Fable 5 vs. Opus 4.8 vs. GPT-5.6 Sol, Terra, and Luna
OpenAI just launched Sol, Terra, and Luna. I compared what all five models can actually do across reasoning, coding, agents, research, vision, tools, writing, and speed.
I had OpenAI's new GPT-5.6 page open on one screen and Anthropic's model guide open on the other.
Five model names. Two companies. A lot of big claims.
The first version of this guide leaned too hard on price. That was the wrong way to compare these models. Price only helps after you know what the model can do.
The real questions are harder. Which model can plan a long job? Which one can code, use tools, browse the web, read a chart, work across a huge file set, or check its own work? Which one is built for one hard project, and which one is built to run the same task all day?
OpenAI made GPT-5.6 generally available today, July 9, 2026. Sol is the flagship. Terra is the balanced tier. Luna is the fast, lower-cost tier. All three are rolling out across ChatGPT, Codex, and the API.
That gives us five models to compare when we add Claude Fable 5 and Claude Opus 4.8.
This guide compares them on the same capability map. You will see what each model can do, where the official tests show a lead, and where the vendor claims stop short of a fair head-to-head answer.
This is for you if:
You use AI for coding, research, writing, or business work.
You want to build agents that can use tools and finish long jobs.
You need to know what changes when you move from Luna to Terra, Sol, Opus, or Fable.
I checked the current model pages, launch posts, context limits, tool lists, and benchmark table. I kept vendor claims separate from my own practical picks. I also left price near the end, where it belongs.
Behind this paywall, you get:
A five-model capability map.
A head-to-head look at reasoning, coding, agents, research, vision, writing, and speed.
The official benchmark scores that show there is no one winner.
A capability-first model router you can copy.
A short price note after the capability decision.
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