Anthropic API 400 — tool_use Without Matching tool_result

What this error means

messages.2: tool_useids were found withouttool_result blocks immediately after is a Anthropic API failure pattern reported for developers trying to fix anthropic api 400 error when tool_use blocks lack corresponding tool_result blocks. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Real production issue where second master chat turn returned 400 from Anthropic API because tool_use blocks were not immediately followed by tool_result blocks in the conversation history.

Common causes

• When building multi-turn agent conversations with tool use, developers frequently mismatch tool_use and tool_result block ordering. The API returns a cryptic 400 error pointing to message indices but not explaining the exact fix.
• Real production issue where second master chat turn returned 400 from Anthropic API because tool_use blocks were not immediately followed by tool_result blocks in the conversation history.

Quick fixes

1. Confirm the exact error signature matches messages.2: tool_useids were found withouttool_result blocks immediately after.
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/drewblik/priorities/pull/11

Evidence note: Real production issue where second master chat turn returned 400 from Anthropic API because tool_use blocks were not immediately followed by tool_result blocks in the conversation history.

Related errors

• Anthropic API tool use conversation history format
• Claude API tool_call without tool_result error
• Anthropic messages API tool sequence validation

FAQ

What should I check first?

Start with the exact messages.2: tool_useids were found withouttool_result blocks immediately after 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 messages.2: tool_useids were found withouttool_result blocks immediately after.