Agent teams fail to launch with Vertex AI backend: prompt not passed to subprocess

Description

When using the experimental agent teams feature (CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1) with Vertex AI as the backend (CLAUDE_CODE_USE_VERTEX=1), spawned teammate agents consistently fail to launch with:

Error: Input must be provided either through stdin or as a prompt argument when using --print

Environment

• Claude Code version: 2.1.50
• Platform: Linux (WSL2, kernel 6.6.114.1-microsoft-standard-WSL2)
• Backend: Vertex AI (CLAUDE_CODE_USE_VERTEX=1)
• Shell: zsh

Steps to Reproduce

1. Set CLAUDE_CODE_USE_VERTEX=1 in environment
2. Enable CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1 in settings or environment
3. Use TeamCreate to create a team and spawn teammates via Task with team_name
4. Observe that spawned agent CLI subprocesses fail immediately

Expected Behavior

Teammate agents should launch successfully and begin processing their assigned tasks.

Actual Behavior

The team system spawns new Claude CLI processes via tmux with commands like:

cd /path/to/project && env CLAUDECODE=1 CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1 CLAUDE_CODE_USE_VERTEX=1 /home/<user>/.local/share/claude/versions/2.1.50 --agent-id <id> --print ...

These subprocesses fail because the prompt input is not being piped or passed as an argument correctly. The --print flag expects input via stdin or as a prompt argument, but neither is provided.

Notes

• Regular Task tool subagents (in-process) work fine — this is isolated to the TeamCreate/tmux-based agent spawning mechanism.
• Reproducible across multiple sessions consistently.
• The issue may be specific to the Vertex AI backend, or it may affect other configurations as well.