5.6 Sol Ultra is the best LLM I've ever used for complex codebases and high effort prompts.

When you spend 1-2 hours thinking about a spec and writing a complex prompt, the most disappointing feeling in the world is when an LLM takes your spec and spits out total junk. Or does 1/20th of the work and then asks if it should keep going.

5.6 Sol Ultra finally - finally! - lives up to my expectations here. If it's not done, it knows it, and it keeps going. It just goes and goes until it is highly confident that it has met your expectations.

This is a model for builders, who have really put a lot of thought into exactly what they want and made it clear to the LLM. In the past, even highly specific, clear-eyed prompts would go unanswered by LLMs, and you'd have to hand-hold it across the finish line. Now, if you know what you want and tell the LLM what you want, you will get it 90% of the time.

I've been following and using this stuff daily at work and for hobbies since ChatGPT first went public. I used the heck out of Fable. I used the heck out of gpt-5.5 and 4.8 and all the other model shots in the past few years. gpt-5.6 Sol Ultra hits different.

My use cases: Python and TypeScript Lambda functions and Fargate containers at work doing backend cloud stuff, and for hobby, porting a legacy C++ MMORPG to a modern Rust/Linux backend and a Rust/WASM web client. Codebases with hundreds of thousands of SLOC from pre-LLM times. Lots of existing technical debt but functional code. Serious need of rearchitecting, refactoring and modernization across the board.

Sol is delivering shippable code at a rate I've never experienced before with no weird hacks needed to achieve it. It's the closest we've had to "it just works" in agentic engineering to this point. And the usage limits are so high that if you can afford the $200/mo plan, you are basically good to go for serious work.