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Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, including changes to agent API behavior and watermark-based text detection.

Model release Provisional 78% confidence first seen

Anthropic released two variants, Claude Fable 5.1 and Claude Mythos 5.1, described as safeguard-layer versions of the same underlying model, along with performance and pricing-related changes (including lower cache-read costs). Coverage also notes the introduction of a statistical text watermark for detecting likely model-generated or processed passages and restrictions for how “thinking blocks” are preserved for accounts created on or after Aug. 31, 2026.

Anthropic Claude Fable 5.1 Claude Mythos 5.1 AWS Claude API Claude Fable 5 Project Glasswing Terminal-Bench-Science 0.1 The New Stack

Source coverage

Anthropic Releases Claude Fable 5.1 and Claude Mythos 5.1: 52.6% on Terminal-Bench-Science and 75% Cheaper Cache Reads MarkTechPost Claude Fable 5.1 watermark: It has a blind spot developers can’t ignore The New Stack

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