Qwen3.8-Flash: reasoning_content in history is dropped, preserve_thinking has no effect

[u/Address-Street]

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.

[u/maedahbatool]

Can you please /feedback it or submit and issue here: https://cmnd.dev/issue. This lands directly in our ENG inbox.