Task tool subagents fail with 'classifyHandoffIfNeeded is not defined'

Bug Description

When launching subagents via the Task tool, some agents fail immediately with:

classifyHandoffIfNeeded is not defined

The error occurs before the agent starts executing any tools. Multiple agents launched in the same session all fail with the identical error.

Steps to Reproduce

1. Start a Claude Code session
2. Launch multiple Task tool subagents (e.g., subagent_type: "Explore") in parallel
3. Some or all agents fail with classifyHandoffIfNeeded is not defined

Observed Behavior

• 3 out of 3 parallel Explore agents failed with the same error
• Task status shows failed with delta: classifyHandoffIfNeeded is not defined
• The error appears to be a framework-internal reference error, not related to the user's codebase
• The output files for failed agents are empty (0 bytes)

Expected Behavior

Subagents should start executing normally without framework reference errors.

Environment

• OS: macOS (Darwin 25.0.0, Apple Silicon)
• Model: claude-opus-4-6
• Date: 2026-02-17

Additional Context

The agents were launched during a planning/exploration phase of a larger implementation task. The main session continued to work correctly — only the subagents were affected. Restarting the session and re-launching agents may or may not reproduce the issue (it appeared intermittently).