Mutmut Survivors

Use this skill for mutation-testing work in this repo.

The repo already has one shared mutmut configuration in pyproject.toml. Use that file as the source of truth for mutation scope and test exclusions.

mutants/ is ignored by git and Bazel. Do not run multiple mutmut jobs at the same time, because they share that state directory.

Workflow

1. Prepare the repo:

- uv run tools/setuplocaldev.py

1. Start or resume mutation testing:

- broad run: uv run mutmut run - focused run: pass specific mutant names to uv run mutmut run - by default, prefer letting mutmut continue until there are no not checked mutants left - if you need a time budget for automation, wrap mutmut externally rather than looking for a mutmut timeout flag

1. Inspect findings:

- uv run mutmut results - uv run mutmut results --all true for the full state - uv run mutmut show MUTANTNAME - uv run mutmut tests-for-mutant MUTANTNAME

1. Produce a survivors report (see Report Generation below)
2. Fix the gap:

- prefer adding or improving tests - avoid changing production code unless the user explicitly asks for it - do not chase low-value survivors that only remove defensive built-in Python exceptions or code that exists only for pyright narrowing, such as isinstance(...) or is None checks whose purpose is type narrowing - use uv run mutmut apply MUTANT_NAME only when you need the exact mutant written into the working tree

1. Verify:

- run the most targeted pytest command that covers the new or changed tests first - rerun the specific mutant or focused mutmut pattern - finish with bazelisk run --noshowprogress //tools:format - finish with bazelisk test --noshowprogress //...

Report Generation

After a broad mutmut run completes, always produce a markdown report file (mutmutsurvivorsreport.md) with the following structure. Use parallel agents to analyze mutants across different source files concurrently.

Report structure

1. Header with run date, total mutants, kill rate, survived count, no-tests

count, timeout count.

1. Triage summary showing how many are interesting vs uninteresting.
2. Root cause sections — group survivors by the underlying reason they

survive (not by file or mutant name). Each section has: - A heading like ## Root Cause N: <short description> (M mutants) - A 1–3 sentence explanation of why these mutants survive - Files: listing the affected source files - Mutants: listing every individual mutant name as a bullet list

1. Uninteresting mutants section at the end, grouped by sub-category (e.g.,

super().init() args, typing.cast() strings, ValueError/KeyError guard text, timing/stats fields, equivalent mutants, unreachable code, type narrowing). Each sub-category lists its mutant names.

Uninteresting mutant categories

These should be classified as uninteresting and listed in the uninteresting section rather than as root causes:

• typing.cast() string arg mutations (runtime no-op)
• isinstance(...) or is None checks used only for pyright type narrowing
• ValueError/KeyError guard message text for states that can never happen
• Timing/stats initial values and arithmetic
• Equivalent mutants (False → None in boolean context, case-insensitive

codec names, removing case _: pass, etc.)

• Unreachable match wildcards

Parallel analysis

When analyzing survivors, launch parallel agents to inspect mutants from different source files simultaneously. For example, if survivors span 5 source files, launch up to 5 agents — one per file — each running mutmut show on its set of mutants and reading the relevant source code and tests. Merge the agents' findings into the final grouped report.

How To Triage A Mutant

Use mutmut show first. It is the fastest way to see what changed without touching tracked files.

Then use mutmut tests-for-mutant to find the tests mutmut considers relevant. Treat that as the first test set to understand, but not necessarily the complete one.

Prioritize these statuses:

• survived: best target for writing a stronger assertion or adding a missing

test case

• no tests: usually means the mutated code path is not exercised by the

selected tests

• timeout or suspicious: often means the mutant triggered a slow or unstable

path and may need manual inspection

Ignore not checked until the run has progressed far enough to produce completed results.

Agent Guidance

For automated work, prefer this loop:

1. Narrow mutmut to one module or function pattern.
2. Read mutmut results.
3. Pick one survived mutant.
4. Read mutmut show MUTANT_NAME.
5. Read mutmut tests-for-mutant MUTANT_NAME.
6. Inspect the affected source file and nearby tests.
7. Add or strengthen tests.
8. Run targeted pytest.
9. Re-run the same mutant pattern with mutmut.
10. Only after the focused mutant is killed, move on or run broader

verification.

Prefer mutmut show over mutmut apply unless the exact on-disk mutated file is necessary. apply is useful, but it dirties the working tree and adds cleanup overhead.