mutation-test

name	mutation-test
description	Run Stryker mutation tests against C# projects and analyze results to find weak spots in test coverage
argument-hint	["test-project-path"]
allowed-tools	Bash, Read, Glob, Grep

Mutation Testing with Stryker.NET

Run Stryker mutation tests to evaluate test suite effectiveness. Mutation testing finds code that can be changed without failing tests—indicating weak coverage.

Arguments

If provided, $ARGUMENTS is the path to the test project or solution. Otherwise, search for test projects.

Workflow

1. Find the Test Project

If no path provided, search for test projects:

find . -name "*.Tests.csproj" -o -name "*.UnitTests.csproj" | head -20

2. Check Stryker Installation

dotnet stryker --version

If not installed:

dotnet tool install -g dotnet-stryker

3. Check for Configuration

Look for stryker-config.json in the test project directory—it may have project-specific settings.

4. Run Stryker

From the test project directory:

dotnet stryker

With options for faster runs:

# Only mutate files changed since main
dotnet stryker --since:main

# Target specific files
dotnet stryker --mutate "src/Services/**/*.cs"

5. Interpret Results

Console output shows:

- Killed:    150  (mutants detected by tests - good)
- Survived:   25  (mutants NOT detected - weak tests)
- Timeout:     5  (caused infinite loop - usually OK)
- No Coverage: 10  (code not covered by tests)

Mutation score: 78.95%

Score interpretation:

Score	Quality
80%+	Excellent
60-80%	Good, some gaps
40-60%	Fair, significant weaknesses
<40%	Poor coverage

6. Find Surviving Mutants

Check the HTML report at StrykerOutput/reports/mutation-report.html for:

• Which files have survivors
• Exact line numbers and mutation types
• What was changed

Common survivor patterns indicate:

• Boundary conditions not tested (> → >= survives)
• Missing negative tests (if (x) → if (true) survives)
• Return values not verified
• Exception paths not tested

Reporting

Provide:

1. Summary: Overall score, killed/survived/no-coverage counts
2. Weak spots: Files with lowest scores, specific surviving mutations
3. Recommendations: Specific tests to add based on survivors

Example:

Mutation Score: 72%
- Killed: 145, Survived: 28, No Coverage: 12

Weak spots:
- UserService.cs (65%): Line 45 `age > 18` → `age >= 18` survived (boundary not tested)
- OrderValidator.cs (78%): Line 23 `&&` → `||` survived (logic branch not covered)

Recommendations:
- Add test for age exactly 18
- Add test where first condition true, second false

Troubleshooting

Too slow: Use --since:main or --mutate to limit scope

No tests found: Run from test project directory, check --project points to source project

Memory issues: Reduce concurrency with --concurrency 2