02Practitioner11 min
Summarize a long document without losing the details
Ask for structure first, then the summary; check it against three facts.
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Start lessonUnderstand · How models work
How models work
What’s a context window?
How much an AI can “see” at once, and why long chats start to drift.
5 min readPlain English
Read it in
In one sentence
The context window is the AI’s working memory: everything it can take into account at once, from your instructions to the chat so far and any files, up to a fixed limit.
Think of it like…
A desk. The AI can work with everything on it: your note, the letter you’re answering, the price list. The desk is big, but it has edges. Pile on more and the oldest papers slide off the back. Even before that, a sheet buried in the middle of the pile is easy to miss.
How it works
Each time you send a message, the app hands the model a bundle: its instructions, the conversation so far, any files, and your new message. The model reads that bundle, and only that, to write its reply.
- 1InstructionsThe app’s own instructions, plus any custom instructions you’ve saved, go in first.
- 2History and filesEarlier messages in this chat, and the files you’ve added (or the relevant parts of them), are sent again every time.
- 3Your messageYour latest question goes in last, and the reply is written from the whole bundle.
- 4The limitThe window is measured in tokens (words and pieces of words). When a chat outgrows it, the app drops or shortens older parts, or asks you to start a new chat.
A new chat starts with a clear desk. The model won’t remember yesterday’s conversation unless the app has saved notes and adds them back in.
Good at, bad at
Good at
- Working from a document you’ve attached
- Following instructions given in this chat
- Comparing a few files side by side
Bad at
- Details from early in a very long chat
- Remembering past chats on its own
- Telling you what fell out of view
Where you’ll see it at work
The drifting chat. Two hours into planning a bakery’s holiday menu, the assistant forgets the “no nuts” rule from the start. Restate it, or start fresh.
The clean restart. A design studio ends a long chat by asking for a summary of what’s been decided, then pastes it into a new chat to carry on.
The long contract. A plumbing firm attaches a supplier contract and asks the AI to quote the clause it’s relying on, to check it read the right part.
Myth vs reality
Myth: A bigger context window means it remembers everything you’ve said.
Reality: Bigger windows hold more, but details in the middle of a long pile still get missed, and nothing carries into a new chat unless the app saves it.
Words to know
Test yourselfNo grades
Forty messages into a chat, the AI starts ignoring an instruction you gave at the start. What’s the best fix?
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