[Bug] API Error 400 - Tool Use Concurrency Issues - Widespread Regression Affecting All Platforms

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

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🚨 Summary

This is a meta-issue to consolidate reports of the widespread API Error: 400 due to tool use concurrency issues affecting users across all platforms.

In the last 48 hours, 10+ duplicate issues have been filed:

• #20555 - Bug: API Error 400 - tool use concurrency issues
• #21281 - [BUG] API Error: 400 due to tool use concurrency issues
• #21100 - API Error: 400 due to tool use concurrency issues
• #21087 - [BUG] API Error: 400 due to tool use concurrency issues
• #21082 - [Bug] Anthropic API Error: 400 Tool Use Concurrency Issues
• #21001 - [Bug] Anthropic API Error: Tool use concurrency conflict in tasks
• #20888 - [Bug] Anthropic API Error: Tool use concurrency validation (400)
• #20839 - [BUG] Error: "API Error: 400 due to tool use concurrency issues"
• #20678 - [Bug] Anthropic API Error: 400 - Tool Use Concurrency Conflict
• #20598 - [BUG] API Error: 400 due to tool use concurrency issues

This suggests a critical regression that needs immediate attention.

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What's Wrong?

Error Message

API Error: 400 due to tool use concurrency issues

When It Occurs

1. After AskUserQuestion - User answers a question, next tool call fails
2. Task/Subagent spawning - Using Task tool with any subagent_type
3. Parallel tool calls - When Claude attempts multiple simultaneous tool calls
4. Custom slash commands - Especially those with multi-step workflows
5. VS Code Extension - Particularly affected, but CLI also impacted

Affected Platforms

| Platform | Reports |
|----------|---------|
| macOS | ✅ Multiple |
| Windows | ✅ Multiple |
| Linux | ✅ Confirmed |

Affected Interfaces

• [x] VS Code Extension
• [x] CLI (Terminal)
• [x] JetBrains Extension (unconfirmed but likely)

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Environment (My Setup)

• OS: macOS (Darwin 25.3.0)
• Claude Code: Latest version (VS Code Extension)
• Model: Claude Opus 4.5 (claude-opus-4-5-20251101)
• Date: 2026-01-27

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Steps to Reproduce

Scenario 1: Task Tool

1. Start Claude Code
2. Ask Claude to use the Task tool (e.g., "explore the codebase")
3. Wait for subagent to spawn
4. Error occurs during or after subagent execution

Scenario 2: AskUserQuestion Flow

1. Trigger a workflow that uses AskUserQuestion
2. Answer the question presented
3. Error occurs on the next tool call after user response

Scenario 3: Parallel Operations

1. Request an operation that requires multiple file reads/searches
2. Claude attempts parallel tool calls
3. Error occurs with concurrency message

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Impact Assessment

Severity: 🔴 Critical

• Frequency: Multiple times per session
• Workaround: None found (retrying doesn't help)
• User Impact: Core functionality broken - tool usage is fundamental to Claude Code
• Productivity Loss: Significant - users cannot complete complex workflows

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Suspected Root Cause

The error suggests the API is rejecting requests due to:

1. Race condition in tool call/response handling
2. Previous tool call state not properly cleared before new call
3. Concurrent message processing conflict
4. Possible backend change affecting tool use validation

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What Should Happen?

Tool calls should execute successfully without concurrency conflicts, regardless of:

• Previous tool call completion timing
• Number of parallel tool calls
• User interaction patterns (answering questions, etc.)

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Request to Anthropic Team

1. Acknowledge this is a known, actively-investigated issue
2. Provide status updates on investigation progress
3. Share workarounds if any are discovered
4. Consider hotfix given the widespread impact
5. Consolidate duplicates under this or #20555 for better tracking

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Related Issues

• #20555 - Bug: API Error 400 - tool use concurrency issues (Original Report)
• #21281 - [BUG] API Error: 400 due to tool use concurrency issues
• #21100 - API Error: 400 due to tool use concurrency issues
• #21087 - [BUG] API Error: 400 due to tool use concurrency issues
• #21082 - [Bug] Anthropic API Error: 400 Tool Use Concurrency Issues
• #21001 - [Bug] Anthropic API Error: Tool use concurrency conflict in tasks
• #20888 - [Bug] Anthropic API Error: Tool use concurrency validation (400)
• #20839 - [BUG] Error: "API Error: 400 due to tool use concurrency issues"
• #20678 - [Bug] Anthropic API Error: 400 - Tool Use Concurrency Conflict
• #20598 - [BUG] API Error: 400 due to tool use concurrency issues

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Claude Model

Opus 4.5

Is this a regression?

Yes - started occurring recently (last few days)

Claude Code Version

Latest (2.x)

Platform

Anthropic API

Operating System

macOS (also affects Windows, Linux)

Terminal/Shell

VS Code integrated terminal

Additional Information

This issue is severely impacting the Claude Code user experience. The volume of duplicate reports in such a short time indicates this is not an isolated incident but a widespread problem requiring urgent attention.

Thank you for your prompt attention to this critical issue. 🙏