Actionable code coverage - detects untested code blocks in recent changes
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Latest commit b2042a6 Dec 12, 2018

undercover 👮‍♂️

Like RuboCop but for code coverage

Inspects files in a git diff and warns on methods, classes and blocks which need test coverage. Use it locally or as part of an automated build to shorten your code coverage feedback loop!


I wanted to create a tool to help others and myself ensure that tests are written for all the recent code changes. This should be useful for any ruby project, but especially those large or legacy codebases that lack testing (and we can't or don't want to invest in full test coverage).

The goal was to provide automated warnings, that are:

  • relevant, so scoped to the actual code changes
  • timely, so we don't end up writing tests long after the implementation
  • actionable, so we can fix them before the code is committed or reaches production

For more background, please read the blog post.


Technically, undercover combines data from git, coverage reports and code structure graphs.

A sample output of undercover ran before a commit may look like this:

screenshot warning

And like this, given that specs were added:

screenshot success

The project is in an early phase, so please expect the unexpected and report any problems or feedback as issues.

Build Status Maintainability codebeat badge


Add this line to your application's Gemfile:

gem 'undercover'

And then execute:

$ bundle

Or install it yourself as:

$ gem install undercover

Setting up required LCOV reporting

To make your specs or tests compatible with undercover by providing an LCOV report, please add simplecov and simplecov-lcov to your test setup.

# Gemfile
group :test do
  gem 'simplecov'
  gem 'simplecov-lcov'

# the very top of spec_helper.rb
require 'simplecov'
require 'simplecov-lcov'
SimpleCov::Formatter::LcovFormatter.config.report_with_single_file = true
SimpleCov.formatter = SimpleCov::Formatter::LcovFormatter
SimpleCov.start do
  add_filter(/^\/spec\//) # For RSpec
  add_filter(/^\/test\//) # For Minitest

require 'undercover'

# ...

Then run your test suite once through to generate the initial coverage/lcov/*.lcov file before you can run the undercover command


Compare method and class coverage with latest commit


Compare with branch/ref

Undercover will list all methods and classes that haven't been covered in specs and have been added since a given git ref. Use the -c --compare ref flag to specify a git ref (commit hash, branch name, tag) to compare against.

This is a recommended usage for CI/CD build environments, as undercover will exit 1 if there are any warnings.

undercover --compare master


undercover -h
Usage: undercover [options]
    -l, --lcov path                  LCOV report file path
    -p, --path path                  Project directory
    -g, --git-dir dir                Override `.git` with a custom directory
    -c, --compare ref                Generate coverage warnings for all changes after `ref`
    -h, --help                       Prints this help
        --version                    Show version

The defaults assume that the program is run from the top level of the project directory.


After checking out the repo, run bundle to install dependencies. Then, run rake to run the tests and RuboCop. You can also run pry -r 'undercover' for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.