OpenAI GPT-5 and GPT-5.5 — The Agentic Turn, the Hallucination Tension, and OpenAI's 2025–2026 Flagship Arc

OpenAI's GPT-5 (August 2025) unified reasoning and speed for the first time. GPT-5.5 (April 2026) became the new default for ChatGPT, with 52.5% fewer hallucinations in high-stakes domains, sharper agentic coding, and a more concise personality. But third-party evals tell a more complicated story. We review the full 5.x arc.