Claude Code Agent API Error 400 — Invalid user_id Pattern Validation Failure

What this error means

API Error: 400 Invalid 'user_id': string does not match pattern. Expected a string that matches the pattern '^[a-zA-Z0-9_-]+$' is a Claude Code failure pattern reported for developers trying to fix claude code agent api 400 error when user_id contains invalid characters. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Claude Code Agent returns 400 API error when user_id doesn’t match pattern ^[a-zA-Z0-9_-]+$. 16 comments indicate multiple affected users. Clear error signature from Anthropic API validation layer.

Common causes

• Developers using Claude Agent (Claude Code’s agent mode) get a 400 error when their user_id doesn’t match the expected pattern. This blocks agent workflows and affects teams with non-standard naming conventions.
• Claude Code Agent returns 400 API error when user_id doesn’t match pattern ^[a-zA-Z0-9_-]+$. 16 comments indicate multiple affected users. Clear error signature from Anthropic API validation layer.

Quick fixes

1. Confirm the exact error signature matches API Error: 400 Invalid 'user_id': string does not match pattern. Expected a string that matches the pattern '^[a-zA-Z0-9_-]+$'.
2. Check the Claude Code account, local tool state, and provider configuration involved in the failing workflow.
3. 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

1. Capture the exact error message and the command, editor action, or request that triggered it.
2. Check whether the failure is account/auth, quota/rate, model/provider, local runtime, or deployment configuration.
3. Review the source evidence below and compare it with your environment.
4. Apply one change at a time and rerun the smallest failing action.
5. 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.

Sources checked

• https://github.com/anthropics/claude-code/issues/56643

Evidence note: Claude Code Agent returns 400 API error when user_id doesn’t match pattern ^[a-zA-Z0-9_-]+$. 16 comments indicate multiple affected users. Clear error signature from Anthropic API validation layer.

Related errors

• Claude Code agent permission denied
• Claude Code API rate limit 429
• Claude Code invalid API key error

FAQ

What should I check first?

Start with the exact API Error: 400 Invalid 'user_id': string does not match pattern. Expected a string that matches the pattern '^[a-zA-Z0-9_-]+$' text and the smallest action that reproduces it.

Can I ignore this error?

No. Treat it as a failed Claude Code workflow until the root cause is understood.

Is this guaranteed to have one fix?

No. The imported evidence supports the troubleshooting path above, but tool behavior can vary by account, plan, version, provider, and local configuration.

How do I know the fix worked?

Rerun the same command, editor action, or request. The fix is working when that action completes without API Error: 400 Invalid 'user_id': string does not match pattern. Expected a string that matches the pattern '^[a-zA-Z0-9_-]+$'.