> ## Documentation Index
> Fetch the complete documentation index at: https://docs.anyfast.ai/llms.txt
> Use this file to discover all available pages before exploring further.
# Claude Fable 5.1
> Use Anthropic Claude Fable 5.1 through AnyFast for demanding reasoning, long-horizon agentic coding, research, and document work.
Claude Fable 5.1 is Anthropic's latest Fable model for demanding reasoning and long-horizon agentic work. Use the AnyFast model ID `claude-fable-5-1` with the native Anthropic Messages API at `POST /v1/messages`.
## Model specifications
| Specification | Value |
| ------------------------- | --------------------------------------- |
| Model ID | `claude-fable-5-1` |
| Context window | 1M tokens |
| Maximum output | 128K tokens |
| Input → output | Text and images → text |
| Thinking | Adaptive, always on |
| Default effort | `high` |
| Effort levels | `low`, `medium`, `high`, `xhigh`, `max` |
| Reliable knowledge cutoff | June 2026 |
## What changed from Fable 5
* **Stronger long-running work** — Improved agentic coding, multistep research, document, spreadsheet, slide, vision, and computer-use tasks.
* **Lower cache-read cost upstream** — Anthropic reduced cache-read pricing while keeping base input and output pricing unchanged.
* **Forced tool use is unavailable** — `tool_choice: {"type":"any"}` and `tool_choice: {"type":"tool"}` return `400`. Use `auto` or `none`.
* **Thinking blocks are conversation-bound** — Keep prior turns append-only and pass thinking blocks back unchanged. Editing the system prompt, tools, or an earlier message can invalidate later thinking blocks.
* **One-way thinking compatibility** — Fable 5.1 can read thinking blocks from earlier Claude models, but earlier models cannot read Fable 5.1 thinking blocks.
## Quick example
```bash cURL theme={null}
curl https://www.anyfast.ai/v1/messages \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"model": "claude-fable-5-1",
"max_tokens": 4096,
"output_config": { "effort": "high" },
"messages": [
{
"role": "user",
"content": "Review this migration plan and identify the three highest-risk assumptions."
}
]
}'
```
```python Python theme={null}
import anthropic
client = anthropic.Anthropic(
api_key="YOUR_API_KEY",
base_url="https://www.anyfast.ai"
)
message = client.messages.create(
model="claude-fable-5-1",
max_tokens=4096,
output_config={"effort": "high"},
messages=[
{
"role": "user",
"content": "Review this migration plan and identify the three highest-risk assumptions."
}
]
)
for block in message.content:
if block.type == "text":
print(block.text)
```
```python Streaming theme={null}
import anthropic
client = anthropic.Anthropic(
api_key="YOUR_API_KEY",
base_url="https://www.anyfast.ai"
)
with client.messages.stream(
model="claude-fable-5-1",
max_tokens=4096,
output_config={"effort": "high"},
messages=[
{"role": "user", "content": "Create a phased rollout plan for a database migration."}
],
) as stream:
for text in stream.text_stream:
print(text, end="")
```
## Core parameters
| Parameter | Type | Required | Description |
| ---------------------- | ------- | -------- | -------------------------------------------------------------------------------------------------- |
| `model` | string | Yes | Must be `claude-fable-5-1`. |
| `messages` | array | Yes | Conversation turns with `user` or `assistant` roles. Text, images, and tool results are supported. |
| `max_tokens` | integer | Yes | Maximum output tokens, including thinking and visible text. Range: 1–128000. |
| `output_config.effort` | string | No | Controls thinking depth. Supports `low`, `medium`, `high`, `xhigh`, and `max`; default is `high`. |
| `thinking.type` | string | No | Omit it or set it to `adaptive`. Thinking cannot be disabled. |
| `tools` | array | No | Tool definitions available to the model. |
| `tool_choice.type` | string | No | Supports `auto` and `none`. Forced `any` or a named `tool` is not supported. |
| `stream` | boolean | No | Enables SSE streaming. Default: `false`. |
| `stop_sequences` | array | No | Custom sequences that stop generation. |
Adaptive thinking is always on. Omit `thinking` or send `{"type":"adaptive"}`. Both manual extended thinking with `budget_tokens` and `{"type":"disabled"}` return `400`. Use `output_config.effort` to control reasoning depth.
Claude Fable 5.1 does not support non-default sampling values. Omit `temperature`, `top_p`, and `top_k`. Only `temperature: 1` and `top_p` from `0.99` to `1` are accepted for backward compatibility; any `top_k` value returns `400`. Assistant-response prefilling is also unsupported.
## Tool use
Keep `tool_choice` at `auto` (the default) or use `none`. Tell the model in the prompt when a tool applies and describe its input schema precisely. Do not use `any` or a named forced tool choice.
## Multi-turn conversations
When you return an assistant turn containing thinking blocks, send the entire turn back exactly as received. Treat the system prompt, tool definitions, and earlier messages as append-only. Changing content before a Fable 5.1 thinking block can return `400` with a conversation-binding error.
Anthropic released Claude Fable 5.1 on September 1, 2026. See the official [model overview](https://platform.claude.com/docs/en/models/fable-5-1/overview), [what's new](https://platform.claude.com/docs/en/models/fable-5-1/whats-new-fable-5-1), and [prompting guide](https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1).
View request fields, response blocks, and the interactive API playground.