Claude Code v2.1.173 adds nested sub-agents, fallback models, and plugin management

Sources: Claude Code releases, What's new
Summary: Claude Code v2.1.173 (2026-06-11) adds a fallbackModel setting for up to three fallback models tried in order when the primary model is overloaded; nested sub-agents up to five levels deep; glob pattern support in MCP deny rules; hardened cross-session security so SendMessage-relayed messages no longer carry user authority; requiredMinimumVersion and requiredMaximumVersion managed settings for enterprise version gating; and /plugin list with --enabled/--disabled filters. v2.1.170 (2026-06-09) added Claude Fable 5 as a selectable model via /model fable.
Why it matters: Multi-level sub-agents and version-gating managed settings enable more complex autonomous pipelines; the cross-session trust boundary fix closes an injection vector for environments that relay messages between Claude sessions.