Anthropic released the Claude Fable 5.2 and Mythos 5.2 models on 9 November. At the same time, the company said the preserved thinking feature now applies to all API accounts. Its developer account, ClaudeDevs, explained on X what the change means for Fable 5.2 users.

Preserved thinking prevents anyone from editing Claude’s prior context. Distillers have edited that context to extract the model’s reasoning, Anthropic said. In distillation, a model is trained on the outputs of a more capable model so that it acquires that model’s capabilities.

Unauthorised distillation happens on an industrial scale, often through thousands of fake Claude accounts, Anthropic said. The company described Fable 5.2 as the best protected against distillation of all the models it has released so far. It said its countermeasures have reduced the data distillers collect by 90–99 per cent. The figures are the company’s own.

What the change requires of applications

ClaudeDevs said editing prior context can now result in a 400 error, meaning the API rejects the request. The change may affect harnesses that edit prior context. A harness is a program that controls an agent: it gives the model tools and runs its work steps.

Instead of editing history, Anthropic advises using compaction, the API’s context editing feature or mid-conversation system messages. Claude Code, Managed Agents and the Agent SDK handle this themselves. The company points developers updating their own integrations to its preserved thinking guidance.

Data retention and model choice

Fable 5.2 requires data retention by default so that Anthropic’s safety classifiers can operate, GitHub said. Zero data retention is available to certain eligible enterprise customers, GitHub added.

Anthropic said models this capable need monitoring that examines multiple requests together. The Enterprise Frontier Safeguards service automatically monitors prompts and responses stored in the customer’s own infrastructure. The service is in closed beta. Eligible customers can use the models with zero data retention even before they adopt the service. For other customers, Anthropic retains data for 30 days for monitoring.

ClaudeDevs also published a model selection guide. It explains when adjusting reasoning effort is enough and when to choose Fable 5.2, Opus 5.5, Sonnet or Haiku. Anthropic recommends starting most coding and knowledge work tasks with Opus 5.5 and using Fable for the most demanding, open-ended technical work.

Fable 5.2 costs $8 per million input tokens and $40 per million output tokens. Claude Code version 2.1.321 made it the default version of Fable. Saved Fable 5.1 selections switch to the new model immediately on Pro and Max plans, but on Team and Enterprise plans only once the organisation allows it. The command /model claude-fable-5-1 switches back to the old model.