# Context Window

Canonical: https://aimastermindlab.com/glossary/context-window

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The context window is the maximum amount of text (measured in tokens) that an LLM can process in a single inference call. Everything outside the window is effectively invisible to the model. Modern models range from 8K to 2M+ token context windows.

## What is a Context Window?

The context window defines how much text an LLM can "see" and "remember" in a single request. It includes the system prompt, the user's message, any retrieved documents, conversation history, and the generated response — all counted together toward the limit.

## Why Context Window Size Matters

- **Long documents** — Analysing a full codebase, legal contract, or research paper requires a large context.
- **Long conversations** — Multi-turn chat applications accumulate history quickly.
- **RAG** — Retrieving many document chunks requires space in the context window.
- **Few-shot examples** — Including many demonstrations for in-context learning consumes tokens.

## Context Window Sizes (2026)

| Model | Context Window |
|---|---|
| GPT-4o | 128,000 tokens |
| Claude 3.5 Sonnet | 200,000 tokens |
| Gemini 1.5 Pro | 1,000,000 tokens |
| Gemini 1.5 Flash | 1,000,000 tokens |
| LLaMA 3 70B | 128,000 tokens |
| Mistral Large | 128,000 tokens |

## The "Lost in the Middle" Problem

Research shows that LLMs tend to recall information at the **beginning and end** of their context more reliably than information in the **middle**. For RAG systems with large contexts, placing the most important retrieved chunks near the beginning or end of the context yields better results.

## Managing Long Contexts

Strategies for working within context limits:
- **Chunking and retrieval** (RAG) — Only load relevant sections.
- **Summarisation** — Compress old conversation history.
- **Sliding window** — Maintain a fixed-size window of recent context.

## Example

If you paste a 300-page PDF (≈150,000 tokens) into a GPT-4o chat (128K context limit), the beginning of the document gets cut off. Gemini 1.5 Pro's 1M token context could handle the entire document at once.
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Source: https://aimastermindlab.com/glossary/context-window — AI Mastermind Lab (https://aimastermindlab.com)