---
title: Using simplecov code coverage with Rails & Minitest
url: https://calvin.my/posts/using-simplecov-code-coverage-with-rails-minitest
published: 2026-01-02
updated: 2026-09-16
category: Development
tags:
- Rails
- Test
- Minitest
- Simplecov
summary: The post explains how to add SimpleCov coverage reporting to a Rails 8 application using Minitest. It outlines installing the coverage tool, initializing it early in the test helper with branch coverage and separate suite names, and optionally filtering short scaffold files or defining custom source groups. Running unit and system tests together merges results, which are written to the coverage output directory.
---

# Using simplecov code coverage with Rails & Minitest

[Simplecov](https://github.com/simplecov-ruby/simplecov) is a Ruby code coverage tool. This article documents the integration with a Rails 8 application, with the minitest test framework.

* * *

## Setup

1. Include the simplecov gem in Gemfile.

   ```yaml
   gem "rails", "~> 8.1"
   gem "minitest", "~> 5"
   gem "simplecov", require: false
   ```

2. Install the gem.

   ```bash
   bundle install
   ```

* * *

## Integrate

1. Edit test/test\_helper.rb and these codes at the top of the file.

   ```ruby
   # frozen_string_literal: true
   
   require "simplecov"
   SimpleCov.start "rails" do
     enable_coverage :branch
     command_name "#{ENV['TEST_SUITE'] || 'unit'}-#{Process.pid}"
     merge_timeout 3600
   end
   ```

2. Optionally, you can add filters to exclude certain files. For example, any files with fewer than 5 lines of code are likely scaffolds.

   ```ruby
   add_filter do |source_file|
     source_file.lines.count < 5
   end
   ```

3. While simplecov already supports default grouping (such as controllers, mailers, models, jobs), you can create new groups as needed. For example, you have a few classes that fall under the providers group, you can do so:

   ```ruby
   add_group "Providers", "app/providers"
   ```

* * *

## Execute

1. To combine the full coverage, you can run all the tests together. In the example below, we run both unit testing and system testing.

   ```bash
   bin/rails db:test:prepare && \
     TEST_SUITE=unit bin/rails test && \
     TEST_SUITE=system bin/rails test:system
   ```

2. The result can be located in the `coverage` directory.