[BUG] Background task completions produce cascading single-line response backlog

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

When multiple background agent completions or system-reminder notifications arrive while Claude is idle/waiting, each one gets processed and responded to individually. This produces a rapid cascade of minimal "Handled." lines instead of a single consolidated response. The same information often gets restated across multiple lines, and most of the output is noise.

Example 1:

- Handled.
- Handled (that was a monitoring check).
- Handled.
- Handled.
- Handled. Let me check if the actual jobs have finished:

Example 2:

- Handled (that was the 3s sequential run we killed in favor of async).
  The data generation is complete — 2,698 new encoding examples ready.
- Handled.
- Handled.
- That's the successful data generation run — 947 enriched + 1,751 synthetic = 2,698 ...
- Handled. Waiting on your go-ahead for the next steps.

What Should Happen?

When multiple notifications queue up while Claude is idle, they should be batched into a single turn rather than processed individually, so the model can produce one consolidated response instead of N separate acknowledgments.

Steps to Reproduce

1. Start a Claude Code session with background agents (using run_in_background)
2. Launch 2+ background agents that will complete around the same time
3. Wait idle for the agents to finish
4. Observe the cascade of individual "Handled." responses as each completion notification arrives

Is this a regression?

I don't know

Claude Code Version

2.1.90

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code Terminal

Claude Model

Opus 4.6 1m

Additional Information

Observed in VS Code extension (v2.1.89-2.1.90) on Linux. Happens consistently when multiple run_in_background agents complete in sequence.