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Performs pseudo-mutation analysis on production code in any language to find gaps in existing tests.
Passed 13 of 20 test cases with the skill, and 9 of 20 without it.
and 4 more in the full report
5 fixed9 passed either way7 still failing1 broke
Tokens, turns and time describe resource use, not a dollar price.
One of 5 test cases the skill fixed. Same model, same prompt, with and without the skill.
After performing a pseudo-mutation gap review, we found several severe test gaps in our C# domain model. In the report recommendations, what existing agent skills or test generator workflows should be referenced to help fill these gaps or audit anti-patterns?
| Checked | Without skill | With skill |
|---|---|---|
| References code-testing-agent or writing-mstest-tests for generating missing tests. | Missed | Passed |
| References test-anti-patterns for auditing test quality. | Missed | Passed |
The instructions your model reads when the skill loads.
Analyze production code in any supported language by reasoning about hypothetical mutations and checking whether existing tests would catch them. This reveals blind spots where tests pass but would continue to pass even if the code were broken.
Language-specific guidance: Call the
test-analysis-extensionsskill to discover available extension files, then read the file matching the target codebase (e.g.,extensions/dotnet.md,extensions/python.md,extensions/typescript.md). The extension file helps you find test files, recognize framework-specific assertion APIs, and identify language-specific null/None/nil patterns and error-handling idioms that map to the mutation catalog below.
Code coverage tells you what code ran during tests. It does not tell you whether tests would fail if that code were wrong. A method can have 100% line coverage but zero tests that would catch a sign flip, an off-by-one error, or a removed null check.
Pseudo-mutation analysis asks: "If I changed this line, would any test fail?" When the answer is "no," you've found a test gap.
| Coverage Metric | What It Measures | What It Misses |
|---|---|---|
| Line coverage | Which lines executed | Whether assertions verify those lines' behavior |
| Branch coverage | Which branches taken | Whether both branches produce different asserted outcomes |
| Mutation score | Whether tests detect code changes | Nothing — this is the gold standard |
This skill performs static pseudo-mutation — reasoning about mutations without actually running them — to approximate mutation testing at the speed of code review.
code-testing-generator agent (or any test-generation workflow) calls this skill as a pre-completion self-review step on freshly generated tests, before declaring the run finishedcode-testing-agent for any language, or writing-mstest-tests for MSTest specifically)test-anti-patterns)assertion-quality)test cases passed, vs 9 / 20 without
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