Subagent fails with 'classifyHandoffIfNeeded is not defined' error

Description

When using the Task tool to spawn subagents (particularly with subagent_type: "general-purpose" or subagent_type: "Explore"), agents frequently fail at completion with the error:

Delta: classifyHandoffIfNeeded is not defined

Observed behavior

• Agents execute their work successfully (file writes, searches, etc.)
• Agents crash at the completion/handoff phase
• The task status shows "failed" despite work being completed
• Files created by the agent exist on disk

Reproduction

1. Spawn a Task agent to create or explore files
2. Agent runs normally and completes its work
3. Agent fails with "classifyHandoffIfNeeded is not defined" when trying to return results

Environment

• Platform: Windows 11
• Claude Code CLI
• Model: claude-opus-4-5-20251101

Impact

• Cannot reliably use subagents for multi-step tasks
• Must manually verify if agents completed work before they crashed
• Affects productivity when parallelizing work with multiple agents

Example failure log

Task aeb9d3d (type: local_agent) (status: failed) (description: Explore tools folder) Delta: classifyHandoffIfNeeded is not defined
Task a83e19e (type: local_agent) (status: failed) (description: Explore new directories) Delta: classifyHandoffIfNeeded is not defined
Task a692ef1 (type: local_agent) (status: failed) (description: Create ARCHITECTURE.md doc) Delta: classifyHandoffIfNeeded is not defined

Multiple agents failed with the same error in a single session.