Claude Models on Google Cloud (Anthropic managed) is generally available with US and Global endpoints. It gives Google Cloud customers the full Claude Platform feature set with Google Cloud IAM, billing and commitment drawdown. Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5 are available, and fast mode for Opus 5.5 costs $8 / $40 per million input and output tokens there.
Claude Fable 5.1 (and Claude Mythos 5.1 for Project Glasswing participants) launched at the same $10 / $50 per million tokens as Fable 5, with cache reads cut to $0.25 per million tokens.
On Fable 5.1 and Mythos 5.1, tool_choice any and tool return a 400 error. For accounts created on or after August 31, 2026, replaying a thinking block after the system prompt, tools or earlier messages changed also returns a 400 error.
Action needed:
Use tool_choice auto with strict tool use, and stop editing context before thinking blocks.
To counter distillation, the API now checks that a thinking block is sent back with the same system prompt, tools and messages. On Fable 5.1, Opus 5.5, Sonnet 5.5 and Haiku 5.5, accounts created on or after August 31, 2026 get an error unless they opt into dropping the block.
Action needed:
Use server-side compaction or mid-conversation tool changes, or opt into non-strict mode.
Claude Fable 5.1 is available on Copilot Pro+, Max, Business and Enterprise. Unlike other Claude models in Copilot it requires data retention so Anthropic can run safety classifiers. Eligible enterprises can use zero data retention through the end of 2026.
