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agents/mutation-tester.md

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+---
+description: Validate test quality through mutation testing
+capabilities: ["mutation-testing", "test-effectiveness", "code-mutations", "survivor-analysis"]
+---
+
+# Mutation Test Runner Agent
+
+Validate test suite quality by introducing code mutations and verifying tests catch them.
+
+## Mutation Testing Concept
+
+Mutation testing modifies ("mutates") code to check if tests detect the changes:
+- **Mutant killed**  - Test failed (good, caught the bug)
+- **Mutant survived**  - Test passed (bad, missed the bug)
+
+## Common Mutations
+
+1. **Arithmetic Operators** - `+` to `-`, `*` to `/`
+2. **Comparison Operators** - `>` to `>=`, `==` to `!=`
+3. **Logical Operators** - `&&` to `||`, `!` removal
+4. **Boolean Literals** - `true` to `false`
+5. **Return Values** - Change return values
+6. **Conditionals** - Remove if statements
+7. **Increments** - `++` to `--`
+
+## Example
+
+```javascript
+// Original code
+function isPositive(num) {
+  return num > 0;
+}
+
+// Mutation 1: Change > to >=
+function isPositive(num) {
+  return num >= 0;  // Should fail test for isPositive(0)
+}
+
+// Mutation 2: Change > to <
+function isPositive(num) {
+  return num < 0;  // Should fail all tests
+}
+```
+
+## Mutation Testing Tools
+
+- **JavaScript**: Stryker Mutator
+- **Python**: mutmut, cosmic-ray
+- **Java**: PITest
+- **C#**: Stryker.NET
+- **Ruby**: Mutant
+
+## Report Format
+
+```
+Mutation Testing Report
+=======================
+Total Mutants: 150
+Killed: 142 (94.7%) 
+Survived: 8 (5.3%) 
+Timeout: 0
+No Coverage: 0
+
+Mutation Score: 94.7%
+
+Survived Mutants:
+  src/utils/validator.js:23
+    - Replaced > with >= 
+    - Suggests missing boundary test
+
+  src/api/users.js:45
+    - Replaced && with ||
+    - Suggests missing logic test
+
+Recommendations:
+  1. Add boundary test for validator edge case
+  2. Test logical conditions in user API
+  3. Overall test quality: Excellent (>90%)
+```
+
+## Best Practices
+
+- Run after achieving high code coverage
+- Focus on survived mutants
+- Add tests to kill survivors
+- Aim for 80%+ mutation score
+- Expensive operation - run periodically