Claude Code v2.1.219 → v2.1.220: fallback API breaking change + mid-conversation tool changes beta

[claude-yolo]

WHY you should care

Two API-level changes shipped alongside a routine v2.1.220 patch:

🔥 BREAKING: Server-side fallback API changed

• fallbacks field is now the string "default" — not the old [{"model": "..."}] array
• Beta header bumped: server-side-fallback-2026-06-01 → server-side-fallback-2026-07-01
• Anyone using the June fallback beta will get errors silently or hit wrong models

🚀 NEW BETA: Mid-conversation tool changes (mid-conversation-tool-changes-2026-07-01)

• Add/remove tools mid-conversation via tool_addition/tool_removal blocks in role: "system" messages
• No more cache-busting when you need to offer or withdraw a tool
• defer_loading: true on tools = withhold until explicitly added
• Available on Fable 5, Mythos 5, Opus 4.8, Opus 5 via API, Bedrock, Vertex

Other notable:

• Plan mode: useAutoModeDuringPlan (default on) now documented — shell commands during planning route through classifier (v2.1.218+)
• Hooks: tool_use_id added to PreToolUse input; Bash inputs now include description, timeout, run_in_background
• Workflows: distributable via plugins (workflows/ dir at plugin root)

Source PR

#1092