Agent crash: classifyHandoffIfNeeded is not defined

Description

When spawning a Plan-type subagent via the Task tool, the agent crashes immediately with a JavaScript runtime error:

classifyHandoffIfNeeded is not defined

The agent never executes any tools or produces output. Status shows as failed.

Reproduction

1. Use the Task tool to spawn a Plan subagent
2. Agent fails immediately with the error above

Context

• Occurred during a multi-step implementation task (renaming a directory across ~40 source files)
• The Task tool call itself succeeded (returned an agent ID), but the spawned agent failed on startup
• Workaround: wrote the plan manually instead of using the Plan agent
• Other agent types (general-purpose with subagent_type: "general-purpose") worked fine in the same session

Environment

• Platform: WSL2 (Linux 6.6.87.2-microsoft-standard-WSL2)
• Model: claude-opus-4-6