🔥 Minitest Heat 🔥

Minitest Heat helps you identify problems faster so you can more efficiently resolve test failures by generating a heat map that shows where failures are concentrated.

For a more detailed explanation of Minitest Heat with screenshots, head over to the wiki for the full story.

Or for some additional insight about priorities and how it works, this Twitter thread is a good read.

Installation

Add this line to your application's Gemfile:

gem 'minitest-heat'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install minitest-heat

And depending on your usage, you may need to require Minitest Heat in your test suite:

require 'minitest/heat'

Prioritizing Your Work

Minitest Heat surfaces the most impactful problems first so you can fix what matters without scrolling through noise.

Issue Priority

Issues are displayed in priority order—you'll always see the most critical problems first:

Priority Type Why First
1 Errors Exceptions in source code—your app is broken
2 Broken Exceptions in test files—fix your tests before trusting them
3 Failures Assertion failures—the core of what tests catch
4 Skipped Only shown when no errors/failures—a reminder, not urgent
5 Slow/Painful Only shown when everything passes—performance cleanup

You never see skips or slow tests when there are real problems. This keeps you focused on what actually needs fixing.

The Heat Map

After listing individual issues, the heat map shows where problems are concentrated across your codebase:

  • Files sorted by severity—most problematic files appear first
  • Line numbers with issue counts—42×3 means 3 issues at line 42
  • Quick hot spot identification—if one file keeps appearing, that's where to focus

The heat map helps you spot patterns. A single file with many issues often points to a deeper problem worth investigating rather than fixing issues one by one.

Configuration

Minitest Heat provides configurable thresholds for identifying slow tests. By default, tests over 1.0s are considered "slow" and tests over 3.0s are "painfully slow."

Add a configuration block to your test_helper.rb after require 'minitest/heat':

Minitest::Heat.configure do |config|
  config.slow_threshold = 1.0        # seconds
  config.painfully_slow_threshold = 3.0
end

Inherently Slow Paths

System tests, integration tests, and other browser-driven tests are inherently slower than unit tests. Rather than raising your global thresholds to accommodate them, you can tell Minitest Heat which paths to exclude from slow test reporting. Tests in those directories still run and report errors and failures normally—they just aren't evaluated against the timing thresholds.

Minitest::Heat.configure do |config|
  config.inherently_slow_paths = ['test/system', 'test/integration']
end

Any test whose file path starts with a listed prefix is excluded. Common paths to consider:

  • test/system — Rails system tests (browser-driven)
  • test/integration — Integration tests with broader scope
  • test/e2e — End-to-end tests

Example: Gem Development

For a gem with fast unit tests, stricter thresholds catch performance regressions early:

Minitest::Heat.configure do |config|
  config.slow_threshold = 0.1
  config.painfully_slow_threshold = 0.5
end

JSON Output

For CI integration, tooling, or programmatic consumption, Minitest Heat can output results as JSON:

bundle exec rake test TESTOPTS="--heat-json"

The JSON output includes:

  • statistics - counts by issue type (errors, failures, skipped, slow, etc.)
  • timing - total time, tests/second, assertions/second
  • heat_map - files with issues, sorted by severity weight
  • issues - detailed issue data with locations and messages

Example usage:

# Capture JSON output (stderr has progress, stdout has JSON)
bundle exec rake test TESTOPTS="--heat-json" 2>/dev/null > results.json

Development

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

To install this gem onto your local machine, run bundle exec rake install. For release instructions, see RELEASING.md.

Running Tests

# Run full test suite
bundle exec rake test

# Run a single test file
bundle exec rake test TEST=test/minitest/heat/issue_test.rb

# Run a single test method
bundle exec rake test TEST=test/minitest/heat/issue_test.rb TESTOPTS="-n /test_error_issue/"

Forcing Test Failures

In order to easily see how Minitest Heat handles different combinations of different types of failures, the following environment variables can be used to force failures.

IMPLODE=true           # Every possible type of failure, skip, and slow is generated
FORCE_EXCEPTIONS=true  # Only exception-triggered failures
FORCE_FAILURES=true    # Only standard assertion failures
FORCE_SKIPS=true       # No errors, just the skipped tests
FORCE_SLOWS=true       # No errors or skipped tests, just slow tests

So to see the full context of a test suite, IMPLODE=true bundle exec rake will work its magic.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/garrettdimon/minitest-heat. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

License

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

Code of Conduct

Everyone interacting in the Minitest::Heat project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.