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Welcome to Gremlins

Gremlins is a mutation testing tool for Go. It has been made to work well on smallish Go modules, for example microservices, on which it helps validate the test suite, aids the TDD process and can be used as a CI quality gate.

As of now, Gremlins doesn't work very well on very big Go modules, mainly because a run can take hours to complete.


Gremlins is still in its 0.x.x release, which, as per SemVer, doesn't guarantee backward compatibility. What this means for Gremlins is that configuration flags and/or configuration files can change among minor releases. This may be important if you use Gremlins in automated builds, for example in continuous integration pipelines.

Also, while in 0.x.x releases, only the current minor release will be maintained; if we are in v0.2.0, no patch releases will be provided for the v0.1.0 line. Once Gremlins will reach it's 1.x.x release, it will start to be backward compatible, and we'll decide how many releases to maintain at the same time (ex. up to two minors before, as Go itself does).

What is Mutation Testing

Code coverage is unreliable as a measure of test quality. It is too easy to have tests that exercise a piece of code but don't test anything at all. Mutation testing works by mutating the code exercised by the tests and verifying if the mutation is caught by the test suite. Imagine gremlins going into your code and messing around: will your test suit catch their damage?


  • Discovers mutant candidates and tests them
  • Only tests mutants covered by tests
  • Can test mutants only in PR changes
  • Supports five mutant types
  • Yaml-based configuration
  • Can run as quality gate on CI