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News · Anthropic · 2 min read

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Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1 with cache reads at $0.25 per million tokens

On September 1, 2026, Anthropic released Claude Fable 5.1 and Claude Mythos 5.1. Both kept the $10/$50 per million token price of Claude Fable 5 and cut prompt cache reads to $0.25 per million tokens.

Archive entry: this covers an event from September 1, 2026, and we published it on November 8, 2026. The details come from sources dated at the time and may have changed since.

On September 1, 2026, Anthropic released Claude Fable 5.1, the successor to Claude Fable 5, and Claude Mythos 5.1, which was limited to Project Glasswing participants, according to its Enterprise Frontier Safeguards post. Both models kept the Fable 5 price and cut prompt cache reads to $0.25 per million tokens.

What changed

  • Claude Fable 5.1 (claude-fable-5-1) targeted long-running agentic coding, knowledge work and research, and Claude Mythos 5.1 (claude-mythos-5-1) launched alongside it.
  • Both models had a 1M token context window by default, 128k max output tokens and always-on adaptive thinking.
  • Pricing was $10 / $50 per million input/output tokens, as for Claude Fable 5. Cache reads cost $0.25 per million tokens, 0.025x the base input price, against 0.1x for other models.
  • Anthropic estimated that Fable 5.1 cost about 25% less than Fable 5 for typical token-billed workloads and up to about 45% less for highly agentic work.
  • Fable 5.1 was available on the Claude API, Claude in Amazon Bedrock, Claude Platform on AWS, Claude Models on Google Cloud (Google managed) and Claude in Microsoft Foundry.

What it means for people building with AI

Teams that ran Fable 5 on long agentic coding jobs kept the same list price and got cheaper cache reads. They also had to handle 400 errors for forced tool use, because neither model supported the tool_choice types any and tool, and, for accounts created on or after August 31, 2026, for replayed thinking blocks after a change to the system prompt, tools or an earlier message.

Access and data retention

Mythos 5.1 was available only through trusted access programs, at launch to a set of US organizations. Both models required 30-day data retention and were not available under zero data retention unless Anthropic expressly authorized it. Fable 5.1 became the default Fable model in Claude Code v2.1.257 on September 1, 2026.

Sources

Written with AI assistance, strictly from the sources listed above. Before it went live, automated checks verified its dates, sources and every language edition.

Anthropic releases Claude Fable 5.1 and Claude Mythos 5.1 with cache reads at $0.25 per million tokens | vibecoding.inc