News tagged "fable-5-1"

Technical diagram showing prompt cache coverage before and after a tool result boundary, with billing tokens highlighted

Claude Code 2.1.260: The Fable 5.1 Prompt-Cache Bug That Cost Every Tool-Using Agent

Fable 5.1 prompt caching silently excluded post-tool-result context, re-sending it uncached on every tool-call turn. Claude Code 2.1.260 fixes it — along with cache-miss diagnostics, a Read() permission revert, and a managed CLAUDE.md governance change.

September 7, 2026via Claude Code Changelog

Abstract diagram of API request routing with a 400 error block and version gate nodes in a technical pipeline visualization

anthropic

Fable 5.1 Breaks Two Operator Assumptions: `tool_choice` Force-Calls and Thinking Block Reuse

Fable 5.1 breaks two patterns operators rely on: `tool_choice: any` and `tool_choice: tool` now return 400, and thinking blocks are version-gated with prefix-mismatch enforcement for new accounts. Migrate both before routing production traffic.

September 3, 2026via Anthropic