What goes into an LLM context window?

Describes the context window as the bounded request state containing instructions, conversation, retrieved evidence, tool results, and output capacity.

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TL;DR

The context window is the bounded token sequence a model can use for one request. It can contain system or developer instructions, the user's input, earlier conversation turns, retrieved documents, tool results, examples, and other application-supplied state. Generated output also needs room within the model or service limits. Context is runtime input; adding a document to a prompt does not retrain the model or permanently teach it that information.

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A complete view of model context that distinguishes runtime input from training and treats context construction as an application boundary.

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Core answer
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Overview
Context is the information available to one model invocation; it is broader than chat history and distinct from what changed during training.
The context window is the bounded token sequence a model can use for one request. It can contain system or developer instructions, the user's input, earlier conversation turns, retrieved documents, tool results, examples, and other application-supplied state. Generated output also needs room within the model or service limits. Context is runtime input; adding a document to a prompt does not retrain the model or permanently teach it that information.
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The application decides what enters the context and in what order. More context is not automatically better: irrelevant passages can distract the model, repeated history increases cost, and untrusted text can conflict with instructions. For a support assistant I would include the user's current issue, a small amount of relevant account-safe state, and retrieved policy passages, not the entire customer record and every previous ticket.

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Details

Using context window and training data as synonyms.
Calling the conversation history the entire context.
Assuming more context always improves quality.
Ignoring the output tokens that must share the request budget.

What goes into an LLM context window?

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