Claude 2.1

Claude 2.1, released November 21, 2023, doubled the context window to 200,000 tokens — about 150,000 words, or more than 500 pages — the largest of any commercially available model at the time. It landed in the same week that OpenAI was consumed by the firing and reinstatement of Sam Altman, and Anthropic used the opening to push long-document work as the thing Claude was for: contract review, financial filings, entire technical specifications in a single prompt.

Anthropic published no new academic benchmark scores with the release, leading instead on reliability: a 2x reduction in false statements compared with Claude 2.0, and a 30% drop in incorrect answers on document-comprehension tasks. The release also brought system prompts and a beta of tool use. Independent needle-in-a-haystack testing published the day after launch found recall degrading once prompts ran past roughly 90K tokens, and varying with where in the document the buried fact sat — an early public demonstration that having a large context window and using all of it reliably are two different problems. Anthropic followed up two weeks later with prompting guidance that recovered much of the gap.