

  Pre-approves these tools, so the agent can use them without asking each time:

  • Read
  • Write
  • Bash
  • Grep

  From allowed-tools in the SKILL.md frontmatter.

• Runs code

  No scripts in the folder and no shell commands in SKILL.md (its code samples are bash).

  From the folder's file list and the shell code blocks in SKILL.md.

• Network

  No URLs in SKILL.md.

  From URLs in SKILL.md, links to its own repository left out.

• Credentials

  Names no API keys, tokens, secrets or passwords.

  From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

mewt and muton Mutation Testing loads about 1.5k tokens when it runs, and up to ~10k if it reads all its reference files. Until then it costs about 67 tokens; SKILL.md has 559 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it

~67

When it runs · the whole SKILL.md, loaded when a task matches

~1.5k

With references · SKILL.md plus every file in references/, read only if the agent opens them

~10k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

• NotePre-approves every shell command (allowed-tools: Bash)SKILL.md

  allowed-tools: Read, Write, Bash, Grep

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from trailofbits/skills at commit 82fe822, republished under its CC-BY-SA-4.0 licence (© trailofbits). 559 words, ~1,549 tokens.

name

mutation-testing

description

Configures mewt or muton campaigns, analyzes surviving mutants, and investigates bugs exposed by testing gaps. Use when setting up mutation testing, reviewing campaign results, identifying equivalent mutants, or finding bugs from surviving mutations.

allowed-tools

Read, Write, Bash, Grep

# Set up and run
mewt init                    # Create config and database
mewt mutate [paths]          # Generate mutants without testing them
mewt run [paths]             # Generate mutants and run the campaign

# Read results
mewt status                  # Overview with per-file breakdown
mewt results                 # Uncaught mutants (default view)
mewt results --all           # Every outcome, not just uncaught
mewt results --format json   # json | sarif | ids | table

# Narrow down (these filters work on both `results` and `print mutants`)
mewt results --target 'src/auth/**'   # Quote globs so the shell does not expand them
mewt results --severity high,medium
mewt results --mutation-types ER,CR
mewt results --status Uncaught        # Uncaught | TestFail | Skipped | Timeout
mewt results --line 42

# Investigate and re-test
mewt print mutant --id [id]              # View the mutated code
mewt test --ids [ids]                    # Re-test specific mutants
mewt test --ids-file uncaught_ids.txt    # Re-test IDs from a file, or '-' for stdin

# Inspect configuration
mewt print config                        # Effective config
mewt print targets                       # Files actually mutated
mewt print mutations --language [lang]   # Mutations and severities for a language

© trailofbits, CC-BY-SA-4.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 11 other files (references, assets) in plugins/mutation-testing/skills/mutation-testing of trailofbits/skills.

• SKILL.md
• agents/openai.yaml
• assets/trail-of-bits-mark.svg
• references/blockchain-patterns.md
• references/equivalent-mutants.md
• references/input-formats.md
• references/optimization-strategies.md
• references/report-template.md
• references/severity-classification.md
• workflows/analyzing-results.md
• workflows/bug-hunter.md
• workflows/configuration.md

Open the folder on GitHubat commit 82fe822