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Claude Fable 5.1 and Mythos 5.1: Anthropic's Top Models Get an Update

On 1 September 2026 Anthropic released Claude Fable 5.1 (generally available) and Claude Mythos 5.1 (limited availability). Fable 5.1 costs $10/$50 per million tokens with unusually cheap $0.25 cache reads, a 1M-token context and 128k output. What it's for, what's restricted, and how to use it in Claude Code.

On 1 September 2026, Anthropic released Claude Fable 5.1 and Claude Mythos 5.1, updates to its most capable model tier.

  • Fable 5.1 (claude-fable-5-1) is generally available.
  • Mythos 5.1 (claude-mythos-5-1) has limited availability, for organisations verified through Anthropic's programmes.

The details

Fable 5.1
Price (per million tokens) $10 input / $50 output
Cache reads $0.25 — 0.025x input, a quarter of the usual 0.1x ratio
Context window 1M tokens
Max output 128k tokens
Thinking Adaptive, always on
Batch API $5 / $25

The cheap cache reads are the notable change. For long-running agents that re-read a large, stable context on every step, caching makes Fable 5.1 far cheaper than its headline price suggests. (Prompt Caching Explained)

Restrictions

  • tool_choice of type any or tool returns a 400 error — the model decides when to use tools.
  • Several features — per-message effort changes, turn-scoped system messages and a thinking "updates" display — are in beta.

Fable vs Opus 5.5

Three weeks after Fable 5.1, Anthropic released Opus 5.5, which it says performs at Fable 5.1's level on most tasks at $4 / $20. In practice:

  • Opus 5.5 for most hard work — it's the default in Claude Code. (Claude Opus 5.5 Released)
  • Fable 5.1 for the hardest, longest-running tasks where the last bit of capability matters.

In Claude Code

Use the fable alias (/model fable), or best, which resolves to Fable where available. Neither Fable model is the default on any plan, and Fable sessions use your plan's usage quickly — keep it for problems that need it. (How to Change the Model, Claude Code Usage Limits)

Source: Claude Platform release notes · Pricing

EasySpawn hosts Claude Code on a persistent server with your own subscription or API key, so you can switch to Fable for a hard problem and back again without changing anything else. See how it works or join the waitlist.

Related: Opus vs Sonnet vs Haiku · What Is a Reasoning Model? · Claude API Pricing Explained · Claude Haiku 5.5 Released

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