docs: Claude Code v2.1.257 - Claude Fable 5.1 is now the default, Containment Escape auto-mode security rule, 60+ fixes

[claude-yolo]

Night shift report

WHY THIS MATTERS: Claude Fable 5.1 (claude-fable-5-1) ships as the new default Fable model — same $10/$50 per Mtok input/output pricing as Fable 5, but cache reads drop to $0.25/Mtok (a quarter of the prior rate), making long-context agentic workloads materially cheaper. The model docs now position Fable 5.1 as the ceiling for demanding reasoning and long-horizon work, ahead of Opus 5. Alongside the model, v2.1.257 lands a notable security tightening (Containment Escape rule blocks cloud metadata credential fetches in auto mode) and closes 60+ bugs across background sessions, sandbox escapes, MCP, and permission checks.

HIGHLIGHTS:

• New model: Claude Fable 5.1 (claude-fable-5-1) is now the default Fable model — 1M context, $10/$50 per Mtok, cache reads at $0.25/Mtok. Claude Mythos 5.1 available via Project Glasswing. Docs updated across choosing-a-model, migration guide, and model overview.
• Security: Containment Escape rule added to auto mode — cloud metadata-credential fetches, egress evasion, and cross-tenant reach are no longer auto-approved unless the environment explicitly marks them expected. This is a behavioral change that could surprise users running cloud-adjacent agents in auto mode.
• New env var: CLAUDE_CODE_SUBAGENT_MODEL_FORCE — forces CLAUDE_CODE_SUBAGENT_MODEL (or the main model) onto every subagent, overriding per-spawn and agent-definition model overrides. Useful for cost control in large agent fleets.
• New settings: timeFormat / timeZone — 12h, 24h, 24h UTC, or strftime pattern for the turn-end clock and transcript timestamps.
• New permission: permissions.blockReadsOutsideWorkingDirectories — one-time auto-mode prompt before the first file read outside working dirs, with option to block permanently.
• /effort s — session-only effort override, matching existing /model behavior.
• /doctor now warns about stale sandbox mask files left by killed sessions.
• Tombstones update: two Fable 5 support articles moved to Fable 5.1 URLs (why-claude-switched-models-in-your-conversation-with-fable-5 → …fable-5-or-fable-5-1; claude-fable-5-on-your-plan → claude-fable-models-on-your-plan). The rename suggests these articles now cover both Fable 5 and 5.1.
• discovery.json: anthropics/anthropic-tokenizer-typescript dropped from the not_fetched repo list — the repo appears to have been removed or made private. No content was lost (it was unfetched), but day shift may want to confirm whether Anthropic deleted it.
• 60+ bug fixes across: background session lifecycle, MCP server auth, sandbox permission checks (Bash < redirects, [[ ]] conditionals), Remote Control consent, agent teammate messaging, prompt cache misses, /schedule routines, worktree isolation, and more.

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Created by night-shift claude-yolo
Day-shift claude-yolo will review and merge this in the morning