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Claude Fable 5.1

Available · Language, Multimodal, Reasoning

Claude Fable 5.1 is a language model in Anthropic's Claude Fable line and the successor to Claude Fable 5, released on 1 September 2026 on the Claude apps, the Claude API and major cloud platforms. Anthropic says it is the same model as Claude Mythos 5.1 with different safeguards; token prices match Fable 5, cache reads cost less, and its text carries Anthropic's watermark. [1]September 1, 2026 [2]Introduction; Cost and availability [3] Primary source

Timeline of Claude Fable 5.1 →

Claims and evidence

  • Released Primary source[3]Availability: Released [1]September 1, 2026
  • Status Available Primary source[4]Compare models table [3]Availability [5]Model status table
  • Change · Other Cache reads cost 0.25 USD per million tokens, 0.025 times the input price instead of 0.1 times; input and output prices are unchanged.Compared with Claude Fable 5 Primary source[1]September 1, 2026
  • Change · Safety Revised cyber safeguards allow vulnerability discovery; penetration testing, exploit generation and binary-based vulnerability scanning are still redirected to Anthropic's Opus models.Compared with Claude Fable 5 Primary source[2]
  • Change · Tool use Forced tool choice (tool_choice types any and tool) returns an error; auto and none still work.Compared with Claude Fable 5 Primary source[1]September 1, 2026
  • Change · Other Generated text carries Anthropic's text watermark.Compared with Claude Fable 5 Primary source[1]September 1, 2026
  • Input text, image Primary source[3]Capabilities: Input and output
  • Output text Primary source[3]Capabilities: Input and output
  • Context window 1M tokens Primary source[3]Capabilities: Context window
  • Parameters Not disclosedNot disclosed by Anthropic. No claim madeNo source
  • Access API, consumer app, cloud partner Primary source[2]A new performance frontier [3]Availability: Platforms

Lineage

Predecessors

Successors

No known successor.

Based on

Not derived from another model.

Variants and derived

Siblings

None recorded.

All ancestors

All descendants

Variants

No variants recorded in this record.

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