GPT-5.5 Launches as OpenAI's First Fully Retrained Base Model Since GPT-4.5

OpenAI launched GPT-5.5 (codenamed 'Spud') on April 23, 2026, as its first fully retrained base model since GPT-4.5. The model is natively omnimodal, processing text, images, audio, and video in a single unified system. GPT-5.5 scores 82.7% on Terminal-Bench 2.0, leading every model by a meaningful margin, while GPT-5.5 Pro achieves 76.9% on SimpleBench and 159 on the Epoch Capabilities Index. GPT-5.4 scores 83.0% on the GDPval benchmark spanning 44 knowledge work occupations. The launch represents a ground-up architectural rebuild rather than an incremental update, pushing the frontier of agentic coding and multimodal AI capabilities.