Background subagent completion notifications not delivered (required relay via teammate message)

Description

While running a multi-agent SDD workflow in a single Claude Code session (main loop dispatching subagents via the Agent tool, tracked with names via SendMessage/idle_notification), two dispatched review subagents (general-purpose, model: sonnet) completed their work and went idle, but the main session never received a completion/idle notification for either of them through the normal channel.

The main session waited (including sending explicit SendMessage check-ins to both agents) for roughly 40+ minutes with no notification arriving. The subagents had, in fact, already completed and were idle well before that.

The two agents' reports only reached the main session because a separate, unrelated Claude Code session in the same environment received their idle_notification/teammate-message events directly and relayed them into the main session's conversation as <teammate-message> blocks.

Impact

• The main session has no reliable way to know a dispatched subagent has finished if its notification is dropped — it either polls wastefully or blocks indefinitely.
• The workaround (another session relaying the message) is not something a user/agent can rely on or trigger deliberately — it happened incidentally because both sessions apparently shared some team/session-routing state.

Environment

• Claude Code CLI, agent-team / subagent (Agent tool) workflow within a single session
• Model: Sonnet 5 (main loop), Sonnet (subagents)
• OS: Linux (Fedora 7.1.8-200.fc44.x86_64)

Expected behavior

Every dispatched subagent's completion (or idle) notification should reliably reach the session that dispatched it, without requiring an unrelated session to relay it.

Actual behavior

Two of several subagent dispatches in the same session never delivered their completion notification to the dispatching session; only a relay from a different session surfaced them.