[BUG] Tool/Agent progress lines repeat hundreds of times instead of updating in place (v2.1.2, macOS, iTerm2)

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)
• [x] I am using the latest version of Claude Code

What's Wrong?

Tool and agent progress indicators repeat hundreds of times instead of updating in place. The spinner line (⏺) should overwrite itself on each update, but instead each update appends a new line, flooding the terminal with duplicate status lines.

This affects:

• WebSearch tool progress
• Task tool subagent status (e.g., cdb-codebase-expert(...))
• Likely all tools with progress indicators

The alternating pattern of lines with/without the ⏺ icon suggests the spinner animation IS cycling correctly, but the ANSI cursor positioning/line clearing is failing.

What Should Happen?

A single progress line that updates in place. The ⏺ spinner should animate by overwriting the same line, showing something like:

⏺ Web Search("query here")

Then when complete:
⏺ Web Search("query here")
⎿ Did 1 search in 29s

Error Messages/Logs

No error messages - this is a visual/rendering bug only.

Steps to Reproduce

1. Start Claude Code 2.1.2 in iTerm2 on macOS
2. Run any command that triggers a tool with a progress indicator (e.g., ask Claude to search the web, or use a custom agent via Task tool)
3. Observe the terminal output during tool execution
4. Instead of a single updating line, hundreds of duplicate lines appear

Example prompt that triggers it:
"Search the web for Python lazy import best practices"

Or spawning a subagent:
"Use the explore agent to find all API endpoints"

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Unknown - issue noticed after updating to 2.1.x

Claude Code Version

2.1.2

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_