Fable 5 executes complex tasks autonomously for hours

Summary

You're no longer bottlenecked by model context windows or token budgets for sustained tasks—Stripe completed a 50M-line codebase migration in one day. But silent capability restrictions mean your error handling must account for degraded performance on security-sensitive workloads.

Why it matters

You're no longer bottlenecked by model context windows or token budgets for sustained tasks—Stripe completed a 50M-line codebase migration in one day. But silent capability restrictions mean your error handling must account for degraded performance on security-sensitive workloads.

Sources

1. 1.Stripe reported that Claude Fable 5 performed a codebase-wide migration on a 50-million-line Ruby codebase — a task that would have taken a full engineering team over two months — in a single day
2. 2.$10 per million input tokens, $50 per million output tokens
3. 3.Fable 5 is not just incrementally better — it's category-expanding
4. 4.when Fable 5 was given access to persistent file-based memory (simple file read/write), its performance improved three times more than Opus 4.8's
5. 5.silently falls back to Claude Opus 4.8 for that response