# Ruby coverage: SimpleCov vs Supercov

> Compare SimpleCov and Supercov for Ruby line, branch, per-test, MC/DC, and assertion coverage, with a concrete missing-test example.

Web page: https://supercov.com/compare/simplecov · Published 2026-09-13, updated 2026-10-12
All Supercov pages for agents: https://supercov.com/llms.txt · Full text: https://supercov.com/llms-full.txt

## What is the difference?

SimpleCov measures Ruby coverage and helps you inspect unexecuted code. Supercov also measures independent conditions and connects executed source to the assertions your tests make.

SimpleCov is not limited to a single line percentage. Version 1.3 supports branch coverage and per-test tracking. If those already answer your questions, you do not need to change tools just to use a coding agent.

| Feature | SimpleCov 1.3.0 | Supercov 4.3.0 |
|---|---|---|
| Code quality scoring | × Not built in | ✓ Supported |
| Security review | × Not built in | ✓ Supported |
| Line coverage | ✓ Supported | ✓ Supported |
| Branch coverage | ✓ Supported | ✓ Supported |
| MC/DC | × Not built in | ✓ Supported |
| Per-test coverage | ✓ Supported | ✓ Supported |
| Assertion coverage | × Not built in | ✓ Supported |
| Reports | ✓ Supported | ✓ Supported |

Standard coverage workflows in the versions shown; custom plugins are outside this table. Supercov supports MRI Ruby: full MC/DC on 3.4+, limited measurement on 3.3. SimpleCov 1.3 requires Ruby 3.3+. SimpleCov’s per-test coverage requires track_tests. Neither tool promises exact per-test coverage for overlapping threaded tests. Per-test coverage means individual tests, not just test files. Supercov needs a supported runner. Assertion coverage is Jev’s judgment, test by test, of whether changing each executed statement would make the test fail; it is not a mutation score or a correctness guarantee.

## Start with your existing Ruby tests

Load SimpleCov before application code so it can measure the files as they load. In your test helper:

```ruby
require 'simplecov'
SimpleCov.start do
  enable_coverage :branch
end

# Load the application and tests after this.
```

To associate coverage with individual tests, enable SimpleCov's `track_tests` option for your runner. See its [configuration guide](https://github.com/simplecov-ruby/simplecov/blob/v1.3.0/docs/Configuration.md). Its [CLI guide](https://github.com/simplecov-ruby/simplecov/blob/v1.3.0/docs/CLI.md) also covers comparing runs and finding affected tests.

## A branch can run without every condition being tested

```ruby
def can_access(signed_in, expired)
  if signed_in && !expired
    return true
  end
  false
end
```

These Minitest checks exercise both outcomes:

```ruby
assert can_access(true, false)
refute can_access(false, false)
```

But they do not test session expiry. Add `refute can_access(true, true)` to check that a signed-in user loses access when the session expires.

In our Ruby fixture, SimpleCov 1.3.0 reported all four lines and both branches covered. Supercov 2.0.0 reported one of two independent conditions covered. Both results can be correct: branch coverage and MC/DC answer different questions.

## When Supercov is useful

Keep your RSpec or Minitest suite. Run its complete command through Supercov, then inspect the gaps:

```sh
supercov -- bundle exec rspec
supercov runs latest gaps
```

You install the Ruby gem first; there is no `npx` step. The [Ruby getting-started guide](https://supercov.com/docs/getting-started?language=ruby) covers setup.

Assertion coverage adds another question: what did the tests actually check? Jev reads what each test ran and judges, statement by statement, whether changing it would make the test fail. It finds unasserted code without generating or running mutants. A judgment is not a guarantee that a test will catch every wrong result.

Choose SimpleCov for an established Ruby reporting workflow. Choose Supercov when you want your agent to investigate condition and assertion gaps as well. Full Supercov MC/DC requires MRI Ruby 3.4 or newer; exact per-test attribution should not be assumed for overlapping threaded tests.


## Feature sources

- [SimpleCov: Coverage and per-test tracking](https://github.com/simplecov-ruby/simplecov/blob/v1.3.0/docs/Configuration.md)
- [SimpleCov: HTML report](https://github.com/simplecov-ruby/simplecov/blob/v1.3.0/docs/Formatters.md)
- [Supercov: Coverage model](https://github.com/supercorp-ai/supercov/blob/v4.3.0/docs/coverage-model.md)
- [Supercov: Supported runners](https://github.com/supercorp-ai/supercov/blob/v4.3.0/docs/supported-suites.md)
- [Supercov: Assertion coverage](https://github.com/supercorp-ai/supercov/blob/v4.3.0/docs/assertions.md)
- [Supercov: Code quality](https://github.com/supercorp-ai/supercov/blob/v4.3.0/docs/quality.md)
- [Supercov: Security](https://github.com/supercorp-ai/supercov/blob/v4.3.0/docs/security.md)
- [Supercov: Reports](https://github.com/supercorp-ai/supercov/blob/v4.3.0/docs/reports.md)
- [Supercov: CLI reference](https://github.com/supercorp-ai/supercov/blob/v4.3.0/docs/cli.md)


Example evidence: [fixture source and recorded results (JSON)](https://supercov.com/downloads/coverage-comparison-fixtures.json). Measured September 13, 2026; Supercov runs repeated with 2.0.0 on September 25, unchanged. These small examples are not a performance benchmark or a general conformance test.