Mutation testing as a hook, not a hope.

mutagate is for individual developers who let Claude Code or Codex write tests (OpenCode and Pi work too). Every time your agent writes a test, mutagate mutates the code under test in the background and tells the agent which bugs slipped through (">= became >; no test noticed"). If the tests catch fewer than 80%, the agent can’t finish yet, for up to two retry rounds by default. Team leads: commit the skill dir and .mutagate/config.json, and every clone gets the same gate.

Hooks run your build and tests outside the agent sandbox; see SECURITY.md.

Status: 0.9.0 is a beta; config keys and report text can change before 1.0. Platforms: macOS and Linux supported; Windows experimental.

Get started

What's supported

✅ validated · 🟡 supported with caveats · ⏳ validation pending · ❌ unsupported

Languages

Language	Mutation engine	Build / test runner	You provide	Engine install	Validated
Java	PIT 1.21.0	Gradle/Maven · JUnit 4/5	JDK 11+	auto, SHA-pinned	✅ Gradle, Gradle multi-module, Maven
Kotlin	PIT + junk filter (Tier B), Arcmutate (Tier A, licensed)	Gradle/Maven · JUnit 4/5	JDK 11+	auto, SHA-pinned	✅ Gradle, Kotlin/Java mixed · ⏳ Tier A
Python	mutmut 2.5.1 / 3.8.0	pytest · pip or uv	mutmut + pytest in project env	project	✅ pip, uv
TypeScript / JavaScript	Stryker 9.1.1	Jest 29 or Vitest 3	@stryker-mutator/core + runner in project	project	✅ Jest, Vitest
Go	gomutants 0.6.1, gremlins 0.6.0 fallback	go test	Go 1.26+	auto, cached	✅ nested-package module · 🟡 scoping caveats
C#	Stryker.NET 5.0.0	dotnet test · xUnit	.NET SDK 10	auto, cached	✅ xUnit · 🟡 slow cold builds

Project-provided engines:

pip install mutmut pytest                                   # Python, pip: in the project venv
uv add --dev mutmut pytest && uv sync                       # Python, uv
npm i -D @stryker-mutator/core@9 @stryker-mutator/jest-runner@9    # TS/JS, Jest (Vitest: @stryker-mutator/vitest-runner@9)

Harnesses

Harness	Wired via	Completion	Agent waiver edits
Claude Code	hooks, .claude/settings.json	native block	asks you
Codex CLI	hooks, .codex/hooks.json (trust in /hooks)	native block	denied
OpenCode	plugin, .opencode/plugins/mutagate.js	continuation	denied
Pi	extension, .pi/extensions/mutagate.ts	continuation	denied

Live eval (npm run eval), natural and forced scenarios:

Harness	Java	Kotlin	Python	TypeScript	Go	C#
Claude Code	✅	✅	✅	✅	✅	✅
Codex CLI	✅	✅	✅	✅	✅	✅
OpenCode	✅	✅	✅	✅	✅	✅
Pi	✅	✅	✅	✅	✅	✅

✅ both scenarios passed · ⏳ missing scenario · ❌ mutagate/harness gap. Each scenario runs twice and passes when one run passes (ADR-0011). Reliability: one of two runs failed for OpenCode C# natural, Pi C# natural, Pi Java natural, Pi Python natural and Pi Python forced. Records: test/evidence.

OpenCode forwards only write/edit/patch/apply_patch tools; Pi only write/edit.

One-shot caveats: OpenCode/Pi need persistent sessions (interactive, opencode serve, pi --mode rpc); opencode run/pi -p cannot reprompt. Without streaming input, claude -p waits for the async hook on each test write.

Quick start

Needs Node 22+, Git and the language toolchain above; no npm dependencies or build step. Project build files/configs stay untouched. Older target frameworks need their runtime. Gate mode is the default. For reports without blocks, set MUTAGATE_MODE=advise or "mode": "advise" in .mutagate/config.json.

Run one block in the project root (Git top level):

# Claude Code
npx skills add keyboardsamurai/mutagate#skill -a claude-code --copy
.claude/skills/mutagate/scripts/mutagate install --agent claude
.claude/skills/mutagate/scripts/mutagate doctor

# Codex CLI (then open /hooks in Codex and trust mutagate)
npx skills add keyboardsamurai/mutagate#skill -a codex --copy
.agents/skills/mutagate/scripts/mutagate install --agent codex
.agents/skills/mutagate/scripts/mutagate doctor

# OpenCode
npx skills add keyboardsamurai/mutagate#skill -a opencode --copy
.agents/skills/mutagate/scripts/mutagate install --agent opencode
.agents/skills/mutagate/scripts/mutagate doctor

# Pi
npx skills add keyboardsamurai/mutagate#skill -a pi --copy
.pi/skills/mutagate/scripts/mutagate install --agent pi
.pi/skills/mutagate/scripts/mutagate doctor

1. doctor checks config, toolchain, jars, adapter and one synthetic mutation. The first run can take 1–3 min (downloads, cold build). Its first line is mutagate 0.9.0; a full pass ends with All checks passed. Restart your agent so it loads the hooks.
2. Restart the agent. Codex: open /hooks and trust mutagate. The session starts with a mutagate 0.9.0 active. runner=… line.
3. Ask the agent to write a test for one production file. A weak test gets the first block, with the survivors.
4. Commit the skill dir and the harness config. Project installs inside Git use repo-relative hook commands (ADR-0013), so clones get the gate.

A successful install ends with Next: run <skill>/scripts/mutagate doctor, then restart your agent so it loads the hooks. (Codex adds In Codex, open /hooks and trust mutagate.). Use an explicit --agent. --agent auto installs for the harness dirs it finds; if it finds none, it prints no harness detected in <repo>; run one of: and one <skill>/scripts/mutagate install --agent <harness> line per harness on stderr, and exits 2.

• Pinned: replace #skill with #skill-v0.9.0.
• Upgrade: run the same npx skills add line again, then install --agent <harness> again (Codex: trust again).
• Uninstall: <skill dir>/scripts/mutagate uninstall --agent <harness>, then npx skills remove mutagate -y.

All commands accept --repo <directory> (Windows: scripts\mutagate.cmd). Install keeps unrelated hooks. User scope (--scope user), a skill outside Git, and Windows write machine-local absolute paths: do not commit them. Claude started in a subdirectory may not load root settings.

After maxRounds, completion warns/allows; a large value keeps blocking until it passes. Infrastructure errors never block; agent-caused red/non-compiling tests block once.

Troubleshooting

<skill> is the skill dir from the Quick start.

Symptom	Cause	Command
No mutagate … active line at session start	Hooks not installed, or agent not restarted	<skill>/scripts/mutagate doctor, then restart the agent
Codex never runs the hooks	Codex does not trust new or changed hooks	Open /hooks in Codex, trust mutagate (after each install)
Claude ignores the hooks	Claude started in a subdirectory, root .claude/settings.json not loaded	Start claude in the Git top level
mutagate inactive: hooks run Node …; need 22+	Hooks found Node < 22	MUTAGATE_NODE=/path/to/node22, or rerun install under Node 22+
jar download failed / offline: missing …	No access to Maven Central (proxy, offline)	node <skill>/scripts/cache-jars.mjs, then export MUTAGATE_OFFLINE=1
C# result: run budget exceeded	Cold dotnet builds take 2–5 min	.mutagate/config.json: "runBudgetSec": 600
Python uv: mutmut missing or uv run fails	Project env not synced	uv add --dev mutmut pytest && uv sync
WARN <harness> version: X differs from tested Y	Installed harness is newer or older than the tested pin	None needed: doctor still passes. Pins: <skill>/references/versions.json; results: Harnesses
Need details on any of the above	—	MUTAGATE_TRACE=1 writes <data dir>/trace.jsonl; runner log: <data dir>/logs/<target>.log (previous: .prev.log)

The data dir is ~/.cache/mutagate/repos/<sha1 of repo path>/ (State). For bug reports attach doctor --json and status --json.

Commands

Command	Behavior
run <test> [--target <fqn>]	Scoped background run; exit 0 = scheduled.
run <test> --sync [--json]	Foreground run/report.
check [--wait <sec>] [--json]	Gate all session targets; --wait also raises the gate deadline.
status [--json]	Scores, staleness, survivor/waiver ids, unresolved tests; JSON adds file/tests, block counts, Kotlin exclusions.
detect	Harness, language, build tool, runner, Kotlin tier (JSON).
doctor [--json]	First line mutagate <version>, then PASS/FAIL/WARN lines; checking <x>… progress on stderr. Config, toolchain, jars, adapter, synthetic mutation; failure hint, passing note fields. WARN (does not fail) when an installed claude/codex/opencode/pi differs from the tested pin. --json: {version, checks, versions}.
waive <id> --reason <text>	Propose; --apply is refused in agent sessions unless allowAgentWaivers.
install / uninstall	Manage hooks/adapters (--agent, --scope project|user).
--version / version	Print mutagate <version>, exit 0.

run --sync/check exits: 0 pass, 1 below threshold/unscored (no mutants to test passes), 2 usage error, 3 result/infrastructure error. Zero-target check: exit 1, explained. Usage errors (missing/outside tests, bad wait/waive arguments, absent survivor ids, unknown commands) are not traced as errors. <command> --help/-h: usage, exit 0. Hooks fail open on exceptions; exit 2: block or Claude async delivery.

Errors: Stop/check/run --sync retry once per fingerprint. Agent-caused errors name registered/glob-matched tests or edited production (Kotlin e: too) or report red baseline tests; reports ask for fixes, async delivery matches low scores. Stop blocks once/target within maxRounds, then warns/allows; other errors warn/allow. Python scoped-copy-only failures fail open; pytest exit 5 is no red baseline. Deleting every registered test: explained unscored target, blocked once like pending. Final unscored reports explain empty, errored, uncovered or budget-limited results. Gate-clipped kills stay pending.

Sessions: Workers/OpenCode/Pi use MUTAGATE_HOOK_SESSION; Codex/Claude supply CODEX_THREAD_ID/CLAUDE_CODE_SESSION_ID. Hookless, export one MUTAGATE_SESSION for run/check across shells. Hook/harness ids win.

Configuration

Share .mutagate/config.json; keys/defaults exported as DEFAULTS:

{"schema":1,"mode":"gate","threshold":0.8,"maxRounds":2,"countNoCoverage":true}

Precedence: flags > MUTAGATE_* env > project > user > defaults (threshold → MUTAGATE_THRESHOLD/--threshold). mode: "advise" never blocks. countNoCoverage: false scores only mutants the registered tests reach (test strength); uncovered mutants are reported, not counted. Budgets: run 300 s, gate deadline 120 s, in-flight wait 60 s; check --wait N raises the gate deadline (CLI only). Reports: ≤10 survivors, ≤1,800 characters. Nonempty testGlobs replaces conventions; mainRoots adds JVM roots. Invalid config warns once/session, uses defaults.

State: CLAUDE_PLUGIN_DATA, else $XDG_CACHE_HOME/mutagate or ~/.cache/mutagate (Windows: %LOCALAPPDATA%\mutagate). Fingerprints: project inputs/dependencies/resources and ancestor build files, not unrelated modules/docs. Production edits invalidate targets; check/status list untested edits. After 6 s idle, async PostToolUse reruns stale targets, suppressing duplicate reports. Wait up to gateWaitSec for repo Maven/Gradle wrapper builds (ps; not Windows) to avoid overlapping compilation. Unresolved tests: announced once, then at completion until selected.

install saves its Node 22+ path in gitignored scripts/.node-path. scripts/mutagate/hook/adapters resolve Node from MUTAGATE_NODE, that file, then PATH; under Node <22 hooks stay inactive with a SessionStart notice. MUTAGATE_TRACE=1: JSONL events; logs: data-dir logs/<target>.log (named in infrastructure errors). MUTAGATE_DEBUG=1 keeps report dirs/verbose PIT output. No uploads.

Runners

Discovery ignores dist/ for every language.

• Java/Kotlin: wrappers honored. Builds supply classpaths without edits; standalone PIT uses incremental history. Undeclared JUnit 5 junit-platform-launcher resolves via the project build/engine version. Maven race: PIT's mvn -q test-compile writes target/; overlapping agent builds may read half-written classes ("Extra bytes at the end of class file"). Rerun or use mvn clean test.
• Python: uv.lock or pyproject [tool.uv], [dependency-groups] or uv_build selects uv run --no-sync (run uv sync first). Mutates a scoped copy, including src-layout packages. conftest.py/__init__.py are never registered.
• TS/JS: nothing installed into the project.
• Go: gomutants mutates only the target file; -skip excludes sibling-test Test\w* names, but sibling Example/Fuzz functions may run. History: data dir. Module-root bin//*.out, *.test, *.exe and extensionless executables never invalidate results. Options: goRunner: "auto" | "gomutants" | "gremlins", goTags: "tag1,tag2", MUTAGATE_GO_RUNNER, --go-tags. Auto: cached/PATH gomutants → install → cached/PATH gremlins; forced gremlins may install. Gremlins mutates one file, runs broader package tests; subdirectory/package names must match or 0.6.0 counts build failures as kills. Excluded: *.pb.go, *_gen.go, zz_generated*.go, mock_*.go, *_mock.go.
• C#: nearest .csproj per source/test (one project/directory). test-case-filter joins FullyQualifiedName~<namespace>.<class> (first namespace/file; comments/strings ignored) per existing registered-test class; substring match: PricingTests also hits PricingTestsExtra. Persistent scratch copy reuses build outputs; source/test-project locks serialize Stryker builds. Test projects (Microsoft.NET.Test.Sdk, MSTest.Sdk, xunit, nunit, mstest references or <IsTestProject>true) yield no target candidates. Target-file mutation only: Standard, no String. Excluded: *.Designer.cs, *.g.cs, *.g.i.cs, Migrations/*.cs, StrykerOutput/, TestResults/. bin//obj/ skip discovery/fingerprints only beside .csproj, else stay inputs. Cold builds take 2–5 min: set "runBudgetSec": 600. Stryker Ignored mutants (some Block removal, configured exclusions) keep ignored: true in raw results; no score/error-count impact.

Property tests (Hypothesis, jqwik, fast-check, rapid, FsCheck) run unchanged. Pin the seed and cap examples in the test itself: results cache per fingerprint, mutants killed by timeout count as kills, the run budget spans every example per mutant. Name property files by module (test_<module>_props.py, FooPropertyTest) or pass --target; status --json shows timed_out.

Centralized .NET UseArtifactsOutput (artifacts/obj without sibling .csproj) needs exclusion: "exclude": ["**/generated/**", "**/build/**", "artifacts/**"]. Else generated inputs invalidate/retry publication. Ambiguous C# targets need --target; ProjectReferences never select.

Go/C# use scratch configs/reports; project mutation configs stay unmerged. Tools: <cache-root>/tools/<name>@<version>/ → PATH → permitted install. MUTAGATE_OFFLINE=1 disables downloads, not build-tool dependency resolution; .NET doctor skips/fails its NuGet smoke offline. Outside v1: .NET Framework, F#, VB.NET, go-mutesting v2, baseline/diff mutation, configurable C# ignored mutations.

PIT jars: pinned in references/jars.json, Maven Central only; Gradle/Maven resolve dependencies separately. Node fetch ignores HTTP(S)_PROXY: preseed <cache-root>/jars/ via curl, then run offline:

node scripts/cache-jars.mjs # optional: --base-url https://repo.maven.apache.org/maven2 or --tier-a
export MUTAGATE_OFFLINE=1

Kotlin

Tier B (default, unlicensed): source-aware checks, references/kotlin-junk.yaml and PIT's Kotlin filter. Data-class synthetics (equals, hashCode, toString, componentN, copy) are excluded only for proven top-level data class owners, including same-file empty marker interfaces. Explicit overrides, inherited/nested/uncertain cases stay scored. Inline-function call-site mutants stay unfiltered. Corpus measurements.

Tier A: arcmutate-licence.txt, ARCMUTATE_LICENCE or kotlinTier: "A" loads pinned Arcmutate instead of Tier B. Bring your license; plugin not bundled.

Waivers

Agents propose (waive <id> --reason <text>); humans approve (--apply or .mutagate/waivers.json). Workflow control, not a sandbox; allowAgentWaivers: true opts out. IDs bind file, line, method, operator, mutation identity. Moving a line invalidates the waiver; the gate reports it.

Links

Evidence, platforms and limitations · Security · Third-party tools · Changelog · Contributing