openclaw - 💡(How to fix) Fix Bug: Subagent completion events lost when parallel spawns finish within 5-8s

Description

When two subagents are spawned in parallel via sessions_spawn and both complete within 5-8 seconds of each other, their completion events fail to arrive at the parent session. The sessions_yield call returns without a subagent completion notification.

Reproduction

Environment: OpenClaw (edge), deepseek/deepseek-v4-pro, Windows 10 x64, openclaw-weixin channel.

Steps:

1. Spawn two subagents simultaneously via sessions_spawn with mode="run"
2. Both subagents complete quickly (25-32s runtime each, 5-8s gap between completions)
3. Call sessions_yield on parent session

Expected: Parent session receives completion events for both subagents.

Actual:

• When completion gap > 40s: both events arrive normally (2/2)
• When completion gap = 8s: only 1 of 2 events arrived (1/2)
• When completion gap = 5s: both events lost (0/2)

Evidence

Transcript analysis: In the lost-event sessions, the last assistant message has an empty stopReason field (should be "stop"), and the transcript file contains no session-end event. The sub-session appears to never properly signal completion.

Impact

This prevents reliable parent-subagent orchestration. Our workaround is to call subagents list after every sessions_yield return to manually check for silently-completed subagents, but this adds latency (60s+ polling intervals).

Suspected Root Cause

Likely a race condition in the event dispatch layer when two sub-sessions complete almost simultaneously -- the second completion signal may be dropped or overwritten by the first.