Mutation-Resistant Coverage Skill

Mutation-Resistant Coverage

Core principle

Branches are not equal. A branch where success vs failure changes observable behavior is worth a test. A branch where two formatting outputs differ by a whitespace is not. You enumerate semantically distinct outcomes, write one high-signal test per outcome, and coverage falls out as a side effect.

You never write a test whose only purpose is to enter a branch.

When to invoke

Pushing a file toward 100% branch coverage.
A mutation-testing run (Stryker, Infection, Pitest, Mutmut) reports surviving mutants and you need to kill them.
Opening a PR where the coverage gate demands more tests and you want to add them without producing cosmetics.

Semantic branch taxonomy

Classify every condition in the target:

| Class | Example | Test required? | |-------|---------|----------------| | success vs failure | try/catch, Result.Ok / Result.Err | Yes, both sides | | empty vs non-empty | items.length === 0, null collection | Yes, both sides | | valid vs invalid | schema validation, type narrowing | Yes, both sides | | authorized vs unauthorized | role / permission check | Yes, both sides | | cached vs uncached | memoization, TTL expiry | Yes, both sides | | primary vs fallback | network → cache, default → override | Yes, both sides | | new vs existing | create vs update, insert vs upsert | Yes, both sides | | defensive | guard where caller already enforces the invariant | Consider deleting the branch | | cosmetic | whitespace-only format variants | No; one test is enough |

Workflow

Step 1 — Enumerate

Read the target file. Produce a branch map:

| Location | Branch | Class | Needs test? | Notes | |----------|--------|-------|-------------|-------| | file.ts:42 | items.length === 0 | empty/non-empty | yes | both sides | | file.ts:88 | user.role === 'admin' | auth/unauth | yes | both sides | | file.ts:104 | if (!input) | defensive | no | callers enforce; propose deletion |

Step 2 — Audit existing tests per branch

For each needs test = yes row, find the existing test(s). Grade them with the judging-test-quality rubric. A branch is "covered" only by a STRONG or ACCEPTABLE test. WEAK or KILL does not count as coverage.

Step 3 — Write the missing tests

Apply the patterns from improving-weak-tests. For every test, name the mutation it catches (in the commit message, not as a comment in the test).

Step 4 — Run coverage

Use the project's tool. Report both numbers:

Branch coverage % — the floor.
Mutation score % (if a tool is configured) — the signal.

Step 5 — Remaining gaps

Any uncovered branch after step 4 falls into exactly one bucket:

| Bucket | Action | |--------|--------| | Architecturally blocked (no seam to reach it) | Hand to reviewing-testability. Do not paper over. | | Dead defensive code | Propose deletion of the branch. | | Genuinely unreachable | Mark with an exhaustive-check in the code (assertNever, pattern-match-all), not a test. |

Hard rules

Never write a test whose sole purpose is to enter a branch.
Never count WEAK/KILL tests as coverage.
Never suppress coverage or analyzer warnings to claim completion.
Never leave a success branch covered without its failure sibling.
Never celebrate a coverage percentage without the mutation score.

If mutation testing is available

Run it. Examples of invocations:

JS/TS: npx stryker run
.NET: dotnet stryker
PHP: vendor/bin/infection
Python: mutmut run
Java: mvn org.pitest:pitest-maven:mutationCoverage

Surviving mutants are your backlog. Each surviving mutant points at a test that should have died and did not. Rewrite that test via improving-weak-tests.

Output

Three sections:

Branch map — the table from step 1.
Tests added — diff blocks per test, each naming the mutation caught.
Remaining gaps — table with bucket and action per remaining branch.

Final line in this exact shape:

Branch coverage: X% (up from Y%). Mutation score: P% (up from Q%). STRONG+ACCEPTABLE tests added: N. Architectural gaps: k. Dead-branch deletions proposed: m.

Upstream cause of why branches are hard to cover: reviewing-testability.
Grading existing tests: judging-test-quality.
Rewriting individual weak tests: improving-weak-tests.

Agent Skills: Mutation-Resistant Coverage

Install this agent skill to your local

Skill Files