Agent Teams: teammate replies stall until lead window gains focus (Windows, run_in_background)

Summary

When a teammate spawned via Agent (with team_name and run_in_background: true) calls SendMessage to the lead, the message is not delivered to the lead as a new conversation turn until the user manually focuses/clicks the lead's CC window. Until that interaction, the lead sits idle with no awareness that a reply is queued.

This contradicts the documented behavior in the SendMessage tool description: "Messages from teammates are automatically delivered to you. You do NOT need to manually check your inbox."

Environment

• Claude Code version: 2.1.132
• Platform: Windows 11 Enterprise 10.0.26200
• Shell: bash (Git Bash on Windows)
• Flag: CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1
• Session ID (one occurrence): 8dbec7f2-2514-448f-84ad-0b3e48185b95

Repro

1. Set CLAUDE_CODE_EXPERIMENTAL_AGENT_TEAMS=1 in env.
2. From a lead session: TeamCreate then spawn a teammate via the Agent tool with team_name set and run_in_background: true.
3. Teammate runs to a point where it calls SendMessage back to team-lead.
4. While the teammate is working, switch focus away from the lead's CC window (alt-tab, click another window).
5. Observe: when the teammate calls SendMessage, the lead does not receive the message as a new turn. The lead sits idle.
6. Click into the lead's CC window. The queued teammate-message immediately surfaces as a new turn and the lead processes it.

Expected

Per the SendMessage tool docs, replies should be pushed to the lead as new conversation turns automatically, regardless of which window has focus. run_in_background: true is supposed to allow asynchronous coordination — focus-dependent delivery breaks that contract.

Observed

• Reproduced repeatedly across two distinct teammates (a "Loremaster" preprocessor and an "Artificer" worker) within the same session.
• Each teammate's SendMessage reply landed in the lead's turn the moment the user clicked into the lead window — never before.
• idle_notification messages from teammates do appear to surface, but actual SendMessage payloads stall.

Impact

Long-running orchestrated pipelines (e.g., the Arcane plugin) stall indefinitely because the lead cannot react to STATUS / QUESTION / BLOCKED / REPORT messages from teammates until the user babysits the lead window. This defeats the purpose of run_in_background: true and breaks any flow that expects the lead to react to teammate output autonomously.

Workaround

Keep the lead's CC window focused, or click into it periodically to flush the message queue.