[BUG] [BUG] API Error 400 "tool use concurrency issues" in interactive mode with Opus 4.5

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 execution gets interrupted after a few seconds of "thinking" with error:
API Error: 400 due to tool use concurrency issues

Found this issue while working with Claude Code itself - Claude helped
diagnose and confirmed it's a known bug affecting multiple users.

## Environment

• Claude Code version: 2.1.19
• OS: Ubuntu (Linux 6.14.0-37-generic)
• Shell: bash
• Model: claude-opus-4-5-20251101

## Configuration

• MCP servers: None ("mcpServers": {})
• Mode: Interactive (NOT -p print mode)
• Clean installation, no custom plugins
• No MCP JSON servers enabled

What Should Happen?

No error !

Error Messages/Logs

Steps to Reproduce

1. Start claude in interactive mode
2. Ask a question that triggers tool use (e.g., file search, code exploration)
3. After a few seconds of thinking, tool execution is interrupted
4. Error appears: "API Error: 400 due to tool use concurrency issues"

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.19

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

What we checked

• ✅ No MCP servers configured
• ✅ Clean ~/.claude.json
• ✅ Latest version (2.1.19)
• ❌ Bug still occurs

This rules out MCP-related causes from #20632.

## Related issues

• #18130 (closed/locked) - similar but in print mode
• #20632 - MCP-related variant
• #732 (claude-code-action) - regression after v1 update

## Additional context

This happens randomly during normal conversation, not tied to a specific
action.