stryker-reporter-llm

Stryker reporter that produces LLM-friendly survivor output (survivors.md).

Installation

pnpm add -D stryker-reporter-llm

Configuration

Add to your stryker.config.*:

export default {
  reporters: ["html", "json", "stryker-reporter-llm"],
  plugins: ["@stryker-mutator/vitest-runner", "stryker-reporter-llm"],
  // ...
};

Output path

You can configure where survivors.md is written via llmReporter.outputPath. Default: .stryker-output/survivors.md.

export default {
  reporters: ["html", "json", "stryker-reporter-llm"],
  plugins: ["@stryker-mutator/vitest-runner", "stryker-reporter-llm"],
  llmReporter: {
    outputPath: "custom/path/survivors.md",
  },
  // ...
};

When using a custom outputPath, ensure any agent or tool that reads the survivors file uses the same path.

Output

The reporter writes .stryker-output/survivors.md (or your configured outputPath) with surviving mutants formatted for LLM consumption. If no survivors exist, the file contains ALL_KILLED.

Enriched output format

Each survivor entry includes context to help an LLM classify likely causes and propose fixes:

| Field | Description | |-------|-------------| | ID | Stable mutant identifier for correlation across runs | | Location | Line and optional column range for precise navigation | | Tests completed | Number of tests run for this mutant | | Covered by | Resolved test names that exercised this code (when Stryker provides testFiles) | | Nearby killed mutants | Killed mutants on the same line, with their failure reasons (helps distinguish "add test" vs "strengthen assertion") | | Original (snippet) | Code snippet at the mutation site |

This helps LLMs distinguish between:

• Missing test — no or weak coverage; add a new test case
• Weak assertion — covered but not asserted; strengthen an existing test
• Equivalent mutant — nearby mutants killed with different reasons; may be ignorable

Example

## File: `src/discount.ts`

### Survivor 1 — Line 27
- **ID:** `2`
- **Mutator:** `ConditionalExpression`
- **Replacement:** `false`
- **Location:** Line 27 (col 7–20)
- **Tests completed:** 3
- **Covered by:** calculateDiscount returns subtotal for bronze tier; calculateDiscount applies silver discount for quantity >= 5; calculateDiscount handles gold tier
- **Nearby killed mutants:** `ConditionalExpression`: expected -1 to be 20 // Object.is equality; `EqualityOperator`: expected -1 to be 20 // Object.is equality
- **Original (snippet):** `if (unitPrice < 0) {`

Optional: STRYKER_SCOPE

When running scoped mutations (e.g. stryker run --mutate "src/auth.ts"), you can set STRYKER_SCOPE so the reporter filters survivors to only include files in scope:

• Format: Comma-separated file paths or globs (e.g. src/auth.ts, src/**/*.ts)
• Behavior: When set, survivors.md includes only mutants from files matching the scope
• Default: When not set, all survivors are included (backward compatible)

Example:

STRYKER_SCOPE=src/auth.ts npx stryker run --mutate "src/auth.ts"