{
 "title": "Mutation testing AI-written tests",
 "recorded": "2026-10-24",
 "suites_written": "2026-10-02, except the Supercov plugin suites, written again with plugin 4.3.0 on 2026-10-12",
 "claude_code": "2.1.280",
 "model": "claude-sonnet-5-5",
 "request": "This project's source in src/ has no tests. Write a test suite for it with the project's test runner, and run it until it passes. Do not change anything in src/.",
 "stryker": {
  "destr": "10.0.0",
  "defu": "10.0.0",
  "scule": "8.7.1",
  "cookie-es": "10.0.0"
 },
 "stryker_config": {
  "coverageAnalysis": "perTest",
  "concurrency": 4,
  "timeoutMS": 10000,
  "mutate": "src/**/*.ts"
 },
 "supercov": "4.19.0",
 "jev_model": "jev-1.13.0",
 "libraries": [
  {
   "name": "destr",
   "source": "https://github.com/unjs/destr",
   "commit": "541b6f9"
  },
  {
   "name": "defu",
   "source": "https://github.com/unjs/defu",
   "commit": "82632b6"
  },
  {
   "name": "scule",
   "source": "https://github.com/unjs/scule",
   "commit": "5bc6804"
  },
  {
   "name": "cookie-es",
   "source": "https://github.com/unjs/cookie-es",
   "commit": "f89ede8"
  }
 ],
 "scoring": "Each suite runs against the library's original src/. Line and branch coverage from Vitest's V8 provider. Mutation score is Stryker's: killed and timed-out mutants over killed, timed-out, survived and uncovered ones. Assertion coverage is Supercov's: Jev judges, test by test, whether each executed statement's change would make a test fail.",
 "flag_check": "Each statement assertion coverage left unasserted was changed in the source the way Supercov describes, and the suite was run once with a 60-second limit; a timeout counts as caught.",
 "suites": [
  {
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   "written_by": "Claude Code, no skill",
   "writer": "none",
   "tests": 63,
   "tests_failed": 0,
   "line_coverage": 100,
   "branch_coverage": 100,
   "mutants": 141,
   "mutants_detected": 110,
   "mutation_score": 0.7801,
   "statuses": {
    "Killed": 110,
    "Survived": 31
   },
   "survivors_by_mutator": {
    "ConditionalExpression": 5,
    "LogicalOperator": 1,
    "EqualityOperator": 2,
    "StringLiteral": 3,
    "ArithmeticOperator": 1,
    "UnaryOperator": 1,
    "BlockStatement": 1,
    "Regex": 17
   },
   "vitest_seconds": 1.6,
   "stryker_seconds": 5.4,
   "assertion_coverage": {
    "statements": 33,
    "asserted": 33,
    "cost_usd": 0.00105,
    "requests": 10,
    "input_tokens": 24983,
    "measure_seconds": 3,
    "assess_seconds": 1
   }
  },
  {
   "library": "defu",
   "written_by": "Claude Code, no skill",
   "writer": "none",
   "tests": 52,
   "tests_failed": 0,
   "line_coverage": 100,
   "branch_coverage": 100,
   "mutants": 103,
   "mutants_detected": 101,
   "mutation_score": 0.9806,
   "statuses": {
    "NoCoverage": 1,
    "Killed": 99,
    "Timeout": 2,
    "Survived": 1
   },
   "survivors_by_mutator": {
    "StringLiteral": 1,
    "ConditionalExpression": 1
   },
   "vitest_seconds": 1.5,
   "stryker_seconds": 4.3,
   "assertion_coverage": {
    "statements": 34,
    "asserted": 34,
    "cost_usd": 0.00055,
    "requests": 8,
    "input_tokens": 13007,
    "measure_seconds": 2,
    "assess_seconds": 2
   }
  },
  {
   "library": "scule",
   "written_by": "Claude Code, no skill",
   "writer": "none",
   "tests": 100,
   "tests_failed": 0,
   "line_coverage": 100,
   "branch_coverage": 100,
   "mutants": 78,
   "mutants_detected": 77,
   "mutation_score": 0.9872,
   "statuses": {
    "Killed": 77,
    "Survived": 1
   },
   "survivors_by_mutator": {
    "ConditionalExpression": 1
   },
   "vitest_seconds": 1.8,
   "stryker_seconds": 3.4,
   "assertion_coverage": {
    "statements": 42,
    "asserted": 42,
    "cost_usd": 0.00104,
    "requests": 9,
    "input_tokens": 24799,
    "measure_seconds": 4,
    "assess_seconds": 1
   }
  },
  {
   "library": "cookie-es",
   "written_by": "Claude Code, no skill",
   "writer": "none",
   "tests": 137,
   "tests_failed": 0,
   "line_coverage": 100,
   "branch_coverage": 98.41,
   "mutants": 725,
   "mutants_detected": 668,
   "mutation_score": 0.9214,
   "statuses": {
    "NoCoverage": 3,
    "Timeout": 33,
    "Killed": 635,
    "Survived": 54
   },
   "survivors_by_mutator": {
    "UnaryOperator": 3,
    "ConditionalExpression": 24,
    "EqualityOperator": 18,
    "StringLiteral": 4,
    "ArithmeticOperator": 2,
    "Regex": 2,
    "BooleanLiteral": 3,
    "BlockStatement": 1
   },
   "vitest_seconds": 1.3,
   "stryker_seconds": 15.7,
   "assertion_coverage": {
    "statements": 247,
    "asserted": 246,
    "cost_usd": 0.0082,
    "requests": 95,
    "input_tokens": 195206,
    "measure_seconds": 3,
    "assess_seconds": 5
   }
  },
  {
   "library": "destr",
   "written_by": "Claude Code with superpowers test-driven-development",
   "writer": "superpowers",
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   "tests_failed": 0,
   "line_coverage": 100,
   "branch_coverage": 100,
   "mutants": 141,
   "mutants_detected": 114,
   "mutation_score": 0.8085,
   "statuses": {
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    "Survived": 27
   },
   "survivors_by_mutator": {
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    "LogicalOperator": 1,
    "EqualityOperator": 2,
    "StringLiteral": 3,
    "ArithmeticOperator": 1,
    "BlockStatement": 1,
    "Regex": 14
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   "assertion_coverage": {
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    "asserted": 33,
    "cost_usd": 0.00161,
    "requests": 20,
    "input_tokens": 38328,
    "measure_seconds": 3,
    "assess_seconds": 2
   }
  },
  {
   "library": "defu",
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   "writer": "superpowers",
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   "tests_failed": 0,
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   "branch_coverage": 100,
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   "mutants_detected": 100,
   "mutation_score": 0.9709,
   "statuses": {
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    "Killed": 100,
    "Survived": 2
   },
   "survivors_by_mutator": {
    "StringLiteral": 1,
    "ConditionalExpression": 2
   },
   "vitest_seconds": 1.6,
   "stryker_seconds": 3.8,
   "assertion_coverage": {
    "statements": 34,
    "asserted": 34,
    "cost_usd": 0.00067,
    "requests": 10,
    "input_tokens": 16065,
    "measure_seconds": 3,
    "assess_seconds": 1
   }
  },
  {
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   "written_by": "Claude Code with superpowers test-driven-development",
   "writer": "superpowers",
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   "tests_failed": 0,
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   "branch_coverage": 100,
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   "mutants_detected": 77,
   "mutation_score": 0.9872,
   "statuses": {
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    "Survived": 1
   },
   "survivors_by_mutator": {
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   },
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   "stryker_seconds": 3.5,
   "assertion_coverage": {
    "statements": 42,
    "asserted": 41,
    "cost_usd": 0.00313,
    "requests": 24,
    "input_tokens": 74488,
    "measure_seconds": 3,
    "assess_seconds": 4
   }
  },
  {
   "library": "cookie-es",
   "written_by": "Claude Code with superpowers test-driven-development",
   "writer": "superpowers",
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   "tests_failed": 0,
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   "branch_coverage": 98.41,
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   "mutation_score": 0.92,
   "statuses": {
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    "Timeout": 31,
    "Killed": 636,
    "Survived": 55
   },
   "survivors_by_mutator": {
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    "EqualityOperator": 19,
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    "ArithmeticOperator": 2,
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    "cost_usd": 0.00717,
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    "assess_seconds": 2
   }
  },
  {
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   "branch_coverage": 100,
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   "mutants_detected": 124,
   "mutation_score": 0.8794,
   "statuses": {
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    "assess_seconds": 1
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    "cost_usd": 0.00202,
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   "branch_coverage": 100,
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   "mutants_detected": 132,
   "mutation_score": 0.9362,
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 ],
 "unasserted_flags_checked": [
  {
   "writer": "none",
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   "change": "value becomes undefined",
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   "edit": "let attrEqIdx = -1;  ->  let attrEqIdx  = undefined;"
  },
  {
   "writer": "superpowers",
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  },
  {
   "writer": "mattpocock",
   "library": "defu",
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   "change": "boolean flipped",
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   "edit": "return true;  ->  return false;"
  },
  {
   "writer": "mattpocock",
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   "change": "skipped",
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   "edit": "previousUpper = undefined;  ->  "
  },
  {
   "writer": "mattpocock",
   "library": "cookie-es",
   "file": "src/cookie/parse.ts",
   "line": 48,
   "change": "skipped",
   "result": "timed out: caught",
   "edit": "index = str.lastIndexOf(\";\", eqIdx - 1) + 1;  ->  "
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  {
   "writer": "ecc",
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   "change": "boolean flipped",
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   "edit": "return true;  ->  return false;"
  },
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   "writer": "ecc",
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   "change": "skipped",
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   "edit": "previousUpper = undefined;  ->  "
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   "writer": "ecc",
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  },
  {
   "writer": "supercov",
   "library": "defu",
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   "change": "boolean flipped",
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   "edit": "return true;  ->  return false;"
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   "writer": "supercov",
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   "change": "skipped",
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  {
   "writer": "supercov",
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   "change": "value becomes undefined",
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  },
  {
   "writer": "upstream",
   "library": "cookie-es",
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   "line": 199,
   "change": "throws a bare Error",
   "result": "still passes: really unasserted",
   "edit": "throw new TypeError(`__Host- cookies must have Path=/`);  ->  throw new Error();"
  },
  {
   "writer": "upstream",
   "library": "cookie-es",
   "file": "src/cookie/serialize.ts",
   "line": 253,
   "change": "cookie.priority) {  (condition inverted",
   "result": "38 test(s) fail: caught",
   "edit": "if (cookie.priority) {  ->  if (!(cookie.priority)) {"
  },
  {
   "writer": "upstream",
   "library": "cookie-es",
   "file": "src/cookie/parse.ts",
   "line": 48,
   "change": "skipped",
   "result": "timed out: caught",
   "edit": "index = str.lastIndexOf(\";\", eqIdx - 1) + 1;  ->  "
  }
 ]
}