[Bug] Background sub-agent output files empty (0 bytes) on Windows

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [ ] I am using the latest version of Claude Code

What's Wrong?

When launching background sub-agents via the Agent tool with run_in_background: true, the completed task frequently displays "0 tokens" in
the status line despite the agent having executed many tool calls and produced significant output.

Example:
├─ superpowers:code-reviewer (Review code quality) · 58 tool uses · 0 tokens ⎿ Done
├─ Agent (Review UI/UX design) · 34 tool uses · 0 tokens ⎿ Done

What Should Happen?

Should display actual token count, e.g.:
├─ superpowers:code-reviewer (Review code quality) · 58 tool uses · 41397 tokens ⎿ Done

Error Messages/Logs

No error messages. The issue is silent - output files are empty but no errors are logged.

Steps to Reproduce

1. Launch multiple background sub-agents with run_in_background: true

Agent
({
subagent_type: "superpowers:code-reviewer",
description: "Review code",
prompt: "...",
run_in_background: true
})

1. Wait for agents to complete
2. Observe the status line showing "X tool uses · 0 tokens"
3. Check task output files in temp directory - most are empty (0 bytes)

Evidence from my session:

• Total output files: 89
• Empty files: 82 (92%)
• Non-empty files: 7 (8%)

Claude Model

Other

Is this a regression?

I don't know

Last Working Version

N/A

Claude Code Version

2.1.80 (Claude Code)

Platform

Other

Operating System

Windows

Terminal/Shell

VS Code integrated terminal

Additional Information

Root Cause Analysis:

After investigating the task output files, I identified a race condition:

Timeline:
T0: Sub-agent completes (e.g., 41397 tokens generated)
T1: completeAgentTask() called
T2: void evictTaskOutput(taskId) ← FIRE-AND-FORGET (doesn't wait!)
T3: enqueueAgentNotification() sends notification immediately
T4: Main session receives notification and reads output file
T5: Disk flush finally completes (too late!)
Result: File is empty at T4 → displays "0 tokens"

The evictTaskOutput() function awaits flush, but the caller uses void evictTaskOutput(taskId) (fire-and-forget). The notification is sent
before disk write completes.

Suggested Fix:
export async function completeAgentTask(result, setAppState): Promise<void> {
// ...update state...
await evictTaskOutput(taskId) // Wait for flush before notifying
}

Note: I'm unable to upgrade to 2.1.126 due to third-party API compatibility, but this race condition is likely still present as it's a
fundamental async/await issue.