Build a Resilient Model Stack Now: fallbackModel Chains, Nested Sub-Agents, and Safe Mode in Claude Code Week 24

TL;DR: Claude Code Week 24 ships fallbackModel for automatic failover, nested sub-agents up to 5 levels deep, /cd to move sessions between projects, and --safe-mode to strip customizations for debugging — the most practically relevant update while Fable 5 is offline.

With Fable 5 still down, fallbackModel is the feature to deploy first. Added in v2.1.166, it configures up to three backup models Claude Code tries in order whenever the primary returns a 529 overload or is unavailable. The switch covers the current turn only — the next message retries the primary first.

Configure a resilient chain in ~/.claude/settings.json:

{
  "fallbackModel": ["claude-sonnet-4-6", "claude-haiku-4-5"]
}

Or for a single session: claude --fallback-model sonnet,haiku

The best alias pairs cleanly: it resolves to Fable 5 when your account has access, otherwise the latest Opus model. Set "model": "best" now, add a fallback chain, and your config will automatically promote to Fable 5 when it returns — no change required.

Nested sub-agents (v2.1.172). Sub-agents can now spawn their own sub-agents — background chains capped at five levels deep, foreground chains self-limiting at any depth. The /agents view shows the full tree with descendant counts and a path back to main. Practical pattern: an orchestrator spawns a research agent that spawns parallel fetch agents — each level scoped to exactly its tools, the top level just waits for results.

/cd (v2.1.169). Move the current session to a different directory without rebuilding the prompt cache. The new directory's CLAUDE.md is appended as a message instead of replacing the system prompt, so context built up in the session survives the hop.

> /cd ../other-project

--safe-mode (v2.1.169). When Claude Code behaves unexpectedly, claude --safe-mode (or CLAUDE_CODE_SAFE_MODE=1) strips CLAUDE.md, plugins, skills, hooks, MCP servers, and custom commands — while keeping auth and built-in tools working. If the problem disappears in safe mode, binary-search which surface introduced it.

claude --safe-mode

Sources: Claude Code What's New — Week 24, Fallback model chains docs, Nested sub-agents docs, Commands reference