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Claude Fable 5.1

Anthropic's Claude Fable 5.1 is live on the gateway with a 1M context window, always-on adaptive thinking, stronger long-horizon agentic work, and cache reads at a quarter of Fable 5's price.

Anthropic's logo on a glowing chip surrounded by a storybook, a crystal, a brain, and a sealed envelope on a circuit board

Long agentic sessions re-read the same prefix hundreds of times, and on a $10-per-million model those cache reads add up faster than the output does. Claude Fable 5.1 extends Claude Fable 5 at the same input and output prices, but cache reads now cost $0.25 per 1M tokens — a quarter of Fable 5's rate — and the model brings stronger long-running agentic coding, multistep research, and document, spreadsheet, and slide work.

Claude Fable 5.1

1anthropic/claude-fable-5-1
Spec Value
Context 1,000,000 tokens
Max output 128K tokens
Input / output $10 / $50 per 1M tokens
Cache read $0.25 per 1M tokens
Cache write $12.50 (5-minute) / $20 (1-hour) per 1M tokens
Thinking Adaptive, always on — steer it with reasoning_effort, low to max
Modalities Text and images in, text out; tool use and JSON schema output

Before you switch a pinned claude-fable-5 workload over, note what carries across and what changed:

  • Thinking cannot be turned off and the raw chain of thought is never returned. reasoning_effort (default high) is the only lever.
  • The model accepts max_tokens and effort only. The gateway strips sampling parameters such as temperature and top_p before forwarding, so a request that sets them still succeeds.
  • New in 5.1: Anthropic rejects forced tool use — tool_choice: "required" or a named function — with a 400. The gateway forwards these requests with auto instead, so existing code keeps working; use response_format with a JSON schema when you need schema-valid output.
  • Anthropic requires 30-day data retention for this model. A provider key from a zero-data-retention Anthropic organization is rejected upstream.
1curl https://api.llmgateway.io/v1/chat/completions \2    -H "Authorization: Bearer $LLM_GATEWAY_API_KEY" \3    -H "Content-Type: application/json" \4    -d '{5        "model": "anthropic/claude-fable-5-1",6        "reasoning_effort": "xhigh",7        "messages": [{ "role": "user", "content": "Plan the migration from SQLite to PostgreSQL." }]8    }'

The model routes through Anthropic directly for now. Bedrock and Microsoft Foundry mappings follow once we have verified them end to end, and Claude Mythos 5.1 — the same model, offered by Anthropic to Project Glasswing participants by invitation only — is not listed until the gateway is enrolled.

Provider cache control docs → | Claude Fable 5.1 on the models page →