Claude Fable 5.1

Overview

Claude Fable 5.1, released by Anthropic on September 1, 2026, is a generally available Mythos-class model in the Claude 5 family for demanding reasoning and long-horizon agentic work. It shares its underlying model with the limited-access Claude Mythos 5.1. On B.AI, use the model ID claude-fable-5-1.

Key Features

• Long-Horizon Agentic Performance: Anthropic reports 52.6% on Terminal-Bench-Science 0.1, 55.8% on Terminal-Bench 4.0, 31.4% on AutomationBench, and 73.4% on CursorBench 3.2.0 with production safeguards enabled.
• Always-On Adaptive Thinking: Adaptive thinking cannot be disabled. The effort control supports low, medium, high, xhigh, and max, with high as the Claude API default.
• 1M Context and 128K Output: Provides a 1,000,000-token context window and up to 128,000 output tokens at standard per-token pricing across the full window.
• Agent-Oriented API Controls: Supports automatic tool use, strict tool schemas, structured output, preserved thinking, per-message effort, turn-scoped system messages, readable progress updates, and content provenance. Several of these controls are beta features.
• Lower Cache-Read Cost: Cache hits and refreshes cost 0.25 Credits/Token, one quarter of the Claude Fable 5 cache-read rate, while base input and output prices remain unchanged.

Best Use Cases

• Long-Running Software Engineering: Repository-scale implementation, debugging, code review, performance work, and autonomous sessions that span many tools and checkpoints.
• Multi-Step Research and Analysis: Research workflows that require search, evidence synthesis, iterative hypothesis testing, and recovery from failed steps.
• Enterprise Knowledge Work: Complex document, spreadsheet, presentation, financial-analysis, and cross-file workflows that benefit from a large context window and vision.
• Browser and Computer-Use Agents: Tasks that operate across applications, maintain state over extended sessions, and provide progress updates between tool calls.

Capabilities and Limitations

Capability	Description
Reasoning	Always-on adaptive thinking with five effort levels. Anthropic reports 60.9% on Humanity's Last Exam without tools and 65.0% with tools.
Creative Writing	Supports drafting, editing, research synthesis, and professional artifact creation.
Multimodal	Accepts text and image input and produces text output. It can analyze charts, diagrams, tables, files, and PDFs.
Context Window	1M tokens.
Max Output	128,000 tokens.
Tool Use	Supports automatic function calling, strict tool use, structured output, browser and computer-use workflows, and preserved thinking. Forced any or named-tool selection is not supported.
Multilingual	Anthropic lists multilingual support across current Claude models.
Knowledge Cutoff	Reliable knowledge cutoff and training-data cutoff: June 2026.

Known Limitations

• Forced tool choice is not supported. Setting tool_choice to any or to a named tool returns a 400 error; integrations should use auto or none and express required tool behavior in the prompt.
• Thinking blocks are forward-compatible only: Claude Fable 5.1 can read thinking blocks from earlier models, but earlier models cannot read its thinking blocks.
• Editing the system prompt, tool definitions, or earlier messages after a Fable 5.1 thinking block can invalidate that block. Long-running integrations should treat conversation history as append-only or use the documented block-dropping controls.
• The model is comparatively slow and costs twice as much per base input and output token as Claude Opus 5.
• Fable safeguards may refuse or route some cybersecurity and biology requests to another Claude model. Fable 5.1 also requires 30-day data retention by default; zero-data-retention use requires express authorization from Anthropic.
• Priority Tier is not supported for Claude Fable 5.1.

Pricing

Model	Input (Credits/Token)	5m Cache Write (Credits/Token)	1h Cache Write (Credits/Token)	Cache Read (Credits/Token)	Output (Credits/Token)	Web Search (Credits/Use)
Claude Fable 5.1	10.00	12.50	20.00	0.25	50.00	-

Caching note

For Claude Fable 5.1, a 5-minute cache write is billed at 1.25x the input rate and a 1-hour cache write is billed at 2x. Cache reads are billed at 0.025x the input rate.

Pricing note

Prices shown in the documentation are B.AI standard reference prices for base billing purposes. B.AI may provide lower actual usage costs through top-up bonuses and account benefits. Specific prices, bonus Credits, and account benefits are subject to the platform display and final billing records.