During February 2025, Esko Lahti focused on backend stability for the promptfoo/promptfoo repository, addressing a critical bug in the OpenAI Chat Provider integration. He implemented robust handling for cases where tool call arrays were empty, resolving an invariant failure caused by malformed llm-rubric responses. Using JavaScript and TypeScript, Esko refactored the message assignment logic and introduced regression tests to ensure future reliability. His work improved the reliability of chat interactions and reduced runtime errors in downstream tooling. The depth of his contribution lay in both the technical fix and the preventative testing, demonstrating strong skills in API integration and backend development.