Claude Fable 5.1

Language modelCard summarized

For demanding reasoning and long-horizon agentic work

Overview

Anthropic · 04Fable

Fable 5.1

For demanding reasoning and long-horizon agentic work

Cost: $10.00 in
Context: 1M

Input / output: $10.00 / $50.00
Max output: 128K
Knows up to: June 2026
Released: 1 Sep 2026

Anthropic · 04Fable

Fable 5.1

claude-fable-5-1

Thinking: Adaptive, always on
Cache read / 1M: $0.25
Inputs: Text, Images, PDF
License: Proprietary

From the system card

Coding, knowledge work and problem-solving, including novel mathematical and scientific reasoning
Largest gains over Fable 5 in terminal-based scientific and engineering work, computer use, and long-horizon agentic and professional work
Prompt injection gets through on fallback turns
Overstates what the user authorized

The general-access configuration of Anthropic's latest and most capable large language model. It shares weights with Claude Mythos 5.1 but adds safeguards that block certain tasks in high-risk dual-use domains such as biology and cybersecurity. It advances coding, knowledge work and problem-solving, with improved novel mathematical and scientific reasoning. p.2

claude-fable-5-1

Input / output per 1M: $10.00 / $50.00
Context window: 1M
Max output: 128K
Knows up to: June 2026
Reasoning: Adaptive (always on)
Weights: Closed

About this card. A model card is the maker's own account of a model: what it's for, how it was trained and tested, and where it falls short. This page condenses Anthropic's System Card: Claude Fable 5.1 & Claude Mythos 5.1. Every claim carries its source page.