First GPT-based app I've built was in summer 2022, right after I got API access to GPT-3, and I was writing first autonomous GPT wrapper right after I got GPT-4 access in February GPT-3. It didn't have "tools" it could use, but it had memory and it was (clumsily) moving along a pre-defined chat scenario. And I'm nowhere near top AI researchers who have got to have had close access much earlier — so I have absolutely no doubt there's got to be people who have been writing exactly what we now call "agents" for 3 years straight.