[BUG] Background agents frequently go idle without delivering their final SendMessage report (re-ping recovers it)

Preflight

• [x] I have searched existing issues and this hasn't been reported yet (searched: "subagent final report not delivered", "agent idle without report SendMessage")
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code (2.1.200)

What's Wrong?

Background subagents that are explicitly instructed to SendMessage their final report to the parent before stopping frequently go idle with the work complete but the report undelivered. The parent receives only a bare idle_notification (no content). A single SendMessage re-ping ("please send your report") recovers the full report in nearly every case — i.e. the agent produced the report content; only the final delivery was dropped.

Observed across different agent types (implementation agents, review agents, search agents), on the order of 10+ instances over four days of heavy multi-agent use. It happens often enough that we adopted a standing operating procedure: on any bare idle notification, verify the agent's actual output via git log/filesystem first, then send exactly one re-ping — because the idle signal alone cannot be trusted to mean "nothing to deliver."

The task-notification result/summary field only partially covers this: it is truncated/summarized relative to the agent's full final report, so it is not a substitute when the report carries structured findings (e.g. a review verdict with per-finding details).

What Should Happen?

The final assistant text of a completing background agent should reliably reach the parent session — either the agent's terminal SendMessage should be flushed before the idle notification is emitted, or the full (untruncated) final message should ride the completion notification itself.

Steps to Reproduce

1. Spawn a background agent with the Agent tool, with an instruction of the form: "Do X, then report your results back with SendMessage before finishing."
2. Let it run a multi-step task (several minutes, multiple tool calls).
3. Intermittently, the parent receives only {"type":"idle_notification", ...} with no preceding report message.
4. Send the agent a SendMessage asking for its report — the full report arrives, demonstrating the content existed at stop time.

Frequency: intermittent but common — roughly 1 in 3–5 agent completions in our sessions; more likely on agents that did several message round-trips with the parent before finishing.

Environment

• Claude Code 2.1.200, Windows 11 Pro (10.0.26200), PowerShell + Git Bash
• Multi-agent: Agent tool with named background teammates + SendMessage