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---
name: test-mutation
description: Analyzes test quality by identifying mutations your tests would NOT catch
argument-hint: <file-path>
user-invocable: true
allowed-tools: Read Grep Glob
effort: medium
---
## Mutation Testing Analysis
Target: **$1**
Since we can't run Stryker/mutmut directly, do a manual mutation analysis:
1. Read the source file and its test file.
2. Apply common mutations mentally and check if existing tests would catch them:
- Change `>` to `>=` (boundary condition)
- Negate boolean conditions (`!`)
- Remove a function call
- Change `&&` to `||`
- Return early / return null instead of computed value
- Off-by-one in loops
3. For each mutation that would NOT be caught:
- Describe the mutation
- Explain why it would go undetected
- Write a test that catches it
4. Report:
```
Surviving mutations found: N
Tests added: N
Estimated mutation score: X% → Y%
```
