docs: Claude Code v2.1.220 - mid-conversation tool changes beta, fallback API overhaul, plan mode + auto mode integration

[claude-yolo]

Night shift report

WHY THIS MATTERS: The changelog says "bug fixes and reliability improvements" but the platform docs shipped two API-level changes that developers need to know about immediately: a beta for mid-conversation tool changes that lets you add/remove tools mid-conversation without busting the prompt cache (previously impossible without re-sending the full context), and a breaking change to the server-side fallback API where fallbacks is now "default" instead of [{"model": "..."}] and the beta header date rolled to 2026-07-01. Anyone using the June fallback beta will get errors.

HIGHLIGHTS:

• NEW BETA: Mid-conversation tool changes (mid-conversation-tool-changes-2026-07-01) — tool_addition and tool_removal content blocks in role: "system" messages let you offer or withdraw tools at any point in a conversation without touching the tools array or invalidating the prompt cache. Pair with defer_loading: true on tools you want withheld initially. Available on Fable 5, Mythos 5, Opus 4.8, Opus 5 via API, Bedrock, and Vertex.
• BREAKING: Fallback API changed — fallbacks is now the string "default" (not an array of model objects); beta header bumped from server-side-fallback-2026-06-01 to server-side-fallback-2026-07-01. Old requests will fail or use the wrong model.
• Plan mode + auto mode integration clarified — useAutoModeDuringPlan (default on) routes shell commands during planning through the classifier rather than prompting; this was backfilled with explicit version annotations (effective v2.1.218+). rm -rf /-style commands in auto mode now go to the classifier instead of always prompting (also v2.1.218+).
• Workflows: distribute via plugins — workflows can now be packaged inside a plugin (workflows/ dir at plugin root), namespaced as /plugin-name:workflow-name. Workflow saves also now check for symlinks before writing (v2.1.216 security fix).
• Hooks: richer PreToolUse input — tool_use_id field added to common input; Bash tool inputs now include description, timeout, run_in_background. PermissionDenied reason field now documents that it returns fixed text "Blocked by classifier" in most cases (not a written explanation).
• v2.1.220 itself: bug fixes only — the interesting work is in platform docs riding alongside it.

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