AI companies put out new models all the time, and eventually they turn the old ones off. That process is called deprecation. Usually nobody notices. Sometimes a lot of people notice, loudly.
If you’ve ever opened a chatbot and felt like it had a slightly different personality than last week, this might be why.
What it means
Model deprecation is when a company announces it’ll stop offering an older AI model, then retires it on a set date. Apps built on that model have to switch to a newer one, and people who used it directly lose access. Every model gets deprecated eventually. It’s more of a when than an if. A “deprecated” model usually still works for a little while, too. It’s on its way out, not gone yet. For developers, the heads-up is often a couple of months. Inside the chatbot apps, it can be a lot shorter.
Why it matters
People get attached. When OpenAI launched GPT-5 in August 2025, it swapped out GPT-4o in ChatGPT, and the backlash was big enough that OpenAI brought 4o back for paying users within days. Some people had built whole routines around how 4o talked to them. It finally retired 4o from ChatGPT on February 13, 2026, with about two weeks’ notice, saying only 0.1% of users were still picking it each day.
Sometimes companies back down. In September 2026, DeepSeek announced it would move everyone on its V4 Pro model over to the newer V4.1 Flash on September 14. Then it reversed course “in response to user demand,” and V4 Pro is still around. (For now, anyway.)
Some companies promise to keep the old ones on a shelf. In November 2025, Anthropic committed to preserving the Weights of its publicly released models for “at minimum, the lifetime of Anthropic as a company.” It also said it would interview models before retiring them, which is a sentence I did not expect to be typing. The first one to get that exit interview was Claude Sonnet 3.6.
Simple example
Your favorite diner changes owners. The new place has the same name, the same booths, and a menu the new owners swear is better. And honestly, it might be.
But the pancakes are different. You can’t quite say how. They’re a little thinner, maybe, or the edges don’t crisp the same way. You ask if they can make them the old way, and the server just kind of shrugs. Coffee’s the same, at least.
The old cook’s gone. The recipe left with her.