DeepSeek's Response API Isn't OpenAI Responses. That One Parser Mistake Drops the Reasoning.

I keep seeing developers use "DeepSeek response API" and "OpenAI Responses API" as if they mean the same thing.

They do not.

That small naming mistake can make your integration look like it works while quietly dropping the most important field in the response: reasoning_content.

I spent time checking the DeepSeek V4 docs and the live TokenMix model catalog. The practical answer is simple:

DeepSeek is OpenAI-compatible at the Chat Completions layer. It is not documented as OpenAI /responses compatible.