Anthropic SDK v0.108.0 Adds Claude Mythos-5 and Fable-5 with Fallback Support
Anthropic released Python SDK v0.108.0 on June 9, 2026, adding support for two new Claude models: Claude Mythos-5 and Claude Fable-5. The update includes both server-side and client-side fallback middleware for handling model refusals.
Anthropic SDK v0.108.0 Adds Claude Mythos-5 and Fable-5 with Fallback Support
Anthropic released version 0.108.0 of its Python SDK on June 9, 2026, introducing support for two previously unannounced Claude models: Claude Mythos-5 and Claude Fable-5.
New Models
The SDK update references two new model identifiers:
- Claude Mythos-5
- Claude Fable-5
No technical specifications, pricing, or capability details have been disclosed for either model. The naming convention breaks from Anthropic's established pattern of Opus, Sonnet, and Haiku tiers.
Fallback Mechanisms
The primary feature of this release is fallback support for handling model refusals:
Server-side fallbacks: When Claude Mythos-5 or Fable-5 refuse a request, the system can automatically route to an alternative model on Anthropic's servers.
Client-side fallbacks: New middleware enables developers to implement fallback logic in their applications for API providers that don't support server-side fallbacks.
The fallback functionality specifically targets refusal scenarios—instances where models decline to respond to certain prompts due to safety filters or content policies.
Implementation Details
The update (commit 6b76649) implements both fallback types as part of the SDK's core functionality. Developers using the Python SDK can now configure fallback chains to maintain service continuity when models refuse requests.
Version 0.108.0 follows v0.107.1. The full changelog is available in Anthropic's GitHub repository.
What This Means
The appearance of Claude Mythos-5 and Fable-5 in SDK code suggests Anthropic is testing new model variants, though the company has made no public announcement. The fallback architecture indicates Anthropic anticipates scenarios where these models may refuse requests more frequently than existing Claude versions, or that they're designed for specialized use cases requiring backup options. The dual naming convention—Mythos and Fable—could signal a departure from Anthropic's performance-based tier system toward use-case-specific models, though this remains speculative without official documentation.
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