Claude Fable 5
Anthropic's Claude Fable 5 is a Mythos-class model featuring a 1M context window and 128K output tokens. It excels at agentic coding and 3D physics.
anthropicClaudeJune 9, 2026About Claude Fable 5
Learn about Claude Fable 5's capabilities, features, and how it can help you achieve better results.
Claude Fable 5 is Anthropic's most powerful generally available model, built on the Mythos architecture class. It is designed for high-stakes autonomous tasks that require deep reasoning and a massive memory buffer. With a 1,000,000 token context window, it can ingest an entire company's codebase or hundreds of research papers in a single prompt. This model is specifically optimized for long-horizon agentic workflows where self-correction is mandatory.
The model introduces a unique 128,000 output token limit, allowing it to write full software modules or expansive technical documentation without truncation. It features a self-verification loop where it uses vision capabilities to check its own generated code, particularly for UI and 3D simulations. While it maintains strict safety filters for high-risk domains like biology, its general reasoning performance reaches senior-engineer levels, outperforming previous iterations in complex system architecture and large-scale migrations.
Developers primarily use Fable 5 for tasks that fail on standard models due to context fragmentation or lack of logic. It combines high-fidelity vision with senior-level software engineering capabilities, enabling it to build complex 3D environments and verify visual outputs against original designs. Technically, it represents a significant leap in multimodal logic and autonomous reliability.
Use Cases
Discover the different ways you can use Claude Fable 5 to achieve great results.
Autonomous Codebase Migration
Migrate 50-million-line legacy repositories to modern frameworks by processing the entire project within the 1M context window.
3D Physics Simulation Generation
Create self-contained C++ or WebGL simulations with complex mesh colliders and fluid dynamics from a single prompt.
Senior Scientific Research Analysis
Synthesize hundreds of PhD-level research papers to identify novel hypotheses while adhering to safety guardrails.
Agentic Strategic Financial Modeling
Drive autonomous agents to process years of market data and generate detailed projections with interactive dashboards.
Real-time Network Visualization
Build backend systems that capture live packets and visualize them as 3D environments to identify security anomalies.
High-Fidelity Technical Content Creation
Generate 100,000-word technical manuals and comprehensive documentation sets in a single pass using expanded token limits.
Strengths
Limitations
API Quick Start
anthropic/claude-fable-5
import Anthropic from '@anthropic-ai/sdk';
const anthropic = new Anthropic({
apiKey: process.env.ANTHROPIC_API_KEY,
});
const message = await anthropic.messages.create({
model: "claude-fable-5",
max_tokens: 1024,
messages: [{
role: "user",
content: "Analyze this codebase for security vulnerabilities and suggest fixes."
}],
});
console.log(message.content[0].text);Install the SDK and start making API calls in minutes.
Community Feedback
See what the community thinks about Claude Fable 5
“Qualitatively, this is a major-version-bump-deserving step change forward. It peaks for long problem-solving sessions on very difficult problems.”
“Fable 5 makes GPT 5.5 feel like a toy. For complex, difficult tasks, it is the new goalpost and the new state-of-the-art.”
“Fable 5 just finished Pokemon Fire Red with Vision alone. Raw screenshots only, no map, no hidden state. That is quite impressive.”
“The 1M context window finally makes large-scale legacy code migration feel like a solved problem. RAG feels optional for most of my projects now.”
“Claude 5 fable (extra high) made a Pokemon clone in 1 hour of reasoning with 8k lines in 1 shot. This is a new era.”
“Anthropic released Fable 5 for general availability and Claude Mythos 5 for restricted research. It is their most powerful model publicly available.”
Related Videos
Watch tutorials, reviews, and discussions about Claude Fable 5
“opus 4.8. So this is basically a matus class model with an extra classifier on top of it and if it triggers the classifier the those specific queries are going to be routed to opus 4.8 instead of feeble 5. So if you see some”
“swing. Okay, so in terms of pricing, this is going to be $10 per million input tokens and $50 per million out output tokens. This is going to be one”
“use this data to train new cloud models or any of non-safety related purposes and we have instituted new privacy protections including logging all human access to the data and ensuring the deletion of this after 30 days. Now this”
“We think the unsafeguarded Mythos 5 can significantly uplift well-resourced threat actors.”
“private simple bench created 2 years ago now and I'm kind of proud it's lasted that long, but we have Claude Fable at almost 82%. That's in number one position and completely apart from all”
“those tasks correct." Well, it is in the lead and cost-effectively way in the lead, except for Gemini 3.5 Flash, which is only 3% behind and four times cheaper, but Fable's top score is only 17%. In other words, 83% of the time it”
“Fable 5 is state of the art across basically every single benchmark. On Swe Bench Pro, it shows around a 20% jump compared to GPT-5.5.”
“This is not a small update either and it's not hype. This is basically Anthropic putting their top-tier Mythos level intelligence into a version that regular users can actually access.”
“just take a look at the comparison of Fable 5 to Opus 4.8 and even GPT 5.5. There is a huge discrepancy in terms of cost to performance in every single domain of coding, guys. As incredible as”
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What to get right first
The decisions that are painful to change later in Claude Fable 5.
Use High-Effort Reasoning Modes
Toggle the model to 'High' or 'Extra High' effort in the API to solve math or logic problems that require deep chain-of-thought.
Leverage Prompt Caching
Use prompt caching for frequently accessed codebases to reduce costs by up to 90% during multi-day autonomous sessions.
Anchor Tasks with Vision
Provide screenshots of desired UIs to allow Fable 5 to use vision to verify its code matches your requirements.
Explicitly Request Self-Verification
Instruct the model to write its own test suite and execute it to identify bugs before returning the final result.
Utilize the 128K Output
Avoid breaking up long requests by asking for the entire backend and frontend in one prompt for architectural consistency.
Testimonials
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Frequently Asked Questions
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