undercover warns about methods, classes and blocks that were changed without tests, to help you easily find untested code and reduce the number of bugs. It does so by analysing data from git diffs, code structure and SimpleCov coverage reports.

Works with any Ruby CI pipeline as well as locally as a CLI.

Build Status Maintainability Downloads

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


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
  enable_coverage(:branch) # Report branch coverage to trigger branch-level undercover warnings

require 'undercover'

# ...

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


Invoked with no arguments, Undercover will flag all untested methods and classes from the current diff:


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 origin/master

Check out docs/ for CI configuration examples:

Merging coverage results (sample gist) is required for parallel tests before processing with undercover.

Code review integrations

A few options exist to provide automated comments from undercover in Pull Request reviews, which is the most streamlined way to add Undercover to your development workflow.


CLI Options

Options can be passed when running the command from the command line:

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

Configuration File

A configuration file named .undercover can be created at the top level of a project's directory containing the same set of options for the CLI. Example file:

-l path/to/different.lcov
-c origin/master

The options set in the file can be overriden by passing arguments when invoking the executable.

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

Ignoring/skipping coverage

Projects with low or nonexistent test coverage are likely to generate large numbers of warnings. While the default workflow would be to address them before the PR approval, your strategy might be different.

In order to acknowledge an untested change and remove the UndercoverCI warning with the intention to improve later (or never), you can wrap the code block with the :nocov: syntax, e.g.

# :nocov:
def skip_this_method
# :nocov:

Read more about the :nocov: syntax in SimpleCov's readme.


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.


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 rubygems.org.


Bug reports and pull requests are welcome on GitHub at https://github.com/grodowski/undercover.


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