mutation-testing-setup

Mutation Testing Setup (Stryker.NET)

Measure whether your tests actually catch bugs — not just execute code — with Stryker.NET, and turn surviving mutants into better tests.

Input — point it at your project

Works on a target: a project/test project or GitHub URL. Find the test project: find <target> -iname "*.Tests.csproj".

Setup

dotnet tool install -g dotnet-stryker
cd path/to/Tests.Project
dotnet stryker

stryker-config.json (scope it so runs are fast):

{
  "stryker-config": {
    "project": "MyApp.csproj",
    "test-projects": ["MyApp.Tests.csproj"],
    "mutate": ["**/Core/**/*.cs", "!**/Migrations/**"],
    "thresholds": { "high": 80, "low": 60, "break": 50 }
  }
}

Reading results

• Mutation score = killed mutants / total. Higher = tests catch more injected bugs.
• Surviving mutant = Stryker changed the code (e.g. flipped > to >=, removed a statement) and no test failed → a real gap in your assertions.
• Timeout/no-coverage mutants = code not exercised at all (a coverage gap, see test-coverage-gap-finder).

For each meaningful survivor: add or strengthen an assertion so the mutation would be caught. A test that runs code without asserting its effect lets mutants survive.

Principles

• Coverage says code ran; mutation says tests would catch a bug. Mutation is the truer quality signal.
• Scope mutation runs to important code (it's slow) — don't mutate the whole solution every CI run.
• Don't chase 100% — focus survivors in critical logic.
• Treat survivors as "missing assertion", not "add a test for coverage".

How to use it & best prompts

"Set up Stryker mutation testing", "are my tests actually catching bugs", "explain these surviving mutants", "improve my tests so these mutants die". Pairs with test-coverage-gap-finder and xunit-test-generator.