Claude Fable 5.1
Anthropic's most capable generally available model for long-horizon reasoning, agentic coding, and research — with stronger safeguards and lower cache costs.
Overview
What is Claude Fable 5.1
Claude Fable 5.1 is Anthropic's most capable large language model for demanding reasoning, long-running agentic work, and complex coding tasks, released on September 1, 2026. It features a 1M-token context window, advanced function calling, vision, and web search, with improved safeguards and a reduced cost for cached inputs.
Running it privately on Venice
On Venice, Claude Fable 5.1 runs with anonymized privacy—your prompts are never stored or used for training. This means you get the full power of a frontier model for sensitive coding, research, or enterprise workflows without sacrificing sovereignty. While not running in a TEE or end-to-end encrypted, Venice ensures zero retention by design, giving builders private, permissionless access to one of the most advanced AI models available.
Assessment
Strengths and limitations
- Exceptional at long-horizon reasoning and sustained agentic workflows, such as multi-step research or complex coding projects.
- Supports vision, function calling, web search, and structured output, making it highly capable for real-world tool-integrated applications.
- Improved safeguards reduce false positives in sensitive domains like cybersecurity and biology, while maintaining strong safety.
- Optimized for enterprise use with zero data retention on Venice, ensuring privacy without sacrificing performance.
- Lower cache read pricing enhances cost-efficiency for highly iterative or agent-loop-heavy workloads.
- Proprietary and closed: cannot be self-hosted, fine-tuned, or audited for internal biases or behavior.
- Higher cost per token than most rivals, especially Claude Opus 5, which Anthropic itself recommends as a starting point.
- Slower latency compared to mid-tier models, making it less ideal for real-time interactive use.
- Not uncensored: includes content filters and safeguards that may block certain outputs, especially in high-risk domains.
Capabilities
What it supports
- Vision (image input)
- Tool use / function calling
- Reasoning
- Web search
- Code-optimized
- Structured output (JSON schema)
- Multiple image inputs
- Random seed on all plan tiers
- Document and web page parsing as reference
- First and last frame to video
Specifications
Datasheet
- Maker
- Anthropic
- Open weights
- No
- License
- Proprietary
- Modes
- Standard, agentic (with tool calling and web search)
- Context window
- 1,000K tokens
- Prompt length
- Up to 1M tokens input
- Input images
- Supported — multiple image inputs via vision capability
- Released
- September 1, 2026
- Architecture
- Transformer-based autoregressive model
- Parameters
- Not disclosed
- Max output
- 128K tokens
- Capabilities
- Vision, Function calling, Reasoning, Web search, Code-optimized
- Privacy on Venice
- Anonymized — prompts not stored
- Available on Venice since
- Aug 2026
API
Call it from your code
Venice exposes an OpenAI-compatible API. Point your base URL at Venice and pass the model id.
curl https://api.venice.ai/api/v1/chat/completions \
-H "Authorization: Bearer $VENICE_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"model": "claude-fable-5-1",
"messages": [{ "role": "user", "content": "Explain quantum tunneling simply." }]
}'Pricing
What it costs on Venice
Billed per token on Venice: $12 per 1M input tokens and $60 per 1M output tokens.
New Venice accounts include a free daily allowance and 500 welcome credits — no credit card required.
Alternatives
How it compares
| Model | Best for | Context | Open weights | Price (Venice) |
|---|---|---|---|---|
| Claude Fable 5.1 | Long-horizon agentic work | 1M tokens | No | $12 in · $60 out / 1M |
| Claude Opus 5 | Complex agentic coding | 1M tokens | No | $6 in · $30 out / 1M |
| Claude Sonnet 4.6 | Balanced performance | 1M tokens | No | $3.60 in · $18 out / 1M |
| DeepSeek V4 Flash 0731 | High-volume, low-cost | 1M tokens | Yes | $0.17 in · $0.35 out / 1M |
Claude Fable 5.1 is the right pick for mission-critical, long-running agentic tasks where reasoning depth and reliability outweigh cost. For most other work, start with Claude Opus 5—it’s half the price and often sufficient.
Use cases
What it is good for
- 01Long-running AI agents that perform research, coding, and multi-step planning over hours.
- 02Enterprise knowledge work requiring vision, code generation, and web search in a single workflow.
- 03High-stakes technical documentation analysis with cross-referencing and structured output.
- 04Cybersecurity and life sciences tasks under trusted access programs via Mythos 5.1.
- 05Agentic coding pipelines where reliability, tool use, and context retention are critical.
Prompting
Getting better results
Use turn-scoped system messages (beta) to guide behavior within specific conversation turns.
Enable per-message effort (beta) to control reasoning depth for complex subtasks.
Leverage display: 'updates' between tool calls to provide user-facing progress during long runs.
Include image inputs with clear captions to improve vision-based reasoning accuracy.
Use content provenance features to trace outputs back to sources when auditing agent decisions.
Structure JSON output requests with clear schema definitions to ensure reliable parsing.
Version history
Predecessor model, with higher cache costs and slightly weaker reasoning.
Current — stronger agentic coding, lower cache cost, improved safeguards.
FAQ
Frequently asked questions
Claude Fable 5.1 is Anthropic’s most capable large language model for demanding reasoning, long-horizon agentic work, and complex coding. Released on September 1, 2026, it features a 1M-token context window, vision, tool use, web search, and improved safeguards, with lower cache read costs for agent-heavy workflows.
On Venice, Claude Fable 5.1 costs $12 per million input tokens and $60 per million output tokens. Cache reads are priced at just $0.30 per million, making it cost-efficient for iterative or agentic workloads.
No. Claude Fable 5.1 is a proprietary model developed by Anthropic. It is not open source or freely available for self-hosting. You can access it via API on Venice or through Anthropic’s platform.
Yes. Claude Fable 5.1 supports vision, allowing it to process multiple image inputs and reason over visual content alongside text. This makes it suitable for multimodal applications like document analysis and UI interpretation.
Yes. Claude Fable 5.1 supports function calling and tool use, enabling it to interact with external systems, retrieve data, and perform actions as part of an agentic workflow.
Claude Fable 5.1 is more capable for long-running, complex tasks, but costs twice as much. Anthropic recommends starting with Opus 5 and escalating to Fable 5.1 only when Opus fails. For most work, Opus 5 is the better value.
Claude Fable 5.1 has a 1,000,000-token context window, allowing it to process extremely long documents, codebases, or conversation histories in a single session.
Yes. On Venice, Claude Fable 5.1 runs with anonymized privacy—your prompts are not stored, profiled, or used for training. This ensures your data remains private while still accessing a frontier AI model.
Run Claude Fable 5.1 privately
No prompt logging. No data used for training.