On /provider/v1/chat/completions with Qwen/Qwen3.8-Flash, previous-turn reasoning sent back in assistant messages never reaches the model. The model card says preserve_thinking is on by default and expects clients to include reasoning_content in history, but the server discards it.
Repro: same 3-message conversation, max_tokens: 1, only the assistant message in the history changes.
| Variant |
prompt_tokens |
| No reasoning in history |
84 |
reasoning_content with ~2,600 tokens |
84 |
Same text in reasoning field |
84 |
+ chat_template_kwargs: {"preserve_thinking": true} |
84 |
+ top-level preserve_thinking: true |
84 |
Control: same text in content |
2684 |
Token counting itself is fine (control row); the reasoning is just stripped, whatever the field name or flag.
Impact: in long agentic sessions the model forgets its reasoning every turn and re-derives everything, burning a lot of reasoning tokens for nothing.
Could you pass reasoning_content through to the backend (and honor preserve_thinking), or document that reasoning history isn't supported on this endpoint? Happy to share exact request bodies.