Claude Sonnet 5.5 / Opus 5 & 5.5 / Fable 5 & 5.1

Change history

12 Oct 2026

Anthropic cuts Claude distillation by 90-99% with new reasoning-protection defenses

Parameter	Before	Now
Reasoning transcript access	Full chain-of-thought reasoning could often be extracted via active prompting or proxy accounts	Reasoning summarized, access restricted to originating organization, and new API accounts (post Aug 31, 2026) cannot edi
Distillation detection	Simple summarization-based defenses, bypassed by active extraction techniques	Two-stage classifier (constitutional classifiers approach) with <0.1% false positive rate, matching bio/cyber safeguard
Extracted reasoning tokens	Baseline high volume harvested by distillation networks	10-100x lower (some days too low to measure) after full rollout

News

Cyber Security· Important

Anthropic cuts Claude distillation by 90-99% with new reasoning-protection defenses

Anthropic published details on how it curbed 'illicit distillation' — large-scale covert operations that scrape Claude's reasoning transcripts to train rival models cheaply. New measures, including reasoning summarization, 'preserved thinking' for new API accounts, and a two-stage classifier system, reportedly cut the volume of usable reasoning data extracted by distillers by 10-100x. Anthropic says a Chinese lab, Zhipu, used distillation (including attempts against Claude) to build GLM-5.3, which US assessors found to be the most cyber-capable open-weight model released so far with safeguards that are easily bypassed.

12 Oct 2026Anthropic