GPT-6 Astra vs GPT-5.6 Sol: Choosing by Completed Work, Not Model Age

GPT-6 Astra and Claude Fable 5.1 launched in early September 2026 with distinct optimizations. Astra excels in coding, scientific tooling, computer use, and execution-heavy agents. Conversely, Fable 5.1 leads in broad reasoning and offers cheaper cache reads for long-horizon workloads. Compared to GPT-5.6 Sol, Astra is preferred for autonomous coding and desktop control. Both Astra and Sol feature 1.05-million-token context windows, supporting structured outputs and function calling, though Astra focuses on higher reliability for completed work.