undercover

undercover

A Claude Code plugin that creates a test coverage feedback loop using the undercover gem.

What it does

After running tests, Claude automatically runs undercover to find untested code changes, then writes tests for any gaps, looping until coverage passes.

Unlike total coverage percentages, undercover checks only the lines you changed, so the feedback is always relevant to the work at hand.

Installation

From marketplace:

/plugin marketplace add grodowski/undercover-claude
/plugin install undercover@undercover-ci

From local clone (development):

/plugin marketplace add /path/to/undercover-claude
/plugin install undercover@undercover-ci

Prerequisites

Your Ruby project needs SimpleCov configured to generate coverage/coverage.json. You don't have to wire this up by hand — let the skill do it.

Quick Setup

Inside Claude Code, run the setup prompt:

/coverage set up coverage for this project

Claude adds simplecov and undercover to your Gemfile, wires SimpleCov::Formatter::Undercover into spec_helper.rb/test_helper.rb, creates .undercover with a compare ref, runs bundle install, and runs your tests once to generate coverage data — then you're ready for the feedback loop.

How It Works

The plugin installs a skill that instructs Claude to run undercover after every test run and act on the results:

1. Run tests
2. Run undercover or undercover --include-files "app/models/foo.rb" when testing a subset of files
3. If gaps found: write tests (or remove dead code)
4. Repeat until undercover exits 0

Usage

Invoke /coverage inside Claude Code. Type what you want to do after the command and the skill routes to the right guide (it defaults to the feedback loop if you say nothing):

• /coverage set up coverage — SimpleCov setup instructions
• /coverage set up CI — CI with UndercoverCI or GitHub Actions
• /coverage parallel tests — coverage merging for parallel runners
• /coverage check this PR — read and address UndercoverCI feedback on a PR

Feature branches and CI

By default undercover checks uncommitted changes only. For feature branches and CI, set a compare ref in .undercover or use the --compare flag:

-c origin/main

Addressing UndercoverCI feedback on a PR

When coverage is reported on a GitHub PR by UndercoverCI, the warnings live in the coverage check's annotations — which gh pr view and the GitHub MCP server don't expose. The skill instructs Claude to pull them directly with gh api, then fix the gaps and verify locally with --compare.

Files

undercover-claude/
├── .claude-plugin/
│   ├── plugin.json           # Plugin manifest
│   └── marketplace.json      # Marketplace catalog (self-hosting)
├── skills/
│   └── coverage/
│       ├── SKILL.md          # Feedback loop + router (loads on every invocation)
│       └── references/       # Guides loaded on demand
│           ├── setup.md      # SimpleCov + Gemfile + .undercover + permissions
│           ├── ci.md         # GitHub Actions / CircleCI / UndercoverCI / merging
│           ├── pr-feedback.md # Reading UndercoverCI PR annotations via gh api
│           └── skipping.md   # :nocov:, add_filter, --exclude-files
└── README.md

License

MIT