What is Context Window?

Everything the model "sees" for one answer must fit in this window: your instructions, any pasted documents, the conversation so far and the space for its response. Exceed it and the oldest material falls out of view.

Two practical consequences:

1. Long documents are why Claude and Gemini can ingest whole reports — huge windows are a headline feature.
2. "It forgot" is usually overflow, not amnesia — the earlier parts of a long chat no longer fit. Summarizing or starting a fresh chat resets it.

A big context window is not memory — the model still learns nothing permanent from your chat. Persistent knowledge needs RAG or fine-tuning.