Claude Code 2.1.257 Makes Fable 5.1 the Default Fable Model, Adds a Containment Escape Rule

Claude Code 2.1.257 shipped on September 1. It adds claude-fable-5-1 as the default Fable model and gives auto mode a rule that stops cloud metadata-credential fetches and cross-tenant reach from being auto-approved unless your environment marks them expected.

Claude-Code Anthropic AI-coding-tools security permissions

Anthropic shipped Claude Code 2.1.257 on September 1, 2026 ¹.

claude-fable-5-1 (Claude Fable 5.1) was added and is now the default Fable model. The release notes list 1M context, $10/$50 per million tokens, and $0.25 per million for cache reads ¹
Auto mode gained a Containment Escape rule: cloud metadata-credential fetches, egress evasion, and cross-tenant reach are no longer auto-approved unless your environment marks them expected ¹
Auto mode now shows a one-time prompt before the first file read outside the working directories, with a setting to block such reads entirely (permissions.blockReadsOutsideWorkingDirectories) ¹

Also in this release: /effort s changes effort for the current session only ¹. The tightening of permissions continues from 2.1.251, which closed several routes around the permission check, so environments that live in auto mode will see approvals surface differently. Anthropic notes that Fable 5.1 defaults to High effort in Claude Code ².

Sources

Claude Code v2.1.257 release notes - GitHub official release notes (September 1, 2026)
Introducing Claude Fable 5.1 and Claude Mythos 5.1 - Anthropic official announcement (September 1, 2026)