mutate

A mutation-testing command that deliberately makes small changes to source code and checks whether the tests detect them. The mutation score shows how many of these artificial faults the tests catch.

In plain words
What is it for?
Checking test quality, creating or using a mutation-testing tool, running mutations against source code, and reporting surviving mutants.
Why use it?
Passing tests do not always prove that tests would catch real mistakes; surviving mutations reveal gaps in test coverage or assertions.

Command

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.

agentmods
npx agentmods add commands/swingerman/engineer/mutate
Clone the repo
git clone --depth 1 https://github.com/swingerman/engineer
Per session 34 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 525 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
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 $0.00034 $0.00525
Opus 5 $0.00017 $0.00262
Sonnet 5 $0.00007 $0.00105
Haiku 4.5 $0.00003 $0.00052

Measured yesterday against content hash b51de204f583, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

mutate 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 yesterday.

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.

commands/mutate.md · 48 lines

What it actually says

Run mutation testing on this project.

Instructions

  1. Check that both test streams are green before proceeding:

    • Run the unit tests
    • Run acceptance tests if a pipeline exists (./run-acceptance-tests.sh)
    • If either fails, report the failure and stop — do not mutate against a red baseline
  2. Detect the project language and check if a mutation tool exists in the project.

    • Preferred: Build a custom mutation tool using TDD — a project-specific module that walks the AST, applies mutations one at a time, runs targeted tests, and reports survivors. See the atdd-mutate skill for the 4-module architecture (mutations, runner, core, hashing/selection) and mutation categories.
    • Alternative: If the user requests a faster setup, install an existing framework (Stryker, mutmut, PIT, etc. — see the atdd-mutate skill).
  3. Configure the tool to:

    • Target source code only (not tests, specs, generated files, or pipeline code)
    • Exclude generated-acceptance-tests/, acceptance-pipeline/, and specs/
    • If $ARGUMENTS is provided, scope mutations to that path
  4. Run the mutation tool. On the custom-tool path, run dae_mutmap.py select first and mutate only the changed functions it returns (differential mutation testing — see the atdd-mutate skill); on the framework path, enable the framework's incremental flag.

  5. Report results:

    • Mutation score (percentage)
    • Total mutants, killed, survived
    • List each surviving mutant with: file, line, mutation description
    • Assessment: strong (90%+), moderate (70-89%), or weak (<70%)
    • On the custom-tool path, run dae_mutmap.py update to refresh mutation-manifest.json; combine fresh results with cached entries for unchanged functions
  6. Ask whether to proceed with killing surviving mutants (invoke /atdd:kill-mutants)

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. yesterday First seen · 48 lines · 34 tokens per session scan A b51de204f583

Subscribe to this mod's changes

mutate is a command published in the GitHub repository swingerman/engineer (144 stars, last pushed 5d ago), licensed MIT. It adds 34 tokens to every session and 525 once invoked, about $0.0002 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 2026-08-30.