Worked on security hardening for the CoT prompt formatter in the UKGovernmentBEIS/control-arena repository, addressing a vulnerability by wrapping chain-of-thought content in <cot> tags and applying HTML escaping to prevent adversarial injections. Used Python to implement these changes, focusing on back end development and adhering to security best practices. Updated and expanded unit tests to cover a range of adversarial input scenarios, ensuring robust validation of the new formatting logic. Delivered a breaking change that required downstream consumers to adapt, with comprehensive test evidence and clear documentation supporting the update. Maintained high code quality and strong test coverage throughout.