What is GPT 5.5?

GPT 5.5 is a large language model developed by OpenAI.

• May 2026 - Nathaniel Whittemore said GPT 5.5 performed comparably to Opus 4.7, slightly ahead on medium settings and slightly behind on extra high and max settings, but at 10 to 60x lower cost.
• Jul 2026 - Andrew Curran said GPT 5.5, along with Claude Opus 4.8 and Kimmy K2.7, could identify the same vulnerabilities as Fable 5 in a report.
• Jul 2026 - Andrew Curran said Fable 5, in his limited experience, outperformed GPT 5.5 and Opus 4.8 significantly.