Claude Fable 5 — Technical Harness Report

Claude Fable 5, released by Anthropic, is a Mythos-class model available for general use, accompanied by an unsafeguarded counterpart, Claude Mythos 5, for vetted partners. This release features several architectural advancements over previous models, such as always-on adaptive thinking, a new tokenizer, and native multi-agent harness patterns. The model employs a two-stage classifier pipeline to manage refusals and has distinct fallback-handling contracts depending on the application surface. The Fable 5 model integrates advanced safety measures, including a refusal stop reason, and offers cost-effective multi-agent deployment for complex tasks. It also emphasizes the importance of effort parameter tuning and prompt iteration to optimize reasoning and control chain-of-thought elicitation. Additionally, the model's API includes mandatory data retention policies and specific configurations to enhance long-horizon execution, while Anthropic's documentation provides detailed harness patterns and practical guidance for effective deployment.

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