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Deep Cover aims to be the best coverage tool for Ruby code:

  • more accurate line coverage
  • branch coverage
  • can be used as a drop-in replacement for the built-in Coverage library.

It reports a more accurate picture of your code usage. In particular a line is considered covered if and only if it is entirely executed:

def foo(something: false)
  bar if something  # => This is considered covered by builtin coverage,
                    #    even though `bar` might not even exist

# somewhere in a test:

Optionally, branch coverage will detect if some branches are never taken. In the following example, test_foo only provides values for x that respond to :to_s, thus the implicit else is never tested (i.e. a value of x that does not respond to :to_s)

def foo(x)
  x = x.to_s if x.respond_to? :to_s
  # ...

def test_foo
  assert_equal something, foo(42)
  assert_equal something_else, foo(:hello)


These examples are direct outputs from our HTML reporter:

DeepCover vs builtin coverage

Feature MRI DeepCover
Line coverage partial
Node coverage no
Branch coverage partial
Method coverage ~
Slowdown < 1% ~20%
Platform support Ruby 2.5+ Ruby 2.1+, JRuby

Line coverage: MRI doesn't cover some lines (e.g. when some_value).

Node coverage: MRI provides no way to tell which parts of any line is evaluated (e.g. 0.times { never_run }). DeepCover covers everything.

Method coverage: MRI considers every method defined, including methods defined on objects or via define_method, class_eval, etc. For Istanbul output, DeepCover has a different approach and covers all def and all blocks.

Branch coverage MRI DeepCover
if / unless / ?:
case / when
❘❘ / && no
{❘foo = 42, bar: 43❘} no
while / until !

Note on loops (!): DeepCover doesn't consider loops to be branches, but it's easy to support it if needed.


Do the appropriate of the installation of the gem, then follow the steps that correspond to your situation.

# if the project is a gem, add this to your .gemspec and then run `bundle install`
spec.add_development_dependency 'deep-cover', '~> 0.7'

# otherwise if using a Gemfile, add this to it and then run `bundle install`
gem 'deep-cover', '~> 0.7', group: :test

# otherwise just run:
gem install deep-cover

Trying deep-cover quickly

An easy way to try deep-cover, without any configuration needed:

deep-cover clone command to run test
# ex:
deep-cover clone rake test

Check the produced coverage/index.html.

Note, this is a bit slower and may cause issues in your tests if your use relative paths that lead outside of the directory (Such as a dependency that is in a parent directory).

Regular setup

  1. Require deep-cover

    You must call require 'deep-cover' before the code you want to cover is loaded.

    Typically, you want to insert that line at the very top of test/test_helper.rb or spec/spec_helper.rb . If deep-cover is required after your code, then it won't be able to detect the coverage.

    Note that if some of your tests run by launching another process, that process will have to require 'deep-cover' also. For example you could insert require 'deep-cover' if ENV['DEEP_COVER'] at the beginning of lib/my_awesome_gem.rb, before all the require_relative 'my_awesome_gem/fabulous_core_part_1', ... Note that the environment variable DEEP_COVER is set by deep-cover exec or DeepCover.start.

  2. Create a config file (optional)

    You may want to create a config file .deep_cover.rb at the root of your project, where you can set the config as you wish.

    # File .deep_cover.rb
    DeepCover.configure do
      ignore_uncovered :default_argument
      # ...
  3. Launch it

    Even after DeepCover is required and configured, only a very minimal amount of code is actually loaded and coverage is not started.

    The easiest way to actually start it is to use deep-cover exec

    For example:

    $ deep-cover exec rspec
    # ...all the output of rspec
    # ...coverage report

Already using SimpleCov / builtin Coverage

To make it easier to transition for projects already using the builtin Coverage library (such as those using SimpleCov), deep-cover can inject itself into those tools so that, while you still only have line-by-line coverage information, it becomes stricter, only marking a line as executed if everything on it has been executed.

You must call require 'deep_cover/builtin_takeover' before you require the coverage tool that you normally use.

For example, the test/test_helper.rb file for SimpleCov users will look like

require 'deep_cover/builtin_takeover'
require 'simplecov'
# rest of `test_helper.rb`

Once this is done, simply generate the coverage as you normally would. In order to get detailed information about why a line is not covered, you will need to the regular deep-cover mode.

Online coverage tools such as Code Climate, Coveralls, Codecov

At the moment, those tools do not support deep-cover. It is however possible to use the takeover system to, at least, make them stricter. Follow the explanation in the above section for injecting into SimpleCov.



configure is used to specify how specific DeepCover should be and which files it should analyse. The following code reflects the default settings:

DeepCover.configure do
  ignore_uncovered :raise, :default_argument
  detect_uncovered :trivial_if
  paths %w[app lib]
  exclude_paths ['fixtures', /^lib\/ignore/]

The file .deep_cover.rb is loaded automatically when requiring deep-cover and is the best place to put the configuration.

Custom filters

deep-cover comes with a few filters that make it possible to ignore certain uncovered codes.

It is easy to add you own filters.

For example, if one wants to ignore uncovered calls to raise but the code uses our_custom_raise instead, the following with work:

DeepCover.configure do
  ignore_uncovered do
    type == :send &&
      receiver == nil &&
      message == :our_custom_raise


After checking out the repo, run bundle then rake dev:install to install dependencies. Then, run rake to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

For detailed analysis:

deep-cover -d -e "if(:try_me); puts 'cool'; end"

To run one of the specs in spec:

bin/cov boolean

More details in the contributing guide.


Please ask questions on Maintainers monitor the tag deep-cover.rb.

Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


This project exists thanks to all the people who contribute. [Contribute].


Thank you to all our backers! 🙏 [Become a backer]


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

Code of Conduct

Everyone interacting in the DeepCover project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.