Takaaki Tsuzuki modernized the testing assertions for the increments/qiita-cli repository by refactoring core unit tests to use updated Jest matchers. Focusing on TypeScript and leveraging skills in refactoring and testing, he replaced legacy matcher functions with toHaveBeenCalledWith and toHaveBeenCalledTimes across several test files. This change improved test reliability, reduced flakiness, and maintained coverage, ensuring that future refactors would be easier to implement. By standardizing assertion syntax, Takaaki accelerated feedback on code changes and increased confidence in release quality. The work demonstrated a methodical approach to maintainability and quality assurance within a focused, one-month development period.