Anthropic API / Anthropic API

Anthropic API 400 Bad Request — Tool Use Concurrency Error in Claude Code

Fix Anthropic API returning unexpected 400 Bad Request during tool use concurrency in Claude Code Includes evidence for Anthropic API troubleshooting demand.

Source-backed Last updated May 13, 2026 1 source Needs local verification

Category: Anthropic API
Error signature: API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation.
Quick fix: Compare the failing environment with a known working setup, then change one configuration value at a time.
Updated: May 13, 2026
Verification status: Source-backed
Evidence: 1 public source URL

Before you change production

This page includes public source URLs in the imported troubleshooting record. Compare those references with your version and environment before applying changes.

Reproduce the smallest failing action and save non-secret logs before changing configuration.
Check versions for Anthropic API, related SDKs, package managers, CI runners, and hosting providers.
Change one setting or dependency at a time, then rerun the same failing command or request.
Avoid destructive commands, credential rotation, billing changes, or security relaxations without a rollback plan.

What this error means

API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation. is a Anthropic API failure pattern reported for developers trying to fix anthropic api returning unexpected 400 bad request during tool use concurrency in claude code. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

270+ comments. Error shows as 'SyntaxError: Unexpected token' in CLI, wrapping an underlying 400 Bad Request from the Anthropic API. The root cause is tool use concurrency limits, but the error message is misleading. Forces conversation recovery via /rewind.

Common causes

Claude Code users encounter a 400 error with tool use concurrency, receiving a misleading 'Bad Request' response with invalid JSON parsing errors in the CLI. The error forces users to run /rewind to recover conversations, losing context. Affects Claude Code v2.0.2+ on macOS.
270+ comments. Error shows as 'SyntaxError: Unexpected token' in CLI, wrapping an underlying 400 Bad Request from the Anthropic API. The root cause is tool use concurrency limits, but the error message is misleading. Forces conversation recovery via /rewind.

Quick fixes

Confirm the exact error signature matches API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation..
Check the Anthropic API account, local tool state, and provider configuration involved in the failing workflow.
Compare the failing environment with a known working setup, then change one configuration value at a time.

Platform/tool-specific checks

Verify the command, editor, extension, or API client that produced the error.
Compare local settings with CI, deployment, or editor-level settings when the error appears in only one environment.
Avoid deleting credentials, local model data, or project settings until the failing scope is clear.

Step-by-step troubleshooting

Capture the exact error message and the command, editor action, or request that triggered it.
Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
Review the source evidence below and compare it with your environment.
Apply one change at a time and rerun the smallest failing action.
Keep the working fix documented for the team or deployment environment.

How to prevent it

Keep provider/tool configuration documented.
Record non-secret diagnostics such as tool version, provider name, model name, and command path.
Add a lightweight check before CI or production workflows depend on the tool.

Diagnostic flow for this page

Match API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation. exactly before applying the quick fix.
Compare the failing environment with Anthropic API versions, account scope, provider settings, and deployment context.
Check the listed common causes in order, starting with the cause that best matches your logs.
Use the evidence status below to decide whether to confirm against public sources or official documentation.
Apply one reversible change, rerun the smallest failing action, and keep rollback notes.

FAQ

What does API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation. mean in Anthropic API?

API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation. indicates the workflow described by "Anthropic API 400 Bad Request — Tool Use Concurrency Error in Claude Code" failed in a Anthropic API context. Start by confirming the exact signature and the component that emitted it.

What should I check first for API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation.?

Check whether this cause matches your environment: Claude Code users encounter a 400 error with tool use concurrency, receiving a misleading 'Bad Request' response with invalid JSON parsing errors in the CLI. The error forces users to run /rewind to recover conversations, losing context. Affects Claude Code v2.0.2+ on macOS.

Is the API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation. guidance source-backed?

The "Anthropic API 400 Bad Request — Tool Use Concurrency Error in Claude Code" page is labeled source-backed. It has 1 public source URL from github.com, so use that evidence status when deciding how much additional verification you need.

When should I avoid the quick fix for API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation.?

For "Anthropic API 400 Bad Request — Tool Use Concurrency Error in Claude Code", avoid applying "Compare the failing environment with a known working setup, then change one configuration value at a time." when the error text differs, the affected version or provider is different, or the change would touch credentials, billing, DNS, production deploy settings, or irreversible data.

What should I collect before debugging API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation.?

For "Anthropic API 400 Bad Request — Tool Use Concurrency Error in Claude Code", collect the failing Anthropic API command or request, non-secret configuration, versions, timestamps, request IDs when available, and the smallest reproduction that still triggers the signature.