Claude Code can sometimes stop with this message: “API Error: 400 due to tool use concurrency issues. Run /rewind to recover the conversation.”
This error often appears after a Bash command, file edit, or tool action. The session may still look active, but the conversation state has broken.
Quick Answer
Claude Code API Error 400 due to tool use concurrency issues means Claude’s tool-call order became invalid. A tool request and tool result may not match correctly. Claude Code suggests /rewind because going back before the broken tool step can recover the conversation.
What Does API Error 400 Due to Tool Use Concurrency Issues Mean?
API Error 400 means the request sent to the API is not valid. Anthropic’s API docs classify 400 errors as invalid_request_error, which happens when the request format or content is invalid.
In Claude Code, this specific message points to a tool-use state problem. The GitHub issue for Claude Code mentions duplicate reports tied to “tool_use ids” without matching “tool_result blocks” in the right place.
In simple words, Claude Code expected one tool flow. The saved conversation had another flow. That mismatch can make the next API request fail.
Why Does This Claude Code Error Happen?
This error usually comes from a broken tool sequence. Claude Code uses tools like Bash, Read, Edit, and Write during coding tasks.
Common causes include:
- A tool call starts but its result does not attach correctly
- Two tool actions overlap in the same session
- The conversation history stores a bad tool-use block
- An IDE or terminal session interrupts the response
- A retry sends an old tool state again
- Claude Code or its extension hits a bug
The issue is not always your code. Sometimes the project files are fine, but the Claude Code conversation state is not.
How to Fix Claude Code API Error 400
Use this order before deleting your session.
- Save your current work
Check any changed files first. Commit, copy, or save important code before resetting anything.
- Run /rewind
Claude Code itself tells users to run /rewind for this error. That command moves the conversation back before the broken point.
- Retry the last request
After rewinding, ask Claude Code to continue from a safer step. Keep the next request simple.
- Avoid running another tool at once
Let one Bash command, edit, or file read finish before asking for more work.
- Restart the terminal session
If /rewind fails, close the Claude Code session and open a fresh one.
- Update Claude Code
Use the latest Claude Code version if the error keeps returning. Tool handling bugs can improve after updates.
What Does /rewind Do in Claude Code?
/rewind rolls the conversation back to an earlier point. It helps remove the broken tool-call step from the active chat history.
Think of it like undo for the Claude Code session. It does not replace good file backups or Git commits.
Use /rewind when:
- Claude Code says to run it
- The same API Error 400 repeats
- A Bash or file tool caused the break
- The session cannot continue normally
Do not keep retrying the same failed prompt many times. That can keep sending the same broken context.
What If /rewind Does Not Work?
Sometimes the session cannot recover cleanly. That can happen when the broken state sits too deep in the conversation.
Try this checklist:
- Copy the last useful answer
- Save edited files
- Open a new Claude Code session
- Summarize the task again
- Ask Claude to inspect only the needed files
- Avoid pasting the full broken log unless needed
A fresh session often fixes the issue because it removes the bad conversation history.
How to Prevent Tool Use Concurrency Errors
You cannot prevent every Claude Code bug. Still, safer workflow habits reduce the chance.
Good habits include:
- run one major command at a time
- wait for Bash output before asking again
- avoid interrupting file edits
- keep prompts short after tool-heavy steps
- use Git before large refactors
- update Claude Code often
- restart long sessions when they feel unstable
Long coding sessions can collect too much tool history. Starting fresh after major tasks can keep the context cleaner.
Claude Code API Error 400 vs Other API Errors
| Error Type | Common Meaning | Best Fix |
|---|---|---|
| API Error 400 | Bad or invalid request | Fix request format or rewind broken state |
| Tool use concurrency error | Tool-call history mismatch | Run /rewind then retry safely |
| API Error 401 | Authentication problem | Check API key or login |
| API Error 429 | Rate limit issue | Wait and reduce request volume |
| API Error 500 | Server-side issue | Retry later or check status page |
This error is different from a normal login or rate-limit issue. The message points more toward a broken tool-use conversation.
Final Thoughts
Claude Code API Error 400 due to tool use concurrency issues usually means the tool-call state broke. Your code may still be safe.
Start with /rewind, then retry with a smaller prompt. If that fails, save your work and start a fresh Claude Code session.
Have you seen this error after Bash, file edits, or another Claude Code tool? Leave a comment with what triggered it.