Antoni Duda developed a new API for the nrfconnect/sdk-zephyr repository to support unregistration of Bluetooth periodic advertising sync callbacks, addressing the need for safer lifecycle management in embedded systems. He implemented the bt_le_per_adv_sync_cb_unregister function in C, ensuring safe iteration over callback lists to prevent concurrent modification issues. Antoni expanded test coverage by introducing regression tests and a dedicated Bash validation script, verifying the robustness of the unregistration process. His work focused on enhancing the flexibility and stability of the Bluetooth host stack, demonstrating depth in Bluetooth development, embedded systems, and testing frameworks, though the scope was limited to a single feature.