Anthropic API / Anthropic API

Claude Code beta tool_runner infinite loop when append_messages called in loop

Fix infinite loop in Anthropic SDK tool runner when calling append_messages() inside the loop iteration — tool results never reach history causing model to re-call same tool Includes evidence for Anthropic API troubleshooting demand.

Source-backed Last updated May 23, 2026 1 source Needs local verification

Category: Anthropic API
Error signature: [Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example)
Quick fix: Compare the failing environment with a known working setup, then change one configuration value at a time.
Updated: May 23, 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

[Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example) is a Anthropic API failure pattern reported for developers trying to fix infinite loop in anthropic sdk tool runner when calling append_messages() inside the loop iteration — tool results never reach history causing model to re-call same tool. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

GitHub Issue #1536 in anthropics/anthropic-sdk-python, fixed by PR #1594 (opened May 23, 2026). Root cause: _messages_modified flag set by append_messages bypasses auto-append of assistant message + tool result. This matches the official documentation example. Not in covered-errors.md.

Common causes

GitHub Issue #1536 in anthropics/anthropic-sdk-python, fixed by PR #1594 (opened May 23, 2026). Root cause: _messages_modified flag set by append_messages bypasses auto-append of assistant message + tool result. This matches the official documentation example. Not in covered-errors.md.

Quick fixes

Confirm the exact error signature matches [Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example).
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 [Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example) 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 [Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example) mean in Anthropic API?

[Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example) indicates the workflow described by "Claude Code beta tool_runner infinite loop when append_messages called in loop" failed in a Anthropic API context. Start by confirming the exact signature and the component that emitted it.

What should I check first for [Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example)?

Check whether this cause matches your environment: GitHub Issue #1536 in anthropics/anthropic-sdk-python, fixed by PR #1594 (opened May 23, 2026). Root cause: _messages_modified flag set by append_messages bypasses auto-append of assistant message + tool result. This matches the official documentation example. Not in covered-errors.md.

Is the [Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example) guidance source-backed?

The "Claude Code beta tool_runner infinite loop when append_messages called in loop" 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 [Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example)?

For "Claude Code beta tool_runner infinite loop when append_messages called in loop", 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 [Bug] append_messages() inside tool runner loop causes infinite loop (advanced usage docs example)?

For "Claude Code beta tool_runner infinite loop when append_messages called in loop", 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.