Bug: API Error 400 - tool use concurrency issues

Found 1 possible duplicate issue:

1. https://github.com/anthropics/claude-code/issues/11421

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🤖 Generated with Claude Code

This has also started happening to me, I did interact with the new Rewind feature on a message but now all my conversations end quickly and abruptly in API Error: 400 due to tool use concurrency issues.. Ive tried removing all my local state to reset or remove any broken commands / state locally but to no avail. I switched to another workspace, reinstalled the vscode extension. Nothing helps. I tried the global CLAUDE.md fix but it didnt help.

2026-01-24 12:12:55.756 [info] From claude: 2026-01-24T11:12:55.756Z [DEBUG] [API:auth] OAuth token check complete

2026-01-24 12:12:56.088 [info] From claude: 2026-01-24T11:12:56.088Z [ERROR] Error in non-streaming fallback: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.1: `tool_use` ids were found without `tool_result` blocks immediately after: toolu_019JokayPpkqsHmYZXExsSGR. Each `tool_use` block must have a corresponding `tool_result` block in the next message."},"request_id":"req_011CXS6yTubqh5GtX3voua3b"}

2026-01-24 12:12:56.088 [info] From claude: 2026-01-24T11:12:56.088Z [ERROR] "Error: Error: 400 {\"type\":\"error\",\"error\":{\"type\":\"invalid_request_error\",\"message\":\"messages.1: `tool_use` ids were found without `tool_result` blocks immediately after: toolu_019JokayPpkqsHmYZXExsSGR. Each `tool_use` block must have a corresponding `tool_result` block in the next message.\"},\"request_id\":\"req_011CXS6yTubqh5GtX3voua3b\"}\n    at generate (B:/~BUN/root/claude.exe:143:37450)\n    at makeRequest (B:/~BUN/root/claude.exe:160:5435)\n    at processTicksAndRejections (native:7:39)"

It stops happening when I switch back to version: 2.1.17 and restart extensions, start a new chat.

Me too. Seems to be a persistent problem that Anthropic is ignoring.

Same for me with newer versions of CC, 2.1.6 working fine and later the issues came

Same for me, literally can't work

same issue!

Same issue - seems to be widespread

✅ WORKAROUND FOUND

The fix: Install VS Code extension version 2.1.17 while keeping CLI at 2.1.19

Steps:

1. In VS Code, go to Extensions
2. Find "Claude Code for VS Code"
3. Click the dropdown arrow next to Uninstall → "Install Specific Version" → Select 2.1.17
4. Uncheck "Auto Update" to prevent it from upgrading
5. Restart VS Code

Result:

• Extension shows: 2.1.17
• claude --version shows: 2.1.19
• It works! No more concurrency errors

What this tells us:
The bug is in the VS Code extension code (2.1.18 or 2.1.19), NOT in the CLI or API. The extension's tool call orchestration is broken in newer versions.

| Configuration | Status |
|---------------|--------|
| Extension 2.1.19 + CLI 2.1.19 | ❌ Broken |
| Extension 2.1.17 + CLI 2.1.19 | ✅ Works |

Hope this helps others!

Can confirm this bug is real, happens when using 2.1.19 and starting new sessions in terminal with prompts fed in.

아진짜 나도 이거때문에 일을 못한다고 일을 씨발 진짜 딱 내가 맥스구독하고 나니까 바로 이러네 아 정신병걸리겠다

Confirmed this bug is real. Also confirmed that dropping back to version 2.1.17 works.

🔴 Also experiencing this - Critical impact on productivity

Environment

• OS: macOS (Darwin 25.3.0)
• Claude Code Version: Latest (VS Code Extension)
• Model: Claude Opus 4.5
• Date: 2026-01-27

Reproduction Scenario

The error occurs frequently during:

1. Task tool usage - When spawning subagents (e.g., statusline-setup, Explore)
2. AskUserQuestion flow - After answering questions, the next tool call fails
3. Parallel tool calls - When Claude attempts to call multiple tools simultaneously

Error Frequency

• Very frequent - happens multiple times per session
• Makes the tool nearly unusable for complex workflows
• No workaround found (retrying doesn't help)

Impact Assessment

Looking at GitHub, there are 10+ duplicate issues filed in the last 48 hours alone:

• #21281, #21100, #21087, #21082, #21001, #20888, #20839, #20678, #20598...

This suggests a widespread regression affecting users across all platforms (Windows, macOS, Linux).

Suspected Root Cause

The error message "tool use concurrency issues" suggests the API is rejecting requests where:

• A previous tool call hasn't fully completed
• Multiple tool results are being processed simultaneously
• There's a race condition in the message/response handling

Suggested Priority

Given the volume of reports and the fact that this blocks core functionality (tool usage is fundamental to Claude Code), this should be treated as a P0/Critical issue.

Request

Could the team please:

1. Acknowledge this is a known issue being actively worked on
2. Provide an ETA or workaround if available
3. Consider a hotfix release given the impact

Thank you for your attention to this matter. 🙏

+1

Same outchea

If your session is stuck in this state and /rewind doesn't recover it, the root cause is likely a dropped tool_result — a tool_use block in the conversation history has no matching result, making the conversation structurally invalid.

Workaround: contextspectre v0.46.0+ can fix this by injecting synthetic tool_result entries:

# Dry run — show orphaned tool_use blocks
contextspectre rewire <session-id>

# Fix — inject synthetic tool_results
contextspectre rewire <session-id> --apply

Install: brew install ppiankov/tap/contextspectre

See #39316 for the specific recovery-failure analysis.

I wish some attention was given to this issue. The only workaround is go back to an old version of VSCode but this has its own problems. Is the Claude team even aware of this?

I am having this issue as well, noticed it after downloading the new claude update.

<img width="967" height="323" alt="Image" src="https://github.com/user-attachments/assets/78c02e15-1565-42c8-b051-301e5c532b43" />

+1 getting the same error after a few turns of successful tool calls and outputs