Coverage Guard Mode Skill

Coverage Guard Mode

When activated, checks coverage after test runs and warns if it drops below the configured threshold.

What It Does

After any test execution (via Bash tool), compares coverage to the threshold in config.json. Warns (doesn't block) if coverage decreased.

Activation

/coverage-guard

Configuration

Edit config.json in this skill directory to set thresholds:

{
  "thresholds": {
    "statements": 80,
    "branches": 70,
    "functions": 75,
    "lines": 80
  },
  "coverageCommand": "npx jest --coverage --coverageReporters=json-summary",
  "coverageFile": "coverage/coverage-summary.json"
}

Hook Configuration

{
  "hooks": {
    "PostToolUse": [
      {
        "matcher": "Bash",
        "hook": ".claude/skills/coverage-guard/scripts/check-coverage.sh",
        "condition": "command contains 'jest' OR command contains 'vitest' OR command contains 'npm test'"
      }
    ]
  }
}

Enforcement Logic

#!/bin/bash
# check-coverage.sh
COVERAGE_FILE="coverage/coverage-summary.json"
THRESHOLD=80

if [ -f "$COVERAGE_FILE" ]; then
  STATEMENTS=$(jq '.total.statements.pct' "$COVERAGE_FILE")
  BRANCHES=$(jq '.total.branches.pct' "$COVERAGE_FILE")

  if (( $(echo "$STATEMENTS < $THRESHOLD" | bc -l) )); then
    echo "WARNING: Statement coverage ($STATEMENTS%) below threshold ($THRESHOLD%)"
    echo "Coverage dropped — check which files lost coverage."
  fi

  if (( $(echo "$BRANCHES < 70" | bc -l) )); then
    echo "WARNING: Branch coverage ($BRANCHES%) below 70%"
  fi
fi

Gotchas

Coverage check runs AFTER the test command — if tests crash, no coverage report is generated
Coverage-summary.json must be configured as a reporter — default Jest config may not include it
Threshold comparisons use floating point — 79.999% will trigger below 80% threshold
Branch coverage is typically 10-15% lower than line coverage — set thresholds accordingly

Agent Skills: Coverage Guard Mode

Install this agent skill to your local