You need to use DeepSeek API directly to gain the extra caching benefits. The DeepSeek provider on OpenRouter is only the 5th-cheapest for V4 Flash, so you have to specify DeepSeek provider when calling OpenRouter. But DeepSeek's API discounts on its models only applies if you call DeepSeek directly. So anyone using OpenRouter to call DeepSeek models is actually losing quite a bit of money.