Claude update: Fable 5.1 gets 1M context and cheaper cache reads

Claude Fable 5.1 is now available across Claude platforms and through the API. The model has a 1 million-token context window, and Anthropic has cut cache-read pricing to $0.25 per million tokens. Claude Code 2.1.257 also makes Fable 5.1 the default model in the Fable tier.

Sources: Anthropic: Introducing Claude Fable 5.1 and Claude Mythos 5.1. Claude Code 2.1.257 release notes.

Claude Fable 5.1 in Claude Code, Cowork, and the API

A context window is the amount of text, code, and tool output a model can keep available during a task. Fable 5.1 has a 1 million-token context window.

Anthropic says Fable 5.1 is available on all platforms, including Amazon Web Services, Google Cloud, and Microsoft Azure. Its API model ID is claude-fable-5-1.

In Claude Code 2.1.257, Fable 5.1 is the default model in the Fable tier. It defaults to High effort in Claude Code and Medium effort in Claude Cowork and on Claude.ai. Effort controls how much computation the model applies to a task, so check it when comparing latency and cost.

Human step: Update Claude Code to 2.1.257 or later. Open /model and choose Fable 5.1 if it is not already selected. In sessions behind a Claude apps gateway, the fable and best aliases may still resolve to Fable 5, so select Fable 5.1 explicitly in the model picker.

Sources: Anthropic's Fable 5.1 launch details. Claude Code 2.1.257 release notes.

Cheaper cache reads for reused context

A cache read lets the model reuse context it has already processed. The price is now $0.25 per million tokens, 75 percent lower than for Fable 5. Other usage prices are unchanged at $10 per million input tokens and $50 per million output tokens.

Anthropic estimates that a typical workload costs about 25 percent less than with Fable 5. For context-heavy, tool-heavy agent workflows, the reduction may reach about 45 percent. Those are Anthropic estimates based on actual usage at default effort levels during August 2026, not a fixed saving for every run.

Source: Anthropic's Fable 5.1 pricing and availability details.

Mythos 5.1 has restricted access

Claude Mythos 5.1 uses the same underlying model as Fable 5.1 but has different safeguards for cybersecurity and life-sciences work. It is not generally available. Anthropic limits access to vetted organizations through its trusted access programs, currently for a set of US organizations.

Source: Anthropic on access to Claude Mythos 5.1.