A Guide to Which AI to Use in the Agentic Era
By Ethan Mollick for One Useful Thing
I have written eight of these guides since ChatGPT came out, but this version represents a very large break with the past, because what it means to “use AI” has changed dramatically. Until a few months ago, for the vast majority of people, “using AI” meant talking to a chatbot in a back-and-forth conversation. But over the past few months, it has become practical to use AI as an agent: you can assign them to a task and they do them, using tools as appropriate. Because of this change, you have to consider three things when deciding what AI to use: Models, Apps, and Harnesses.
Models are the underlying AI brains, and the big three are GPT-5.2/5.3, Claude Opus 4.6, and Gemini 3 Pro (the companies are releasing new models much more rapidly than the past, so version numbers may change in the coming weeks). These are what determine how smart the system is, how well it reasons, how good it is at writing or coding or analyzing a spreadsheet, and how well it can see images or create them. Models are what the benchmarks measure and what the AI companies race to improve. When people say “Claude is better at writing” or “ChatGPT is better at math,” they’re talking about models.
Apps are the products you actually use to talk to a model, and which let models do real work for you. The most common app is the website for each of these models: chatgpt.com, claude.ai, gemini.google.com (or else their equivalent application on your phone). Increasingly, there are other apps made by each of these AI companies as well, including coding tools like OpenAI Codex or Claude Code, and desktop tools like Claude Cowork.
Harnesses are what let the power of AI models do real work, like a horse harness takes the raw power of the horse and lets it pull a cart or plow. A harness is a system that lets the AI use tools, take actions, and complete multi-step tasks on its own. Apps come with a harness. Claude on the website has a harness that lets Claude 4.6 Opus do web searches and write code but also has instructions about how to approach various problems like creating spreadsheets or doing graphic design work. Claude Code has an even more extensive harness: it gives Claude 4.6 Opus a virtual computer, a web browser, a code terminal, and the ability to string these together to actually do stuff like researching, building, and testing your new website from scratch. Manus (recently acquired by Meta) was essentially a standalone harness that could wrap around multiple models. OpenClaw, which made big news recently, is mostly a harness that allows you to use any AI model locally on your computer.
Until recently, you didn’t have to know this. The model was the product, the app was the website, and the harness was minimal. You typed, it responded, you typed again. Now the same model can behave very differently depending on what harness it’s operating in. Claude Opus 4.6 talking to you in a chat window is a very different experience from Claude Opus 4.6 operating inside Claude Code, autonomously writing and testing software for hours at a stretch. GPT-5.2 answering a question is a very different experience from GPT-5.2 Thinking navigating websites and building you a slide deck.
It means that the question “which AI should I use?” has gotten harder to answer, because the answer now depends on what you’re trying to do with it. So let me walk through the landscape.
For more, click here.