Anthropic SDK: append_messages() in Tool Runner Loop Causes Infinite Loop

What this error means

append_messages() inside tool runner loop causes infinite loop is a Anthropic API failure pattern reported for developers trying to fix anthropic sdk append_messages infinite loop / tool runner loop stuck. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Reported 2026-05-13: append_messages() in tool runner loop causes infinite loop. Official docs example triggers the bug.

Common causes

• Calling append_messages() inside the tool runner loop (as shown in the official Advanced Usage docs example) causes an infinite loop. Developers following the official documentation hit this production-blocking bug.
• Reported 2026-05-13: append_messages() in tool runner loop causes infinite loop. Official docs example triggers the bug.

Quick fixes

1. Confirm the exact error signature matches append_messages() inside tool runner loop causes infinite loop.
2. Check the Anthropic API 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/anthropic-sdk-python/issues/1536

Evidence note: Reported 2026-05-13: append_messages() in tool runner loop causes infinite loop. Official docs example triggers the bug.

Related errors

• Anthropic SDK tool use loop hangs
• Anthropic messages API stuck processing

FAQ

What should I check first?

Start with the exact append_messages() inside tool runner loop causes infinite loop text and the smallest action that reproduces it.

Can I ignore this error?

No. Treat it as a failed Anthropic API 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 append_messages() inside tool runner loop causes infinite loop.