Most "free token counter" tools in your bookmarks are not running the model's tokenizer. They're running a character-ratio estimate and labeling the output "tokens". For OpenAI's GPT family the official tokenizer is open and easy to ship in a browser. For Claude, Gemini, and most others it isn't. Here's what that means for your context-window math. Up-front disclosure on this one: the tool I link to below is one I built. I got tired of paste-counter-paste-counter loops where the same input produced different numbers, and tired of tools that claim to support every model but quietly use one tokenizer for all of them. Free, client-side, no signup. I'm linking to it because it's what I use, and because I'd rather show you how it works than pitch it. If you've ever opened three "GPT token counter" tabs and gotten three different numbers, you're not crazy and the tools aren't all wrong. They're doing different things and labeling them the same way.…