In November 2023, Anthropic released Claude 2.1 with a feature that redefined what AI assistants could do: a 200,000 token context window.

To put that in perspective: you could paste an entire novel, ask questions about it, and Claude would understand all of it.

The Context Revolution

200,000 tokens equals roughly:

• 150,000 words
• 500 pages of text
• An entire book
• Hundreds of emails
• Dozens of documents

GPT-4 had 32K tokens (extended to 128K). Claude 2.1's 200K was a massive leap.

Why This Mattered

Document analysis: Upload entire codebases, legal contracts, research papers Conversation memory: Maintain context through very long discussions Data processing: Analyze large datasets without chunking Complex tasks: Work with interconnected information across many sources

The possibilities expanded dramatically.

Tool Use

Claude 2.1 added another critical feature: tool use.

What it meant: Claude could call external APIs, retrieve information, and take actions beyond just generating text.

Use cases:

• Search databases
• Make calculations
• Retrieve real-time data
• Interact with other software

This moved Claude from "chat assistant" toward "AI agent."

Reduced Hallucinations

Anthropic reported significant improvements in accuracy:

• 50% reduction in false statements
• Better at saying "I don't know"
• More reliable citations
• Improved consistency

For enterprise use, this reliability improvement mattered as much as new capabilities.

Where Are They Now?

Claude's context window has remained industry-leading, with Claude 3 maintaining 200K tokens. The tool use feature evolved into more sophisticated agent capabilities.

November 2023 was when Claude proved that longer context windows weren't just a feature—they were a fundamentally different way to use AI.