It claims to be 5.1, I do not know how to check it
The first response is claimed to by gpt-5-1.
The second response is claimed to by gpt-5-1-t-mini (thinking for 5 seconds).
there is no version selection.
If I switch to a free ChatGPT account, I can still select “Thinking” on the website by click on the plus next to the input box. That then routes me to gpt-5-1-t-mini.
Alternatively you can append “think hard” to your prompt, which will usually route you to gpt-5-1-t-mini too. I tried this with your prompt and it worked.
If you want to try your prompt against frontier LLMs, you could try LMArena. Your prompt will be shared with researchers and trained on, but you can try gpt-5.1-thinking-high, opus-4.5-thinking and gemini-3-pro-preview for free. The Gemini App and Google AIStudio also give free access to gemini-3-pro-preview. Or you buy a subscription.
The first response is claimed to by
gpt-5-1. The second response is claimed to bygpt-5-1-t-mini(thinking for 5 seconds).If I switch to a free ChatGPT account, I can still select “Thinking” on the website by click on the plus next to the input box. That then routes me to
gpt-5-1-t-mini. Alternatively you can append “think hard” to your prompt, which will usually route you togpt-5-1-t-minitoo. I tried this with your prompt and it worked.Note: with free ChatGPT the context window of your
gpt-5-1is limited to 16k tokens.If you want to try your prompt against frontier LLMs, you could try LMArena. Your prompt will be shared with researchers and trained on, but you can try gpt-5.1-thinking-high, opus-4.5-thinking and gemini-3-pro-preview for free. The Gemini App and Google AIStudio also give free access to gemini-3-pro-preview. Or you buy a subscription.