# Claude Fable 5.1

> Anthropic's model for demanding reasoning and long-horizon agentic work, with adaptive thinking that is always on.

## Key facts

- Model author: Anthropic
- Input: $10.00 per 1M tokens
- Output: $50.00 per 1M tokens
- Cached input: $0.25 per 1M tokens
- Context window: 1,000,000 tokens
- Maximum output: 128,000 tokens
- Input modalities: image, text
- Output modalities: text
- Capabilities: reasoning, tools, vision
- Released: 2026-09-01
- Knowledge cutoff: Jun 2026
- Model IDs: `anthropic/claude-fable-5-1`
- Model page: https://www.realrelay.ai/model/anthropic/claude-fable-5-1

## Overview

Claude Fable 5.1 is a model from Anthropic designed for demanding reasoning and long-horizon agentic work, where an agent plans, calls tools and keeps working across many steps. Adaptive thinking is always on, so the model decides for itself how much to reason before it answers. It accepts text and image input and returns text.

Claude Fable 5.1 has a 1M-token context window and can generate up to 128K output tokens. It accepts text and image input and returns text output. It was released on September 1, 2026. Its knowledge cutoff is June 2026.

## Capabilities

Claude Fable 5.1 supports reasoning, tool calling and image understanding. It accepts text and image as input and produces text as output.

Adaptive thinking is always on, so the model reasons before it answers without a separate switch. Tool calling lets it act inside an agent loop, and image input lets it read screenshots, charts and scanned pages alongside text.

## Pricing

Claude Fable 5.1 costs $10.00 per 1M input tokens and $50.00 per 1M output tokens. Cache reads are $0.25 per 1M tokens.

Claude Fable 5.1 is available through RealRelay's OpenAI-compatible API.

## Use cases

Use Claude Fable 5.1 for long-horizon agent runs, such as a coding agent that works through a multi-file refactor, calls tools to run tests and keeps its plan in view across the 1M-token context window.

It also fits demanding knowledge work: reading long reports together with charts and screenshots, reasoning through the findings and writing a structured answer.

## Call it

OpenAI-compatible chat completions:

```bash
curl https://www.realrelay.ai/v1/chat/completions \
  -H "Authorization: Bearer $REALRELAY_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model": "anthropic/claude-fable-5-1", "messages": [{"role": "user", "content": "Hello"}]}'
```

## FAQ

### What is Claude Fable 5.1?

Claude Fable 5.1 is a model from Anthropic designed for demanding reasoning and long-horizon agentic work, where an agent plans, calls tools and keeps working across many steps. It supports reasoning, tool calling and image understanding and accepts text and image input.

### How much does Claude Fable 5.1 cost?

Claude Fable 5.1 costs $10.00 per 1M input tokens and $50.00 per 1M output tokens, with cache reads at $0.25 per 1M tokens.

### What is the context window of Claude Fable 5.1?

Claude Fable 5.1 has a 1M-token context window and can generate up to 128K tokens in a single response.

### What inputs and outputs does Claude Fable 5.1 support?

Claude Fable 5.1 accepts text and image input and returns text output.

### Does Claude Fable 5.1 support tool calling?

Yes. Claude Fable 5.1 supports tool calling, so it can call functions you define and use the results in multi-step tasks. It also supports reasoning and image understanding.

### When was Claude Fable 5.1 released?

Claude Fable 5.1 was released on September 1, 2026. Its knowledge cutoff is June 2026.

### How do I call Claude Fable 5.1 via the API?

Call Claude Fable 5.1 through RealRelay's OpenAI-compatible API using the model ID anthropic/claude-fable-5-1.