The official DeepSeek proxy URL for Janitor AI is https://api.deepseek.com/chat/completions, used with the model deepseek-flash.
Both /chat/completions and /v1/chat/completions reach DeepSeek's server; a bare base URL fails because Janitor AI does not add the path.
Test the key with a curl call first; if it works, any remaining error is in the Janitor AI form.
OpenRouter lists deepseek/deepseek-v4.1-flash, and no DeepSeek model with a :free suffix, so older free-model guides are stale.
Never give a third party your official DeepSeek key; match key, URL and model to one provider.