Claude Fable 5 Exits Free Plans Today — API Breaking Changes Developers Must Handle

Summary: Claude Fable 5, launched June 9, ends its complimentary window in Pro, Max, Team, and seat-based Enterprise plans today—billed at API rates going forward—while carrying two API-level changes that require code updates for any existing integration.

Key Facts

• Billing change: Usage now draws from credits at $10/M input and $50/M output tokens — less than half the price of Claude Mythos Preview; refused requests that produce no output are not charged
• New response type: Declined requests return HTTP 200 with stop_reason: "refusal", not an error; Anthropic offers a server-side fallbacks parameter (beta) and SDK middleware for automatic retry with fallback credit covering prompt-cache costs
• Thinking mode: Adaptive Thinking is always on — thinking: {"type": "disabled"} is not supported; raw chain-of-thought is never exposed, only a summary or omitted block
• Claude Mythos 5 (claude-mythos-5): Same model, safety classifiers removed, limited to Project Glasswing approved customers; successor to Claude Mythos Preview
• Context: 1M-token default window, up to 128K output tokens; 30-day data retention, no zero-retention option

Why It Matters

The grace period ending is an immediate cost event for millions of subscribers and a code-level breaking change for integrations that treat all non-200 responses as failures. Teams migrating from Opus 4.8 also need to rework thinking-block handling since raw reasoning is no longer returned. At current pricing, Fable 5 positions Anthropic to compete directly with GPT-5.5 on cost-per-token at frontier capability.

Read More

• Introducing Claude Fable 5 and Claude Mythos 5 — Anthropic Docs