For engineers who don't trust their test suite: catch real bugs and kill the flake.
Reach for this when your suite is green but you don't believe it - high coverage that misses the bug, tests that flake in CI, mocks that pass while production breaks. It takes a suite from "runs" to "trustworthy": prove which gaps actually matter, root-cause flakiness instead of retrying around it, and use mutation testing to expose assertions that verify nothing. Pull it in before a release cut or when a passing build still ships regressions.
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Arranged in the author's recommended order. Walk through them in sequence, or open any one on its own.
Root-causes intermittently failing tests and eliminates the hidden dependency at its source instead of retrying around it.
Produces a risk-ranked list of untested critical paths and branches from a real branch-coverage report crossed with git churn, naming the specific missing cases and the smallest test that buys the most safety.
Builds realistic domain fixtures, factories, and edge-case datasets with the builder pattern and valid defaults, so each test owns exactly the data it asserts on.
Writes consumer-driven contract tests at service and API boundaries – Pact consumer tests, provider verification, broker publishing, and can-i-deploy gates – so an incompatible change fails a build instead of breaking integrations in production.
Converts an acceptance criterion or user story into one maintainable Playwright or Cypress end-to-end test that reads like the journey it covers.
Designs the minimal set of test doubles for a unit or integration test and decides, per dependency, whether to stub, fake, spy, mock, or exercise the real thing.
Runs mutation testing on already-covered code and turns each surviving mutant into a specific missing assertion, exposing tests that execute code but verify nothing.
Writes and reviews end-to-end Playwright tests that survive UI refactors and never fail on timing – resilient locators, web-first assertions, isolated state, and a flake policy with quarantine rules.
Drive development with strict Red-Green-Refactor discipline – write one failing test, write the minimum code to pass it, refactor on green – including test-list planning and a worked kata cycle.