[BUG] Terminal UI rendering corruption when running 18 parallel Task subagents

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 launching many parallel subagents (18-30+) using the Task tool, the terminal UI experiences severe rendering corruption. Issues include:

• Text merging: Status text corrupts into "ReDone", "LiennDone", "RiDone" instead of clean "Done" labels
• Tree structure corruption: Hierarchical characters mix improperly like • C├── and • J LiennDone
• Column overlap: Token counts and stats truncate (e.g., "t ol uses" instead of "tool uses", "tokensens" instead of "tokens")
• Line bleeding: Agent descriptions merge across lines, making the output unreadable

What Should Happen?

• Clean hierarchical tree display of all running agents
• Status indicators ("Done", spinners) render correctly on their own lines
• Token counts and tool use numbers display without truncation or overlap
• Each agent's output line remains distinct and readable

Error Messages/Logs

No explicit error messages - this is a visual rendering issue only.
The agents complete successfully, but the UI display is corrupted.

Steps to Reproduce

1. Open Claude Code in a terminal
2. Run a prompt that spawns many parallel subagents:

``
review all 54 prompts using subagents -> for each prompt launch subagent to check if changes affect the CLI codebase
``

1. Observe the UI as 20+ Task agents spawn and run concurrently
2. Watch the display corruption appear as agents complete (especially when multiple complete around the same time)

Note: The bug becomes visible after ~15-20 agents are running simultaneously. It worsens when agents complete at similar times, causing many concurrent status line updates.

Claude Model

Opus

Is this a regression?

I don't know

Claude Code Version

2.1.23 (Claude Code)

Platform

Anthropic API

Operating System

macOS (Darwin 24.6.0)

Terminal/Shell

macOS Terminal (needs confirmation from reporter)

Additional Information

Screenshots showing the corruption:

<img width="80%" alt="Image" src="https://github.com/user-attachments/assets/24eef790-3c51-4639-9be7-6dbfa459efd1" />

(Note: Screenshots need to be uploaded manually after issue creation)

Observations:

• Issue appears specific to high parallelism (20+ concurrent subagents)
• Likely related to ANSI escape sequence handling or terminal buffer management during rapid concurrent status updates
• Workaround: Batching subagents into smaller groups (5-6 at a time) avoids the corruption, but defeats the purpose of parallel execution