Context Window
A context window is the total amount of text a language model can take into account while writing a reply, measured in tokens and including the reply itself. It is the model’s working memory for one conversation, and it is separate from the data the model was trained on.
How it fills
Everything in the request counts. The system prompt, every earlier turn of the conversation, any documents attached, the definitions of the tools the model may call, and the results those tools return, which for an assistant with web retrieval means the pages it fetched. The model’s own output, including any reasoning it does before answering, counts too. Anthropic’s context window documentation gives the current sizes as 200,000 tokens or one million depending on the model, and notes that accuracy and recall degrade as the window fills, so more room is not the same as better use of it.
What it means for a fetched page
When an assistant retrieves a page to answer a question, that page enters the window as text, alongside the other pages it retrieved and the conversation so far. The page is competing for a finite budget. A long page costs more of it and, past a point, is truncated or summarized before the model reads it. What survives that process is the material that was already compact: a sentence that answers the question by itself, a table with its caption, a stated number with its source. A homepage that reaches the answer in its ninth paragraph may never have that paragraph read.
A question readers ask
Is the context window the same as training data?
No. Training data is what the model learned from, fixed before release and impossible to update from a website. The context window is what the model reads right now, in this conversation, and a page fetched with retrieval enters it. A site cannot change what a model was trained on; it can change what the model has in front of it when it answers.
Related terms
Sources
Definition, what counts toward the window, and sizes by model: Anthropic’s context window documentation. No figures of this site’s own appear on this page.