[BUG] Frequent "API Error: 400 due to tool use concurrency issues" in VSCode extension

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?

Getting frequent "API Error: 400 due to tool use concurrency issues" errors when using Claude Code in VSCode.

Environment:

• VSCode Extension (not CLI)
• Windows
• Started happening: [תאריך - למשל January 25-26, 2026]

Steps to reproduce:

1. Open any project in VSCode with Claude Code extension
2. Ask Claude to perform any task that uses tools (read files, edit, etc.)
3. Error appears frequently, sometimes on every task

Expected behavior:
Tool calls should complete without concurrency errors

Actual behavior:
Tasks fail with "API Error: 400 due to tool use concurrency issues"
This happens multiple times per session, making the extension unusable.

Frequency:
Almost every task triggers this error

What Should Happen?

Tool calls (Read, Edit, Write, Bash, etc.) should complete successfully without errors. Claude should be able to perform tasks that require multiple tool uses without concurrency issues.

Error Messages/Logs

API Error: 400 due to tool use concurrency issues

Steps to Reproduce

1. Open any project in VSCode with Claude Code extension (v2.1.19)
2. Start a conversation with Claude
3. Ask Claude to perform any task that requires tool usage, for example:

• "Read the file package.json"
• "Create a new file called test.ts"
• "Edit this function..."

1. Error appears: "API Error: 400 due to tool use concurrency issues"

The error happens on almost every task that involves tool calls.
No specific file or code triggers it - it's a general issue with tool usage.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.67

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Terminal.app (macOS)

Additional Information

Version mismatch detected:

• VSCode Extension: 2.1.19
• CLI (claude --version): 2.0.67

The error occurs frequently when Claude attempts to use multiple tools.
Started happening around January 25-26, 2026.
Previously worked without issues.