Host: Codex CLI — This skill was designed for Claude Code and adapted for Codex. Cross-reference commands use installed skill names in Codex rather than /octo:* slash commands. Use the active Codex shell and subagent tools. Do not claim a provider, model, or host subagent is available until the current session exposes it. For host tool equivalents, see skills/blocks/codex-host-adapter.md.

Test Coverage Audit

Overview

Trace every codepath in a diff, map each path against existing tests, visualize coverage gaps, and auto-generate tests for uncovered paths.

Core principle: Trace codepaths in changed files -> Map against existing tests -> Score coverage quality -> Generate tests for gaps -> Report before/after counts.

Caps and Limits

These hard limits prevent runaway analysis:

• 30 code paths max per audit. If a diff yields more than 30, prioritize by complexity and risk (error paths, security-sensitive branches, public API surfaces first).
• 20 tests generated max per audit. Focus on highest-impact gaps first.
• 2-minute per-test exploration cap. If understanding a single test path takes longer than 2 minutes, mark it as "needs manual review" and move on.

Phase 1: Codepath Tracing

Step 1: Identify Changed Files

Determine the diff scope. Use the most relevant source:

# PR diff
git diff --name-only main...HEAD

# Staged changes
git diff --name-only --cached

# Last commit
git diff --name-only HEAD~1..HEAD

Filter to source code files only (exclude configs, docs, generated files).

Step 2: Trace Data Flow Through Every Branch

For each changed file, you MUST trace:

1. Conditionals -- Every if/else, switch/case, ternary, and pattern match. Each branch is a separate codepath.
2. Error paths -- Every catch, throw, error return, validation failure, and early return with error. WHY: Error paths are the most common source of untested bugs.
3. Function calls -- Every function invoked from changed code. Trace one level deep into callees to identify integration boundaries.
4. Loop boundaries -- Empty collection, single item, and multi-item paths through loops.
5. Guard clauses -- Every early return, null check, and permission gate.

Step 3: Build the Codepath Inventory

Produce a structured inventory:

## Codepath Inventory: [filename]

| # | Path Description | Type | Risk |
|---|-----------------|------|------|
| 1 | validateUser() happy path | conditional | low |
| 2 | validateUser() missing email | error | medium |
| 3 | validateUser() invalid format | error | medium |
| 4 | processOrder() empty cart guard | guard | high |
| 5 | processOrder() payment timeout | error | high |
| 6 | processOrder() success | conditional | low |

Type categories: conditional, error, guard, loop-boundary, integration, async

Risk assessment: high = user-facing failure or data loss, medium = degraded behavior, low = cosmetic or logging

Phase 2: Test Mapping and Quality Scoring

Step 1: Search for Existing Tests

For each file in the diff, search the test directory for related tests:

# Find test files that reference the changed file or its exports
# Search by filename pattern
find tests/ -name "*[changed_file_stem]*" -type f

# Search by import/require of the changed module
grep -rl "import.*from.*[module_name]" tests/
grep -rl "require.*[module_name]" tests/

# Search by function name references
grep -rl "[function_name]" tests/

Step 2: Score Test Quality

For each codepath, assess existing test coverage with this rubric:

Step 3: Produce Coverage Map

Map each codepath to its test coverage:

## Coverage Map: [filename]

| # | Codepath | Test File | Rating | Notes |
|---|----------|-----------|--------|-------|
| 1 | validateUser() happy path | test-user.sh:42 | ★★★ | Asserts valid + invalid inputs |
| 2 | validateUser() missing email | test-user.sh:58 | ★★ | Tests missing, not malformed |
| 3 | validateUser() invalid format | -- | ☆ | No test for format validation |
| 4 | processOrder() empty cart guard | -- | ☆ | Guard clause untested |
| 5 | processOrder() payment timeout | test-orders.sh:30 | ★ | Checks no crash, no assertions |
| 6 | processOrder() success | test-orders.sh:15 | ★★★ | Full integration test |

Phase 3: Coverage Diagram

After completing the map, produce an ASCII coverage summary. This is the primary output artifact.

COVERAGE: 5/12 paths tested (42%)
  Code paths: 3/5 (60%)
  User flows: 2/7 (29%)
GAPS: 7 paths need tests

Break down by category:

BY TYPE:
  conditional:  3/4 tested (75%)  ████████░░
  error:        1/5 tested (20%)  ██░░░░░░░░
  guard:        0/2 tested  (0%)  ░░░░░░░░░░
  integration:  1/1 tested (100%) ██████████

BY RISK:
  high:    1/3 tested (33%)  ███░░░░░░░
  medium:  2/5 tested (40%)  ████░░░░░░
  low:     2/4 tested (50%)  █████░░░░░

Use full block for covered and light shade for uncovered. 10-character bar. Always show exact fractions and percentages.

Phase 4: Auto-Generate Tests

Step 1: Detect Project Test Conventions

Before generating any tests, you MUST detect the project's testing patterns:

**Detected Test Conventions:**
- Framework: [jest/vitest/pytest/bash/go test/etc.]
- Location: [tests/ | __tests__/ | src/**/*.test.* | etc.]
- Naming: [test-*.sh | *.test.ts | *_test.go | etc.]
- Style: [BDD describe/it | xUnit | TAP | custom]
- Helpers: [test-utils.ts | conftest.py | helpers/ | etc.]
- Assertion library: [built-in | chai | assert | etc.]

Step 2: Generate Tests for Uncovered Paths

For each no-test and smoke-only codepath, generate a test that:

1. Follows project naming conventions -- same directory structure, same file naming pattern
2. Uses existing test helpers -- import from the same test utilities the project already uses
3. Tests behavior, not implementation -- assert observable outcomes, not internal state
4. Covers the specific gap -- targets the exact branch or error path identified in Phase 1
5. Includes edge cases -- aim for full coverage on each generated test

Step 3: Present Generated Tests

For each generated test, show:

### Generated: test for [codepath description]
**Covers:** Codepath #N from [filename]
**Raises coverage:** from no-test to full coverage

[test code block]

Step 4: Report Before/After

After generating all tests, show the coverage change:

BEFORE: 5/12 paths tested (42%)
AFTER:  11/12 paths tested (92%)
  New tests generated: 6
  Remaining gaps: 1 (manual review needed)

Integration with Other Skills

With flow-deliver / skill-code-review

Coverage audit runs as a complement to code review. When invoked during deliver phase:

1. Code review assesses quality and correctness
2. Coverage audit assesses test completeness
3. Both feed into the ship/no-ship decision

With skill-tdd

If coverage audit finds gaps in new code, recommend the user adopt TDD for the next iteration. Coverage audit fixes existing gaps; TDD prevents future ones.

With skill-verification-gate

After generating tests, use skill-verification-gate to run the test suite and confirm the new tests pass.

Red Flags -- Do Not Do This

Quick Reference

1. TRACE   -> Identify all codepaths in the diff (max 30)
2. MAP     -> Find existing tests for each path
3. SCORE   -> Rate coverage quality (no-test / smoke / happy-path / full)
4. DIAGRAM -> ASCII coverage visualization
5. GENERATE -> Auto-create tests for gaps (max 20)
6. REPORT  -> Before/after test counts