Deepseek via Anthropic api doesn't show reasoning blocks

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Describe the bug When using deepseek-reasoner via openrouter and Openai-chat api, I see reasoning blocks. When I use deepseek official anthropic api I don't.

To Reproduce

1. configure deepseek as a custom provider with anthropic api
2. use deepseek-reasoner model
3. ask something - no thinking blocks visible

Expected behavior See thinking blocks.