API Error: 400 due to tool use concurrency issues when reading multiple files

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?

When Claude Code attempts to read multiple files in parallel (e.g., using the Read tool on 2 files simultaneously), it returns:

API Error: 400 due to tool use concurrency issues

Steps to Reproduce

1. Start a new Claude Code session
2. Ask Claude to check progress or read multiple files
3. Claude attempts parallel Read operations like:

• Read file1.md
• Read file2.md

1. Error occurs immediately

Environment

• OS: Windows 11
• Claude Code: VS Code Extension & Antigravity environment
• Behavior: Reproducible in both VS Code and Antigravity IDE

Expected Behavior

Parallel tool calls should execute successfully or fall back to sequential execution.

Actual Behavior

API returns 400 error with message about "tool use concurrency issues"

Additional Context

• The error occurs consistently when multiple tools are called in parallel
• Single tool calls work fine
• The terminal/CLI version of Claude Code works normally

What Should Happen?

Parallel tool calls should execute successfully without errors. When Claude Code reads multiple files simultaneously (e.g., Read file1.md and Read file2.md in parallel), all files should be read and returned correctly. If parallel
execution is not supported, the system should automatically fall back to sequential execution instead of returning a 400 error.

Error Messages/Logs

Steps to Reproduce

1. Open VS Code with Claude Code extension installed 2. Open any project folder
2. Start a new Claude Code chat session
3. Send a message like: "Check the progress of my work by reading these files: file1.md and file2.md"
4. Claude Code attempts to read both files in parallel using the Read tool
5. Error appears: "API Error: 400 due to tool use concurrency issues"

Alternative reproduction:

1. Ask Claude: "Good morning, let me check yesterday's work"
2. Claude tries to use Glob + Read tools in parallel
3. Same error occurs

Note: Single file reads work fine. The error only occurs when multiple tools are called simultaneously in a single
response.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

v2.1.19

Platform

Other

Operating System

Windows

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_