Anthropic recommends Claude Fable 5 as the upgrade path from Opus 4.8 for agentic and long-context workloads. This migration guide covers a safe production transition.
Step 1 — Audit dependencies
List all API calls using claude-opus-4-8 or legacy Opus aliases. Note max context used, tool-calling patterns, and whether you disable thinking today.
Step 2 — Update model ID
Switch to claude-fable-5. Adaptive thinking is always on — remove thinking: disabled if present. Use the effort parameter to control cost.
Step 3 — Add refusal handling
Fable 5 can return stop_reason: refusal. Implement server-side fallbacks to Opus 4.8 or Sonnet during rollout. Use Anthropic's fallback credit to avoid double cache charges.
Step 4 — Test agent loops
Re-run your top 20 agent workflows. Fable 5's 1M context may change how you chunk documents — often you can remove manual RAG splitting.
Step 5 — Monitor costs
Fable 5 is cheaper per token than Opus 4.8 ($10/$50 vs $15/$75) but longer outputs can increase bills. Set task budgets (beta header) for cap control.
