[BUG] Error: "API Error: 400 due to tool use concurrency issues"

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?

Issue: Persistent "API Error: 400 due to tool use concurrency issues" appearing after every tool call. Error displays to user after tool calls. Tool calls appear to actually succeed (responses are returned). Error persists across all tool invocations.

What Should Happen?

Tool calls should complete without displaying an error message to the user. Since the tools are actually executing successfully and returning results, no error should be shown. The "API Error: 400 due to tool use concurrency issues" message should not appear when there is no actual failure.

Error Messages/Logs

"API Error: 400 due to tool use concurrency issues"

Steps to Reproduce

Steps to reproduce:

Open Cursor with Claude Code extension
Start a conversation
Trigger any tool call (e.g., ask Claude to run a command or read a file)
Error appears despite tool completing successfully

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

Unknown / Please advise which version to roll back to

Claude Code Version

Claude Code v2.1.19 (darwin-arm64)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_