Named background subagents complete without delivering their final message to the parent; their messages briefly render in the user's UI

Environment: Claude Code CLI on macOS (darwin 25.5.0), model claude-fable-5, agent teams / named background subagents via the Agent tool.

What happened

1. The main conversation spawned three named background subagents (subagent_type: code-reviewer, run_in_background default) in a single message.
2. All three completed their work, but their final reports were never delivered to the parent session — no task-notification, no agent result. The parent polled for ~15 minutes (TaskList showed no tasks; TaskOutput with both the agent name and name@session ID returned "No task found").
3. Much later, the parent received idle_notification teammate messages (idleReason: "available") from all three agents, revealing they had finished long before. Only after the parent explicitly messaged them via SendMessage did they resend their final reports.
4. Separately, the human user reported that the subagents' messages addressed to the parent session briefly appeared in their UI view for a moment before disappearing — subagent-to-parent traffic seems to have been transiently rendered as user-facing output.

Expected

• A background subagent's final message should be delivered to the parent as the Agent tool result / a task notification when it completes.
• Agent-to-agent messages should not render in the human user's view.

Impact

The parent session assumed the agents had died silently and redid their work; the user watched a long idle wait. The transient rendering of inter-agent messages in the user's view is confusing and could leak internal coordination content.