Mutation testing

A test that cannot fail is worse than no test, because it still reports coverage. Mutation testing is the only mechanical way to find those: it changes the code on purpose and re-runs the suite. A mutant that survives is a hole.

This skill is about what to do with the survivors. Running the gate is bin/harness mutation; deciding what each survivor means is judgement, and it is the whole job.

The two gates, cheapest first

gate	cost	finds
bin/harness antipatterns <files>	instant	assertions that are structurally incapable of failing
bin/harness mutation	seconds to minutes	assertions that could fail but do not, because nothing exercises them

Run the first before the second. It costs nothing and removes whole categories before the expensive gate sees them.

bin/harness mutation runs Infection on the diff only (--git-diff-lines), so the budget stays bounded. Thresholds: MSI 70, covered MSI 80. They live in bin/harness, not in infection.json5 — one place. infection.json5 owns only paths.

bin/harness mutation                              # the diff the push would send
HARNESS_MUTATION_BASE=HEAD bin/harness mutation   # versus your own edit

The base is the branch's upstream (@{upstream}), so a run judges the commits the push adds rather than the branch it sits on. origin/HEAD is origin/main in this fleet, which reads a one-line edit as the whole migration; HARNESS_MUTATION_BASE scopes a run to something else, and bin/harness counterfactual reads the same base.

Before you read a survivor, know what the mutation reaches

A survivor is a statement about the tests that cover the mutated line, and the cheapest way to get that wrong is to reason about the wrong set of callers. Ask the graph first:

bin/harness graft callers <the method the mutant lives in> --depth 2

That is who exercises the line, transitively, read from the code rather than from your reading of the test files. A mutant that escapes because no caller ever reaches the branch is a test-shaped hole; one that escapes because the branch is genuinely unreachable is a finding about the code, and the two want opposite fixes. The graft skill is the full command table.

Reading a run

76 mutations were generated:
      56 mutants were killed by Test Framework
      20 covered mutants were not detected
       Mutation Code Coverage: 100%
              Covered Code MSI: 73%

Mutation Code Coverage is how much of the mutated code the suite executes. Covered Code MSI is how much of what it executes it can actually tell apart. A high first number with a low second one is the signature of a tautological suite: 100% of the lines run, and a fifth of the logic could be inverted without a single failure.

That is not hypothetical. Updatronix_Security measured exactly those numbers, and nine of its twenty survivors were LogicalAnd variants: the tests assert what the method returns, and never exercise the case where one branch of the condition is false.

The three kinds of survivor

Every survivor is one of these. Deciding which is the job; guessing is how a real bug gets closed as "equivalent".

1 · The test is too weak. Most common. The assertion checks the outcome for one input, and the mutation changes behavior for an input nobody tried.

Fix: assert on the branch. If the condition is $a && $b, there are four cases and the test suite should name all four in its intent, even when two share an assertion.

2 · The mutant is equivalent. The change genuinely cannot alter behavior: logic that is already dominated by an earlier return, a defensive branch that cannot be reached, $x = 1 where $x is unused.

Fix: none in the test. Consider deleting the dead logic instead — an equivalent mutant is usually a small piece of code that does nothing, and removing it is better than documenting it. If it must stay, say why in the code, not in the mutation config.

3 · The code is wrong or unreachable. The branch is never taken, or the guard contradicts something upstream. This is a bug, and the survivor found it for free.

Fix: report it. Do not quietly write a test that pins the current behavior — that converts a bug into a specification.

Procedure

1. Run bin/harness mutation. Read the escaped list from .cache/infection/infection.json; group it by mutatorName before looking at individual mutants. Twenty survivors are usually three causes, not twenty.
2. For each group, open the mutant's line and read its originalCode and mutatedCode. The mutation says exactly which decision the suite cannot see.
3. Classify it: weak test, equivalent, or real bug. Say which, out loud, for each group. A classification nobody stated is a classification nobody can check.
4. For a weak test, write the case that distinguishes the two versions, and confirm it fails against the mutation before it passes against the code.
5. Re-run. The number moving is the proof; a test added without re-running is an assumption.

Thresholds

Never lower a threshold to make a run pass. The threshold is the only thing making the gate real, and a gate that moves on request is not a gate.

If the current threshold genuinely cannot be met yet, the honest move is to record the measured number and raise the threshold as the suite improves — in a commit that says so. A number in a file with no explanation is the next person's guess.

What this skill kept from upstream

Distilled from trailofbits/skills/mutation-testing, which targets the mewt and muton tools rather than PHP.

• Equivalent mutants are a category, not a failure. Upstream devotes a reference to cataloguing them; that distinction is the single most useful idea in the source and it transfers unchanged.
• Group survivors before analysing them. Twenty findings are usually a handful of causes.
• Severity determines effort. Not every survivor deserves a new test.
• Survivors are a bug-hunting tool, not only a test-quality metric. The third category above comes from there.

What was dropped, and why

• Every mewt/muton command, flag and config file. Wrong toolchain: this fleet uses Infection with PHPUnit. Keeping them would have produced a skill that reads well and misleads.
• Blockchain and Solidity references. No target here.
• The foreign-tool input formats (slither-mutate, mull, dextool-mutate). Infection writes its own JSON.
• The report template. A formal analysis document does not fit a gate that runs on every push.

What upstream was missing

• The antipatterns first pass. Structural bans are instant and remove the cheapest categories before the expensive run.
• --git-diff-lines and the budget it protects. Upstream assumes you run a campaign; here it runs inside a pre-push hook.
• --initial-tests-php-options. Infection spawns PHPUnit as a plain php, so a coverage driver loaded only in Infection's own invocation does not reach the child. That failure reads as "no code coverage generator has been detected", which is misleading. bin/harness mutation passes the flags through; a hand-rolled invocation must too.
• The classification-is-mandatory rule. Upstream assumes an analyst who will label things. An agent needs to be told, because "all twenty are equivalent" is a very easy sentence to write.

Verification

The skill was followed correctly when:

• every escaped mutant has been classified and the classification was stated;
• the MSI moved, or the run was explained rather than excused;
• no threshold in bin/harness or infection.json5 was changed to make the run pass.