Which model? Just tried a bunch of ChatGPT, OpenAI's API, Claude, Anthropic's API and DeepSeek's API with both chat and reasonee, every single one replied with a single "hi".
gpt-4.5-preview-2025-02-27 replied with "Hi!"
I got "hi", as expected. What is the full system prompt + user message you're using?
https://i.imgur.com/Y923KXB.png
> gpt-4.5-preview-2025-02-27
Same "hi": https://i.imgur.com/VxiIrIy.png
Say just 'hi'
I used empty system prompt.