Chris Lies
← AI Lab

What Is My Context Window

September 26, 2026

The Question

  • What is my context window?

Setup

  • General research

Results

  • A context window is the amount of information an LLM can "see" at one time when generating an answer.
  • Tokens are not equal to words.
  • Exact context window sizes are different depending on the LLM being used.
  • Context window has to accommodate both input and output.

Claude context window sizes as of 09/26/2026

| Claude model      |   Context window |
| ----------------- | ---------------: |
| Claude Opus 5     | 1,000,000 tokens |
| Claude Sonnet 5   | 1,000,000 tokens |
| Claude Fable 5    | 1,000,000 tokens |
| Claude Opus 4.8   | 1,000,000 tokens |
| Claude Opus 4.7   | 1,000,000 tokens |
| Claude Opus 4.6   | 1,000,000 tokens |
| Claude Sonnet 4.6 | 1,000,000 tokens |
| Claude Sonnet 4.5 |   200,000 tokens |
| Claude Haiku 4.5  |   200,000 tokens |

OpenAI context window sizes as of 09/26/2026

| Model         |   Context window |
| ------------- | ---------------: |
| GPT-5.6 Luna  | 1,050,000 tokens |
| GPT-5.6 Terra | 1,050,000 tokens |
| GPT-5.6 Sol   | 1,050,000 tokens |
| GPT-5.5       | 1,050,000 tokens |
| GPT-5.4       | 1,050,000 tokens |
| GPT-5         |   400,000 tokens |

What I Learned

  • Context is important when deciding what to feed your agents.
  • Choosing the correct LLM is important.

GIT Repository

  • N/A

Log

  • 2026-09-26 — Researching context windows.