During February 2026, Hangjun contributed to the huggingface/transformers repository by addressing a stability issue in backend parameter handling. He focused on the convert_rope_params_to_dict function, where he resolved a TypeError that occurred when the ignore_keys_at_rope_validation parameter was provided as a list rather than a set. By normalizing this input to a set using Python, Hangjun ensured consistent and error-free set operations, particularly during JSON deserialization of model configurations such as Qwen3.5 via vLLM. His approach preserved backward compatibility and maintained expected behavior, demonstrating careful attention to robustness and correctness in backend development with Python.