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What is a context window and what happens when you exceed it?

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01

Understand the problem

The model's working memory: what fits, what falls out, and how apps deal with the limit.

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02

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03

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The context window is the maximum number of tokens (prompt + generated output) a model can attend to in one request. Exceeding it causes an API error or forces truncation; apps handle the limit by summarizing, trimming old turns, or retrieving only relevant context. Long contexts also degrade attention quality ('lost i

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04

Read the code

Guarding the window before a chat call
const WINDOW = 200_000;
const RESERVED_OUTPUT = 4_096;

function fitMessages(system: string, turns: Msg[], docs: Doc[]) {
  let budget = WINDOW - RESERVED_OUTPUT - countTokens(system);
  const keptDocs  = takeWhileUnderBudget(rankByRelevance(docs), 0.4 * budget);
  const keptTurns = takeRecentUnderBudget(turns, budget - tokensOf(keptDocs));
  return { system, docs: keptDocs, turns: keptTurns };
}
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