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.

Source-backed Last updated May 15, 2026 1 source Needs local verification

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: May 15, 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

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.

Diagnostic flow for this page

Match append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes 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 append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes mean in Anthropic API?

append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes indicates the workflow described by "Anthropic Python SDK: append_messages() inside tool runner loop causes infinite loop or dropped assistant turn" failed in a Anthropic API context. Start by confirming the exact signature and the component that emitted it.

What should I check first for append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes?

Check whether this cause matches your environment: 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).

Is the append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes guidance source-backed?

The "Anthropic Python SDK: append_messages() inside tool runner loop causes infinite loop or dropped assistant turn" 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 append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes?

For "Anthropic Python SDK: append_messages() inside tool runner loop causes infinite loop or dropped assistant turn", 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 append_messages() inside tool runner loop causes infinite loop (plain context message) or dropped assistant turn (assistant message replacement) — two failure modes?

For "Anthropic Python SDK: append_messages() inside tool runner loop causes infinite loop or dropped assistant turn", 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.