Glossary Context window
Context window
AgenticAI
Definition
The fixed amount of text a model can attend to at once — the whole conversation, the files it has read, the tool results — beyond which the earliest material falls out or is summarized
Measured in tokens. Everything an agent knows about the current task lives here; nothing in it survives the session unless it is written somewhere. Handoffs exist because context windows end.
The working memory of a session, and the reason long sessions get worse before they end: the useful material from the start is what gets compacted first. The Dictionary of AI Coding has a longer treatment.