docs: Claude Code v2.1.212 - /fork becomes background sessions, runaway loop caps, MCP auto-background

[claude-yolo]

Night shift report

WHY THIS MATTERS: v2.1.212 reshapes how Claude Code handles parallel work and runaway agents. /fork now creates a true background session (its own row in claude agents) instead of an in-session subagent — this is a breaking behavior change for anyone scripting around it. More importantly, two new safety caps (200 WebSearch calls/session, 200 subagent spawns/session) with configurable env vars address a real failure mode where agents enter delegation or search spirals and burn tokens indefinitely. Long MCP calls (>2 min) now auto-background so the main session stays responsive.

HIGHLIGHTS:

• /fork behavior changed: now copies conversation into a new background session with its own claude agents row; the old in-session subagent behavior moves to the new /subtask command — existing workflows using /fork for quick sub-tasks need updating
• Runaway loop protection: CLAUDE_CODE_MAX_WEB_SEARCHES_PER_SESSION (default 200) and CLAUDE_CODE_MAX_SUBAGENTS_PER_SESSION (default 200) cap search and delegation spirals; /clear resets the subagent budget
• MCP auto-background: tool calls running >2 min move to background automatically; configurable via CLAUDE_CODE_MCP_AUTO_BACKGROUND_MS
• Security fix: plan mode was auto-running file-modifying Bash commands (touch, rm, etc.) without a permission prompt or canUseTool callback — now fixed
• Security fix: worktree creation could follow a committed symlink at .claude/worktrees and write files outside the repository — now fixed
• /resume picker: now shows all past sessions including deleted ones, resuming as a background session
• Prompt caching improvement: mid-conversation system block now works behind LLM gateways and custom base URLs (Bedrock, Vertex, 1P)
• Agent SDK docs: MCP and session examples now include Python alongside TypeScript; query() error-handling pattern clarified with try/catch
• Enterprise: forceLoginMethod now enforced for VS Code extension, SDK, setup-token, and install-github-app — not just terminal
• OTel fixes: HTTP exports no longer rejected by Azure Monitor (chunked encoding fixed); trace_id/span_id now present in OTLP records when TRACEPARENT is set
• Task tool mode deprecated: subagents now inherit the parent session's permission mode by default
• Package size grew ~3.5% (159 KB → 165 KB), consistent with new features

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