Background Task Notifications Not Delivered When Multiple Agents Complete Simultaneously (v2.1.19)

Bug Description

When running multiple background agents (run_in_background: true) in parallel, completion notifications are not consistently delivered to the main session. Only 1 out of 3 agents sent a notification upon completion.

Environment

• Claude Code Version: 2.1.19
• OS: macOS Sequoia 15.2 (Darwin 25.2.0)
• Model: claude-opus-4-5-20251101
• Terminal: iTerm2

Steps to Reproduce

1. Launch 3 background agents simultaneously:

Task({
  subagent_type: "general-purpose",
  description: "Task 1",
  prompt: "...",
  run_in_background: true
})
// Repeat for Task 2 and Task 3

1. While agents are running, send a user message (e.g., "checking status")

1. Wait for all agents to complete

Expected Behavior

All 3 agents should deliver <task-notification> messages upon completion.

Actual Behavior

• Agent 1 (a34bb07): ✅ Notification received
• Agent 2 (a0fc025): ❌ No notification (but completed successfully)
• Agent 3 (a194492): ❌ No notification (but completed successfully)

All agents completed successfully (verified via tail on output files), but only 1 notification was delivered to the main session.

Evidence

Output files confirm completion:

$ grep -c "stop_reason" ~/.claude/projects/.../subagents/agent-*.jsonl
agent-a0fc025.jsonl:22  # Completed
agent-a194492.jsonl:41  # Completed  
agent-a34bb07.jsonl:42  # Completed (notification received)

Suspected Cause

Race condition in notification queue processing:

1. Multiple agents complete around the same time
2. User sends a message while notifications are queued
3. Queue processing is interrupted or notifications are "swallowed"

Workaround

Manually check agent output files:

tail -20 /private/tmp/claude/-Users-*/tasks/*.output

Or wait a few seconds after agents complete before sending any messages.

Impact

• Users cannot reliably track background task completion
• Workflow automation based on notifications becomes unreliable
• Parallel agent execution loses observability

Related

This may be related to the notification queue handling introduced in v2.1.16+ with the new task management system.

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Thank you for looking into this! Happy to provide additional logs or context.