It's branded as a separate model, but really it's like an 'ultrathinking' mode of OAI's model.
The details are not public but it's parallel test time compute and multi search, aka the model reasons along different vectors and compares answers.
If 'feels like' a smarter model, even though I'm pretty sure it's a synthetic variation of Astra.
It is by far the 'smartest model' if you were to benchmark it, and many of OAIs released were based on this.
There are few or no benchmark scores on it because it's not suitable for harness testing or coding, it's almost designed to be 'one shot' or 'few shot'.
It takes 8-25 to answer, and if you give it a sufficiently sophisticated prompt, it will think for 1 hour.
It requires OAI's 'pro' account aka the $200/month one.
It's the best open secret in AI, I can't recommend it enough.