GPT-5.5 remains one of my favourite models I have worked with.

It was the first reasoning model that made speed feel like a feature rather than a compromise. Instead of spending twenty minutes thinking about the task, it could spend a few seconds reasoning and then start implementing.

That changed my workflow.

Efficient Reasoning

I do not normally want a model sitting on XHigh or Ultra for every task. Most of my work is better served by Low or Medium reasoning, a clear plan, and a fast implementation loop.

GPT-5.5 understood that style of work. You could explain what you wanted, and it would normally get moving without turning the conversation into an interview.

When it was good, it was astonishingly good. It understood the goal, implemented quickly, and gave you the feeling that the entire codebase was moving at once.

The problem was consistency.

Sometimes 5.5 behaved brilliantly. Sometimes it behaved like a maniac. You could never be completely certain which version had arrived for the session.

Great Starts, Weak Finishes

GPT-5.5 could implement most of a plan extremely well, then become strangely lazy near the end. The broad architecture would be there, the important files would be changed, and the last ten percent would quietly turn into placeholders, missed states, or details it claimed were complete.

That meant I had to keep checking the actual result. A confident summary was not proof that the work was finished.

This is one of the clearest improvements in GPT-5.6 Sol. Sol is slower to begin because it asks more questions, but it is much more likely to keep working until the plan is genuinely complete.

Front-End Design Was the Weak Point

GPT-5.5 had about as much natural front-end taste as a toddler with a box of crayons.

Without a visual reference, the interfaces could be horrific. Generic gradients, strange spacing, too many cards, and design decisions no human had requested.

There was one reliable trick: give it a screenshot of the intended design. Once it had something concrete to follow, it became far more capable. The problem was never that it could not write CSS. The problem was that it could not consistently decide what good CSS should produce.

GPT-5.6 is a major improvement here. It still needs direction, but the default result is much closer to something I would willingly show another person.

A Warmer OpenAI Model

GPT-5.4 often felt determined to reject whatever you proposed. GPT-5.5 softened that personality and became much more pleasant to work with.

It would still challenge ideas, which I value, but it felt warmer and more collaborative. That balance is one of the reasons I stayed with it even when newer alternatives appeared.

The Verdict

GPT-5.5 was fast, understandable, occasionally brilliant, and occasionally completely unhinged.

It taught me how valuable efficient reasoning could be. It also taught me never to trust an agent's completion message without checking the final interface.

GPT-5.6 Sol is more dependable, more thoughtful, and much better at finishing. But 5.5 still deserves credit for being the model that made this style of fast, agent-led development feel genuinely fun.