Takao Kashima focused on backend development and testing for the ethereum-optimism/op-geth repository, addressing a persistent issue with flaky rollback tests. By explicitly assigning nonces to transactions in the Go-based simulated ethclient, Takao eliminated nonce collisions that previously caused instability. Additionally, Takao optimized test performance by reducing the polling interval in the pendingStateHasTx function, resulting in faster test execution and more reliable CI feedback. This work improved the reliability and clarity of rollback test scenarios, leveraging skills in Go, Ethereum infrastructure, and test stability engineering. The solution demonstrated a targeted approach to enhancing test robustness within a complex codebase.