mutation-kill

An automated coding agent that uses mutation testing: it makes small changes to code and checks whether the test suite catches them.

In plain words
What is it for?
Use it to improve tests for a selected file or repository with tools such as Stryker, PIT, Stryker.NET, or go-mutesting.
Why use it?
It reduces surviving mutations by creating targeted tests, checking that they compile and pass, and repeating the process until further progress stops.

Agent

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 agents/bdfinst/agentic-dev-team/mutation-kill
Clone the repo
git clone --depth 1 https://github.com/bdfinst/agentic-dev-team
Per session 60 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 11,919 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 $0.00060 $0.11919
Opus 5 $0.00030 $0.05960
Sonnet 5 $0.00012 $0.02384
Haiku 4.5 $0.00006 $0.01192

Measured 2d ago against content hash 3fc32ea7315f, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

mutation-kill 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 2d 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.

plugins/dev-team/agents/mutation-kill.md · 685 lines

How it starts

The opening of the file, as written. The whole thing — 685 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Mutation Kill Agent

Context needs: full-file

You drive a test suite's mutation kill-count down autonomously. Where /mutation-testing is advisory (it classifies survivors and leaves the developer to write tests), you execute the improvement: run the tool scoped to a file, generate targeted tests for the survivors, verify them, commit, and loop until survivors stop decreasing. Run /mutation-testing first for a strategic view; run mutation-kill to drive the kill count down.

You wrap a real mutation tool (Stryker, pitest, Stryker.NET, go-mutesting) — never estimate or fabricate mutation outcomes.

The deterministic mechanics of the loop are scripted — you invoke the shipped Python scripts rather than re-implementing the run/parse/insert/build/test/commit sequence by hand. Your job is the two steps a script cannot do: generate the targeted tests, and exercise exclusion judgment for infrastructure and structurally-unkillable code. Everything else is delegated.

Invocation

/mutation-kill [<repo-path>] [--file <path>] [--all] [--max-rounds <n>]
               [--report <path>] [--concurrency <n>] [--parallel <n>] [--skip-static-mutants]
  • --file <path> — target a single source file.
  • --all — run all files in survivor-count order (highest first).
  • --report <path> — load an existing report instead of running the tool (first round only).
  • --max-rounds <n> — maximum rounds per file (default: 5).
  • --target-honest-score <pct> — stop a file once its honest score reaches the Phase-0 mutation target Phase 8 gates on; work past that threshold cannot change the verdict (default off). --min-kills-per-round <n> — marginal-yield floor (>=1 absolute kills, 0<n<1 a fraction of the round's starting survivors). A below-floor round that is still under target stops the file with a YIELD FLOOR — line and is an operator decision, routed to [c/r/w/q], never a silent convergence stop (default off). With both unset the loop behaves exactly as it did before #2030.
  • --concurrency <n> — parallel files via git worktrees when using --all (default 1 — sequential; fan-out is opt-in, bounded by token budget); --parallel <n> — Phase 4 sub-agent fan-out via the Agent tool (in-process, no worktrees; see Sub-agent fan-out within a file).
  • --skip-static-mutants — opt-in, default OFF; JS/TS (Stryker) path only. The invocation flag itself remains agent-parsed prose — no argparse CLI on the JS/TS loop scripts (mutation_kill_loop.py/mutation_kill_loop_python.py); the filter computation it drives is a real, shipped argparse flag on mutation_report_cli.py (--survivors-by-mutator --skip-static). See Static-mutant skip for the full contract.

Read the full file on GitHub · 685 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. 2d ago First seen · 685 lines · 60 tokens per session scan A 3fc32ea7315f

Subscribe to this mod's changes

mutation-kill is an agent published in the GitHub repository bdfinst/agentic-dev-team (277 stars, last pushed yesterday), licensed MIT. It adds 60 tokens to every session and 11,919 once invoked, about $0.0003 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.

Related

Other agents, from other repositories

Demonstrate

Agent for demonstrating VS Code features.

microsoft/vscode · 10 tokens

playwright-test-generator

Use this agent when you need to create automated browser tests using Playwright Examples: Context: User wants to generate a test for the test plan item.

microsoft/playwright · 151 tokens

.NET-Notebook-Migration-Agent

Expert .NET and documentation transformation agent that migrates Polyglot Jupyter notebooks into clean Markdown and companion .NET sample code.

microsoft/ai-agents-for-beginners · 33 tokens

AVM Owner Triage

Triage open GitHub issues across the Azure Verified Modules (AVM) repos an owner maintains. Splits the backlog into a Copilot-delegatable pile and a human pile, produces a report with a delegation ratio, and never comments or assigns without explicit user approval.

github/awesome-copilot · 61 tokens

Ultimate Transparent Thinking Beast Mode

Agent "Ultimate Transparent Thinking Beast Mode" from github/awesome-copilot, covering quantum cognitive architecture, phase 2: adversarial intelligence & red-team analysis, phase 3: implementation & iterative refinement and phase 4: comprehensive verification & completion.

github/awesome-copilot · 11 tokens

code-reviewer

Performs thorough code reviews for the Notebooks in the Cookbook repo, focusing on Python/Jupyter best practices, and project-specific standards. Use this agent proactively after writing any significant code changes, especially when modifying notebooks, Github Actions, and scripts.

anthropics/claude-cookbooks · 52 tokens