Anthropic API / Anthropic API
Anthropic Python SDK: append_messages() inside tool runner loop causes infinite loop or dropped assistant turn
Fix Anthropic SDK tool_runner loop hanging infinitely or dropping responses when append_messages() is called during tool execution Includes evidence for Anthropic API troubleshooting demand.
- Category
- Anthropic API
- Error signature
append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes- Quick fix
- Compare the failing environment with a known working setup, then change one configuration value at a time.
- Updated
What this error means
append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes is a Anthropic API failure pattern reported for developers trying to fix anthropic sdk tool_runner loop hanging infinitely or dropping responses when append_messages() is called during tool execution. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.
Why this happens
Reported on anthropics/anthropic-sdk-python#1536, fixed in PR#1538 (2026-05-13). Calling append_messages() inside a tool runner loop had two failure modes: plain context messages caused infinite loop, assistant message replacements caused dropped turns. Critical for agentic workflows using tool_runner. Category: Anthropic API (SDK runtime bug).
Common causes
- Reported on anthropics/anthropic-sdk-python#1536, fixed in PR#1538 (2026-05-13). Calling append_messages() inside a tool runner loop had two failure modes: plain context messages caused infinite loop, assistant message replacements caused dropped turns. Critical for agentic workflows using tool_runner. Category: Anthropic API (SDK runtime bug).
Quick fixes
- Confirm the exact error signature matches
append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes. - 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.
Sources checked
Evidence note: Reported on anthropics/anthropic-sdk-python#1536, fixed in PR#1538 (2026-05-13). Calling append_messages() inside a tool runner loop had two failure modes: plain context messages caused infinite loop, assistant message replacements caused dropped turns. Critical for agentic workflows using tool_runner. Category: Anthropic API (SDK runtime bug).
Related errors
- Anthropic API
FAQ
What should I check first?
Start with the exact append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes 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 (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes.