Claude Fable 5 Launches as Anthropic Retires Older Models
Anthropic releases Claude Fable 5 and Claude Mythos 5 with one-million-token context windows while retiring Claude Sonnet 4 and Claude Opus 4. The platform update also brings improved tooling for code execution and web searches.
Anthropic introduces Claude Fable 5 as its most capable widely released model, accompanied by Claude Mythos 5 for Project Glasswing participants. Both models feature a one-million-token context window, 128k maximum output tokens, and always-on adaptive thinking. These new models use a tokenizer introduced with Claude Opus 4.7, which generates roughly thirty percent more tokens for the same text compared to previous versions.
The Claude Platform simultaneously retires the older Claude Sonnet 4 and Claude Opus 4 models, meaning all requests to these retired versions now return an error. Anthropic recommends developers upgrade to Claude Sonnet 4.6 and Claude Opus 4.8 respectively. Researchers who need ongoing access to the older models can request permission through the External Researcher Access Program.
Additional platform updates bring enhanced functionality to various developer tools. The code execution tool now discloses its 90-second per-cell time limit so Claude can budget long-running tasks, while updated web search and web fetch tools add a parameter to drop consumed result blocks for smoother agentic workflows. Furthermore, a new endpoint for listing pending work in self-hosted sandboxes is now available on Claude Platform on AWS.