Claude Fable 5 Is Now Available in Puter.js

Puter.js now supports Claude Fable 5, Anthropic's most capable publicly available model—a frontier, Mythos-class system tuned for the hardest coding, vision, and long-horizon reasoning tasks.

What is Claude Fable 5?

Claude Fable 5 is a safety-hardened, generally available version of Anthropic's frontier Mythos-class model, released on June 9, 2026. According to Anthropic's official announcement, its capabilities exceed those of any model the company has previously made generally available, and it is state-of-the-art on nearly every benchmark Anthropic tested—with its lead over prior Claude models growing the longer and more complex the task.

Here's what stands out, according to Anthropic's model documentation:

• Most capable Claude model available — outperforms Opus 4.8 across software engineering, knowledge work, vision, and scientific reasoning
• State-of-the-art coding — 80.3% on SWE-bench Pro, well ahead of Opus 4.8 (69.2%); during early testing, Stripe used it to run a codebase-wide migration in a day that would have taken a team over two months by hand
• State-of-the-art vision — extracts precise numbers from scientific figures and rebuilds a web app's source from nothing but screenshots
• Longest autonomous operation yet — stays focused and on-task across millions of tokens, longer than any previous Claude model
• 1 million token context window — process entire codebases, lengthy documents, and complex datasets in a single prompt
• 128,000 output tokens — generate long-form analyses, refactors, and detailed reports in one response
• Effort levels — performance scales with reasoning compute, and at the highest effort the model validates its own work

How It Compares to Opus 4.8

We compared the published benchmarks for Fable 5 and Opus 4.8 side by side. On the harder, longer-horizon tasks, Fable 5 comes out clearly ahead:

• Coding — 80.3% on SWE-bench Pro vs Opus 4.8's 69.2%
• Knowledge work — top scores on Hebbia's Finance Benchmark for senior-level financial reasoning and data analysis
• Vision — the new state-of-the-art for image-heavy tasks, from reading scientific charts to reconstructing apps from screenshots
• Long, complex tasks — the harder and longer the job, the wider Fable 5's lead grows

Pricing is $10/$50 per million input/output tokens—less than half the cost of Claude Mythos Preview.

One behavior worth knowing about up front: Fable 5 ships with safeguards on sensitive topics. In our testing, prompts touching cybersecurity and biology were declined, with the model noting it was switching to Opus 4.8 to handle the request instead. Anthropic says this fallback triggers in under 5% of sessions on average—but if your app works in those areas, expect to see it.

Examples

Hard, Multi-Step Reasoning

puter.ai.chat(
  "Prove that for every positive integer n, the number 2^(2n) - 1 is divisible by 3. Then explain the intuition behind the proof.",
  { model: 'claude-fable-5' });

Agentic Coding Across a Whole Codebase

puter.ai.chat([
  { role: "user", content: largeCodebase + "\n\nMigrate the data layer from the callback-based API to async/await across every module. Keep behavior identical and flag anything ambiguous before changing it." }
], { model: 'claude-fable-5' });

Vision: Read a Chart

puter.ai.chat([
  { role: "user", content: [
    { type: "file", puter_path: "~/revenue-chart.png" },
    { type: "text", text: "Extract every data point from this chart as a table, then summarize the trend." }
  ]}
], { model: 'claude-fable-5' });

Streaming

const response = await puter.ai.chat(
  "Design a distributed task scheduler with fault tolerance and exactly-once execution guarantees",
  { model: 'claude-fable-5', stream: true }
);

for await (const part of response) {
  puter.print(part?.text);
}

Get Started Now

Just add one library to your project:

// npm install @heyputer/puter.js
import { puter } from '@heyputer/puter.js';

Or add one script tag to your HTML:

<script src="https://js.puter.com/v2/"></script>

No API keys and no infrastructure setup. Start building with Claude Fable 5 immediately.

Learn more:

• Free, Unlimited Claude API
• Puter.js AI Documentation
• Claude Fable 5 and Claude Mythos 5 — Anthropic