API Error: 400 due to tool use concurrency issues in VS Code extension

Description

Experiencing repeated API Error: 400 due to tool use concurrency issues errors in the VS Code extension panel. The errors require saying "Keep going" multiple times to recover, significantly disrupting the workflow.

Environment

• Claude Code version: 2.1.19
• Installation: Homebrew cask (/opt/homebrew/Caskroom/claude-code/2.1.19/claude)
• Platform: macOS Darwin 24.6.0
• Interface: VS Code extension panel

Error Details

{
  "type": "assistant",
  "message": {
    "model": "<synthetic>",
    "role": "assistant",
    "stop_reason": "stop_sequence",
    "content": [
      {
        "type": "text",
        "text": "API Error: 400 due to tool use concurrency issues."
      }
    ]
  },
  "error": "invalid_request",
  "isApiErrorMessage": true
}

Reproduction

1. Use Claude Code in VS Code extension panel
2. Make requests that trigger multiple tool calls
3. Error occurs intermittently, sometimes in rapid succession

Frequency

In a single session, this error occurred 15 times within a 12-minute window (21:21-21:33), requiring the user to type "Keep going" after each occurrence to continue.

Timeline of errors

2026-01-26T21:21:23.940Z
2026-01-26T21:22:50.659Z
2026-01-26T21:24:05.035Z
2026-01-26T21:24:07.638Z
2026-01-26T21:25:18.338Z
2026-01-26T21:25:39.885Z
2026-01-26T21:25:51.623Z
2026-01-26T21:26:02.563Z
2026-01-26T21:26:15.050Z
2026-01-26T21:26:22.612Z
2026-01-26T21:26:31.682Z
2026-01-26T21:26:40.725Z
2026-01-26T21:30:01.785Z
2026-01-26T21:30:12.897Z
2026-01-26T21:33:28.861Z

Expected Behavior

Tool calls should either:

1. Execute without concurrency errors
2. Be automatically retried by the client
3. Provide clearer guidance on how to avoid the issue

Workaround

Using the CLI instead of VS Code extension panels appears to avoid this issue.

Additional Context

• The CLI session running simultaneously did not experience these errors
• The error appears to be an API-level rejection (HTTP 400) rather than a client-side issue
• Session ID for reference: 12a29e1e-547c-4f9c-acf6-085995693f18