Claude Fable 5: The Mythos-Class Masterclass

What the Mythos-class tier is, how Fable 5 differs from every Claude before it, and how to make your first API call against the new request shape.

• What Is Claude Fable 5? The Mythos-Class Tier Explained

  15m
• Environment Setup & Your First Fable 5 API Call

  20m
• The New API Contract: What Changed, What 400s

  20m

Master Fable 5's adaptive thinking model: the five-level effort dial, summarized thinking display, and choosing the right depth for every workload.

• Adaptive Thinking Fundamentals

  15m
• The Five Effort Levels: low to max

  20m
• Summarized Thinking in Practice

  15m

Build agents that run for hours: tool loops designed for minutes-long turns, parallel sub-agent delegation, and memory files that persist across sessions.

• Designing Long-Horizon Agent Loops

  25m
• Parallel Sub-Agent Delegation

  25m
• Memory Files: Persistence Across Sessions

  20m

Handle safety-classifier refusals gracefully, wire up server-side fallbacks to Opus 4.8, and run Fable 5 within its retention and latency constraints.

• Understanding Classifier Refusals

  20m
• Server-Side Fallbacks: The Recommended Pattern

  20m
• The Production Envelope: Retention, Timeouts, Monitoring

  20m

Move real Opus 4.8 workloads to Fable 5 with a mechanical migration playbook, an honest cost model at $10/$50 per million tokens, and a staged rollout strategy.

• The Opus-to-Fable Migration Playbook

  25m
• The Fable 5 Cost Model: $10/$50 and Where ROI Lives

  20m
• Staged Rollout & Your Model Portfolio

  20m