OpenAI’s Claude Mythos competitor GPT-5.6 Sol launches under government-controlled access it calls unsustainable

OpenAI’s new flagship GPT-5.6 Sol beats Anthropic’s Claude Mythos 5 in coding benchmarks, but the US government is forcing a restricted rollout. OpenAI isn’t happy about it.

OpenAI has unveiled GPT-5.6 Sol, a new generation of models built to compete with Claude’s Mythos class. The limited preview is only open to select partners through the API and Codex, at the explicit direction of the US government. The same government previously yanked Anthropic’s Mythos-class model Fable 5 off the market. OpenAI isn’t subtle about its frustration. “We don’t believe this kind of government access process should become the long-term default. It keeps the best tools from users, developers, enterprises, cyber defenders, and global partners who need them.”Ad GPT-5.6 also brings a new layered naming scheme that looks a lot like Claude’s. The number (x.6) marks the generation, while Sol, Terra, and Luna are permanent performance tiers that can evolve on their own. Sol is the flagship. Terra matches GPT-5.5 at half the cost. Luna is the budget option. On top of that, there’s a “max” mode for deeper reasoning and an “ultra” mode that farms out complex tasks to sub-agents running in parallel.AdDEC_D_Incontent-1 OpenAI’s benchmark numbers put Sol ahead of Anthropic’s Claude Mythos 5 in agentic coding. On Terminal-Bench 2.1, Sol scores 88.8 percent. Sol Ultra hits 91.9, Claude Mythos 5 lands at 88 percent, and Fable 5 trails at 84.3. Sol also shows gains in biology. On GeneBench v1, a benchmark for genomics and quantitative biology, it beats GPT-5.5 (30 percent vs.