Coverage Ratchet

Coverage Ratchet

A build/test gate proves the tests that exist pass. It says nothing about whether tests were *written* — a phase with no test task ships green on build success alone. The coverage ratchet closes that hole deterministically: line coverage is measured on every verify, compared against a committed baseline, and may never drop. It only rises.

Why a Ratchet, Not a Target

An absolute threshold ("80%") fails one of two ways on a real codebase: it fails day one (so it gets disabled), or it gets set below current reality (so it gates nothing). A ratchet starts *from wherever the project actually is* and only tightens. Nobody argues with "don't get worse."

When This Skill Activates

Use this skill when:

• A project has a test target but coverage has never been measured
• Agent-written features keep arriving with build-passing, test-free code
• Setting up the deterministic gauntlet (with testing/fitness-functions/ and swift/code-size/)

Setup

• Copy templates/coverage-gate.sh into the project (e.g. Scripts/coverage-gate.sh), make it executable, and fill in the config block (scheme, destination, app target name).
• Bootstrap the baseline:

Scripts/coverage-gate.sh --init     # runs tests with coverage, writes .coverage-baseline

• Commit both the script and .coverage-baseline.

.coverage-baseline holds a single decimal fraction (e.g. 0.6231) — the app target's line coverage. Trailing whitespace ignored; nothing else in the file.

The Gate

Scripts/coverage-gate.sh            # exit 0 = pass, exit 1 = coverage dropped

• Runs the test suite with coverage enabled, extracts the app target's line coverage from the xccov JSON report.
• Fails if current < baseline − 0.0025 (the epsilon absorbs measurement jitter from unchanged code).
• Suggests a ratchet-up when current > baseline + 0.01: update .coverage-baseline to the new number in the same commit as the tests that earned it. That's the ratchet clicking forward.
• Fails loudly on an unexpected report shape or a missing target — a broken measurement must never read as a pass.

Ratchet-up etiquette: raising the baseline is routine and encouraged; *lowering* it requires a written reason in the commit message (e.g. deleting a well-covered module) — treat it like widening a fitness-function allowlist.

CRAP Report (Advisory)

templates/crap-report.sh combines per-function cyclomatic complexity (SwiftLint JSON output) with per-function coverage (the same xccov report) into the CRAP score — Change Risk Anti-Patterns:

CRAP(f) = complexity(f)² × (1 − coverage(f))³ + complexity(f)

A complex *and* untested function scores explosively; simple-and-tested stays near its complexity. The report prints the top 20 — the exact functions where a test is worth the most. Advisory only, never a gate: the name-join between the two tools is heuristic.

Common Pitfalls

| Pitfall | Problem | Solution |

|---------|---------|----------|

| Absolute threshold on existing code | Fails day one or gates nothing | Ratchet from measured reality |

| Gating total project coverage | Test-target/generated code pollutes the number | Gate the app target's own lineCoverage |

| Silent pass on tool failure | Broken measurement reads as green | Script fails loudly on unexpected JSON |

| Chasing the number with assertion-free tests | Coverage without verification | Pair with mutation spot-checks (testing/mutation-testing/) |

| Never ratcheting up | Baseline fossilizes at day-one level | Raise it in the same commit as new tests |

References

• testing/mutation-testing/ — do the covering tests actually assert anything (advisory)
• swift/code-size/ — the complexity half; small functions are coverable functions
• testing/fitness-functions/ — architecture invariants as tests
• generators/ci-cd-setup/ — CI wiring for the gate