Qwen 3.5 fine-tune hits tag confusion

Qwen 3.5's native reasoning mode uses `<<<reasoning_start>>>` and `<<<reasoning_end>>>`, leading to direct conflict when users attempt to fine-tune using alternative tags like `<think>`.

• –The model's behavior shows strong structural priors from its RLHF/SFT training that prioritize the official reasoning delimiters.
• –Retraining with the new tags while still seeing `</think>` suggests a lingering tokenizer or chat template mismatch that hasn't been fully overwritten.
• –This highlights the difficulty of re-mapping reasoning behaviors in models where the chain-of-thought process is deeply integrated into the pre-training and alignment phases.
• –Developers should prioritize aligning their datasets with the model's native special tokens rather than forcing new ones on "thinking-heavy" architectures.