mutant-survival-triage

Turns "this mutant survived" into "here is the specific test that would kill it, and here is the input that separates the mutant from the original."

What this owns, and what it does not

Owns: one surviving mutant at a time. Reading the mutated line and the tests that already covered it, deciding which of five reasons explains the survival, and writing a concrete test proposal (inputs plus the assertion) that would flip the mutant to killed.

Does not own: installing or configuring a mutation testing tool, choosing mutate globs, picking a mutation-score threshold, or wiring a build gate. Those are per-tool configuration decisions and are settled before a report exists. This work starts at the report and ends at a test proposal a human writes.

Step 1 - Normalize the survivor into one record

Every tool reports the same underlying facts in a different container. Flatten each survivor into one record before classifying, so the heuristics below stay tool-independent:

interface SurvivedMutant {
  tool: 'stryker' | 'pit' | 'mutmut' | 'mull';
  file: string;
  line: number;
  column?: number;
  mutator: string;      // tool-native operator name, kept verbatim
  original: string;     // the source text before mutation
  mutated: string;      // the source text after mutation
  coveredBy: string[];  // tests that executed the line and did not kill it
}

Where each field comes from per tool, and why the non-killed statuses must not be merged (Survived vs NoCoverage / no-coverage, Timeout, and the rest), is in references/tool-normalization.md. One rule from there is load-bearing for Step 2: a no-coverage mutant is never a weak-assertion finding, because no assertion ran.

Step 2 - Classify why it survived

Work the classes in this order. The cheap, tool-asserted ones come first, and equivalent-mutant comes last because it is the only class that cannot be proven (Step 5).

The missing-case versus weak-assertion split is the whole decision: both look identical in the report, and only reading the covering test tells them apart. Ask one question: did any covering test already run the separating input? If yes the test is the defect; if no the suite is.

Step 3 - Per-mutator heuristics

The four mutation families below cover most survivors. Tool operator names differ, so match on the family, not the string. The full operator-name-to-family mapping across StrykerJS, PIT, mutmut, and Mull, with sources, is in references/tool-normalization.md.

Conditional boundary

if (qty > maxQty) throw new Error('Cap exceeded');   // original
if (qty >= maxQty) throw new Error('Cap exceeded');  // mutant, survived

The separating input is always the boundary value itself. Here only qty === maxQty distinguishes the two: the original does not throw, the mutant does. Any test at qty < maxQty or qty > maxQty behaves identically under both and can never kill this mutant, no matter how strict its assertion.

Propose: one test at exactly the boundary, asserting the original behavior.

Arithmetic operator

const total = subtotal + tax;   // original
const total = subtotal - tax;   // mutant, survived

Two survival causes, and they need different fixes:

• Every covering test uses the operator's identity value, so both forms agree (tax === 0 here; 1 for a multiply-to-divide mutant). That is missing-case: add a test with a non-identity operand.
• A covering test does use a non-identity operand, but asserts a range or a truthiness check instead of the value, for example expect(total).toBeGreaterThan(0). That is weak-assertion: replace it with exact equality on the computed number.

Propose: the non-identity operand plus an exact-value assertion.

Statement or call removal

notifyUser(orderId);   // original
                       // mutant: the call is gone, and it survived
return success;

A removed call survives whenever its only effect is outside the value under assertion. The return value is unchanged, so an assertion on the return value can never kill it. This is almost always missing-case for an unobserved side effect, not a loose assertion.

Propose: verify the effect, not the return. A test double on the collaborator asserting it was called with orderId, or an assertion against the state the call was supposed to change.

Constant or literal

const PAGE_SIZE = 42;  // original
const PAGE_SIZE = 0;   // mutant, survived

A surviving constant mutation means no assertion is coupled to the constant's value. The common shapes: the behavior the constant drives (here, pagination) has no test at all, or the test hard-codes its own copy of the value and never reads the production one. The second shape is the trap, because the test looks like coverage.

Propose: an assertion on the behavior the constant drives, reading the production constant rather than restating the literal. Note that PIT's default return mutators (EMPTY_RETURNS, NULL_RETURNS, PRIMITIVE_RETURNS) produce the same shape at the return boundary (PIT mutation operators).

Step 4 - Propose the specific test

A proposal is complete only when all four are present:

1. The separating input. The concrete value on which the original and the mutant diverge. If you cannot name one, the classification is equivalent-mutant, not missing-case.
2. The observable that differs. Return value, thrown error, persisted state, or a collaborator call. A test that cannot observe it cannot kill the mutant.
3. The exact assertion, with the expected value written out.
4. Whether an existing test should be tightened instead of a new one added. Adding a second test next to a weak one leaves the weak assertion in place.

Emit one block per survivor:

**Surviving mutant:** `src/cart.ts:42` - Equality Operator / GreaterThanBoundary
**Original:** `if (qty > maxQty) throw new Error('Cap exceeded');`
**Mutated:**  `if (qty >= maxQty) throw new Error('Cap exceeded');`
**Class:** missing-case (boundary)

**Covering tests that did not kill it:**
- `cart.spec.ts > addItem qty=1`   - `1 > 100` and `1 >= 100` are both false; identical behavior.
- `cart.spec.ts > addItem qty=101` - `101 > 100` and `101 >= 100` are both true; identical behavior.

**Separating input:** `qty === maxQty === 100`. Original: no throw. Mutant: throws.

**Proposed test (new):**
```ts
it('accepts a quantity exactly at the cap', () => {
  const cart = new Cart({ maxQty: 100 });
  expect(() => cart.addItem({ qty: 100 })).not.toThrow();
  expect(cart.items).toHaveLength(1);
});
```

**If a boundary test already exists and the mutant still survived,** the suite is
not the only suspect. Either the assertion does not observe the throw (for example
it wraps the call in a try/catch and asserts nothing), or the production boundary
is off by one and the existing test encodes the wrong expectation. Confirm the
intended cap semantics against the specification before changing either side.

That last paragraph is the point of the whole step: a survivor is evidence that the test and the production code disagree with the specification somewhere, and "add another test" is only one of the two possible resolutions.

Step 5 - Equivalent mutants and the residual survivor rate

equivalent-mutant is a judgment, not a proof: finding equivalent mutants is undecidable in general (Straubinger, Degenhart and Fraser, 2024, citing Budd and Angluin, Acta Informatica 18, 31 to 45, 1982). StrykerJS agrees "there is no definitive way for Stryker to find and ignore them" and to "accept that you won't make 100%" mutation score (Stryker equivalent mutants); PIT calls them "equivalent mutations" and filters some by not mutating lines that call common logging frameworks (PIT basic concepts).

Practical consequences for triage:

• A non-zero survivor count is the expected steady state, not an open defect queue. Track the count of survivors classified equivalent-mutant separately from the ones classified as test gaps, or the backlog never reaches zero and the team stops trusting it.
• Every equivalent-mutant call ships with its reasoning, naming why no input can separate the two forms. A second reader has to be able to disagree. An unexplained exclusion is indistinguishable from a missed test.
• Recurring equivalent shapes are worth fixing at the source. StrykerJS's own examples include neutral operations such as number1 += 0 and comparisons where both sides are provably equal, and it suggests rewriting those code patterns (Stryker equivalent mutants).

If you quote a mutation score, state the convention. StrykerJS documents two: detected / valid * 100, which counts uncovered mutants against you, and detected / covered * 100, which does not (mutant states and metrics). Neither excludes equivalent mutants, because no tool can identify them reliably, so a score is always an underestimate of test strength by an unknown margin. Comparing two scores computed under different conventions is meaningless.

Output format

## Mutation survivor analysis - <run id>

**Tool:** stryker | pit | mutmut | mull
**Survivors analyzed:** N
**Mutation score:** X% (convention: detected / valid)

| Class | Count | Recommended action |
|---|---:|---|
| missing-case | 14 | Add one test per separating input. |
| weak-assertion | 7 | Tighten the named existing assertion. |
| equivalent-mutant | 3 | Exclude, reasoning recorded below. |
| unreachable / no coverage | 2 | Remove the code or document the path. |
| flaky-killer | 1 | Stabilize the covering test, then re-run. |

### Per-survivor detail

(one Step 4 block per survivor)

### Top priority (5 to 10)

1. `file:line` - class - one-line description of the test to write.

Rank by blast radius of the mutated code, not by class. A surviving boundary mutant in a pricing rule outranks ten survivors in a formatter.

Anti-patterns

Limitations

• Classification is heuristic. Every class except unreachable (which the tool asserts via its no-coverage status) is a judgment from reading code, and equivalent-mutant is undecidable in general (Step 5).
• Business intent is not recoverable from source. Whether the original boundary or the mutated one is correct is a specification question, not a code question.
• Coverage links vary by tool. StrykerJS reports coveredBy and killedBy per mutant (report schema), while Mull's survivor lines carry no per-mutant test list (Mull tutorial). Where the link is missing, the missing-case versus weak-assertion split needs a manual re-run of the suspected test against an applied mutant.
• Operator coverage differs by tool. A family absent from one tool's operator set produces no survivors of that shape there, which is a gap in the signal, not evidence of a stronger suite.