What changed
Anthropic launched claude-fable-5 as its most capable generally available model, and claude-mythos-5 for Project Glasswing participants only. The prior public frontier was claude-opus-4-8; Fable 5 and Mythos 5 share the same underlying model, with Fable 5 running Anthropic's safety classifiers on every request and falling back to Opus 4.8 for requests in high-risk categories (cybersecurity, biology, chemistry, distillation), while Mythos 5 skips the classifiers entirely and is restricted to vetted partners.
TL;DR
Claude Fable 5 launches at $10/$50 per MTok (input/output), 1M token context window, 128k max output tokens, always-on adaptive thinking, with SWE-bench Verified at 95.0%, SWE-bench Pro at 80.3%, MMLU Pro at 91.50%, and LiveCodeBench at 89.78%.
Developer signal
Fable 5 is not a drop-in replacement for Opus 4.8 — review the Breaking Changes section above before upgrading any production integration. Once you've addressed those migration items: (1) API access — use model ID claude-fable-5; it's available today on the Claude API, Claude Platform on AWS, Amazon Bedrock, Vertex AI, and Microsoft Foundry. (2) Thinking — adaptive thinking is always on and cannot be disabled; control depth via the effort parameter (low, medium, high). Default display is "omitted" (thinking blocks exist but are empty); set thinking.display: "summarized" if you want readable summaries of the reasoning. Pass thinking blocks back unchanged in multi-turn conversations. (3) Token counting — Fable 5 uses the tokenizer introduced with Claude Opus 4.7; the same text produces roughly 30% more tokens than with pre-4.7 models. Recalibrate any fixed-length prompts, token budget estimates, and prompt caching breakpoints before deploying at scale. Use the token counting API with model: "claude-fable-5" to measure your actual prompts. (4) Fallback — the fallbacks parameter (beta, Claude API and Claude Platform on AWS only) lets you specify fallback models to retry on when Fable 5 refuses; alternatively, use the SDK middleware for TypeScript, Python, Go, Java, or C#. (5) GitHub Copilot — Fable 5 is available today in GitHub Copilot for Pro+, Max, Business, and Enterprise tiers under usage-based billing at provider list pricing; the 1M context window is available and requires opting in to the extended context size selector.