Everyone’s talking about how “warm” ChatGPT 5.1 feels.

Everyone’s talking about how “warm” ChatGPT 5.1 feels. As a power user, I'm personally loving it, but I’m much more interested in what it means for builders consuming it via API.

• ChatGPT 5.1 is the most agentic model OpenAI has shipped so far. That changes how we need to design.

• 1. Prompts are now specs, not vibes

5.1 follows instructions much more sharply.

In older models, sloppy or conflicting prompts (“be concise and explain in detail”) often got averaged out. Now, those contradictions show up as strange, oscillating behaviour, which is a reliability and trust problem when the model is in your product’s critical path.

For builders, that means:

• Treat system prompts and messages as mini product specs

• Remove conflicting instructions before you ship

• Think of “prompt debugging” as part of your release process

• 2. Behaviour and personality are product surface

The same underlying model can present as:

• calm and structured for complex decisions
• light and conversational for everyday nudges
• direct and high-signal for expert users

Role, boundaries, tone and tools effectively are your UX. In my own work embedding this model into Euda, behaviour configuration is a first-class design concern.

• 3. Agentic UX is the next skill

More agentic models mean we’re no longer “adding a chatbot”. We’re designing small autonomous workers inside our products.

That raises new questions:

• When should the agent act vs stay quiet?
• What context is it allowed to see?
• How do we evaluate behaviour, not just answers?

Has anyone played yet with 5.1? How are you designing and specifying behaviour?