Anthropic Says New Defenses Cut Distillation Harvest 90-99%

Reasoning transcripts, not final answers, are the prize for distillers. In an October 12 research post, Anthropic reports that a model distilled from Claude's full reasoning transcripts scored 80.4% on SWE-bench Verified, versus 71.8% when distilled from only summarized reasoning.

The company says it has cut what distillers can harvest by 90-99%, through a bundle of changes rolled into Claude Fable 5.1 and Opus 5.5. The core move is a feature called 'preserved thinking': new API accounts, created after August 31, 2026, cannot edit Claude's prior context, which the post identifies as the technique attackers use to pull long reasoning transcripts out.

The post names two Chinese labs. Zhipu is said to have been 'distilling cyber capabilities into GLM-5.3'; Moonshot's Kimi models, per the post, 'secretly forwarded' requests to Claude, including 'surveillance analysis requests potentially linked to China's People's Liberation Army'.

A two-stage classifier now covers Sonnet 5.5, Opus 5 and 5.5, and Fable 5 and 5.1, with false positive rates 'below 0.1%'. Repeat attempters get stricter rate limits, and associated accounts are banned 'within two hours of detected distillation campaigns'. Anthropic says tens of thousands of accounts have been banned and that reasoning token extraction is down 10-100 fold. High-confidence blocked requests have been billed since September 24.

No third party has verified the 90-99% figure; the post is Anthropic's own. The company concedes 'revenue loss and substantial internal resource investment' and occasional false positives for legitimate users. It is the 344th Anthropic story we have tracked in the last 90 days.