[BUG]

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?

Title: API Error 400: "tool_use ids must be unique" when making parallel tool calls

What's happening:
When Claude Code makes multiple tool calls in parallel (in a single response), the API returns error 400 with message: "tool_use ids must be unique". This happens consistently when 2+ tools are called simultaneously.

Actual behavior:
Claude Code generates duplicate tool_use IDs, causing the API to reject the request.

Environment:

Platform: macOS (Darwin 24.5.0)
Claude Code: VS Code Extension
Model: claude-opus-4-5-20251101
Workaround:
Sequential tool calls work fine. Only parallel calls trigger this bug.

What Should Happen?

When Claude Code needs to execute multiple tool calls in a single response (parallel execution), each tool call should be assigned a unique tool_use ID. The API should successfully process all parallel tool calls and return their results.

For example, when asking Claude to "search for .tsx and .css files", it should:

Generate unique IDs for each tool call (e.g., toolu_01ABC... and toolu_01XYZ...)
Execute both Glob searches in parallel
Return combined results without errors
Currently, Claude Code generates duplicate IDs causing: API Error: 400 - "tool_use ids must be unique"

Error Messages/Logs

Steps to Reproduce

Steps to reproduce:

Start a Claude Code session
Ask Claude to perform a task that triggers multiple parallel tool calls (e.g., "search for all .tsx and .css files")
Error appears: API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.XX.content.X: tool_use ids must be unique"}}
Expected behavior:
Parallel tool calls should work without error - each tool_use should have a unique ID generated.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.19 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_