Working at the frontier: How Rakuten builds agents overnight with Claude Fable 5

2026-07-20 · Source: Claude Blog · Field: Technology & Digital — Artificial Intelligence & Machine Learning, Software Development & Engineering, Emerging Technologies & Innovation · Depth: Expert, medium

Summary

Rakuten's General Manager of AI for Business, Yusuke Kaji, has been testing Claude models since September 2024, finding Claude Fable 5 a significant advancement for long-running enterprise agents. Since March 2025, Rakuten has used Claude for software development, deploying agents across product, sales, marketing, and finance as part of its "AI-nization" initiative. Fable 5 distinguishes itself by its enhanced autonomy, allowing agents to run unattended overnight, checking their own work, and completing nuanced tasks. This self-verification capability, where the model re-checks assumptions and returns to first principles, prevents early errors from compounding, enabling agents to close issues approximately 10x faster. Rakuten balances Fable 5's frontier cost by assigning it complex tasks where its advanced capabilities yield better outcomes, while using smaller models for simpler work.

Key takeaway

For AI/ML Directors evaluating agentic AI solutions, Claude Fable 5's self-verification capabilities significantly reduce the need for human oversight in long-running tasks. You can delegate entire complex workflows, freeing your team to focus on strategic decisions rather than constant course-correction. Consider implementing a tiered model strategy, using Fable 5 for high-value, complex tasks and more cost-effective models for routine operations to optimize both performance and budget.

Key insights

Claude Fable 5's self-verification and taste alignment enable agents to autonomously complete complex, long-running enterprise tasks.

Principles

• Agents should re-check assumptions and return to first principles.
• "Taste alignment" between model and team improves judgment on ambiguous calls.
• Balance frontier capability with cost by matching model to task complexity.

Method

Agents with Claude Fable 5 can be delegated entire tasks, reflecting at each step, correcting bad assumptions, and navigating to outcomes without human steering.

In practice

• Deploy agents across product, sales, marketing, and finance.
• Delegate multi-hour tasks to self-correcting agents.
• Match model capability to task complexity for cost efficiency.

Topics

• AI Agents
• Claude Fable 5
• Enterprise AI
• Self-Verification
• AI-nization
• Anthropic

Best for: Machine Learning Engineer, AI Product Manager, Product Manager, Director of AI/ML, AI Engineer, MLOps Engineer

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Editorial summary, takeaway, and curation by AIssential. Original article published by Claude Blog.