Anthropic API 400 — Duplicate tool_result Blocks for Same tool_use

What this error means

HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id is a Anthropic API failure pattern reported for developers trying to fix duplicate tool_result blocks error in anthropic claude api agent loops. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

Goose agent project hit this error with Databricks provider and direct Anthropic — long sessions with many tool calls fail because multiple tool_result blocks share the same tool_use ID.

Common causes

• Long-running agent sessions with many tool calls accumulate duplicate tool_result blocks, causing the API to reject the entire message history. Developers struggle to deduplicate without losing conversation context.
• Goose agent project hit this error with Databricks provider and direct Anthropic — long sessions with many tool calls fail because multiple tool_result blocks share the same tool_use ID.

Quick fixes

1. Confirm the exact error signature matches HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id.
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/aaif-goose/goose/pull/9133

Evidence note: Goose agent project hit this error with Databricks provider and direct Anthropic — long sessions with many tool calls fail because multiple tool_result blocks share the same tool_use ID.

Related errors

• Anthropic API tool_result deduplication
• Claude API conversation history management for agents
• tool_use tool_result pairing best practices

FAQ

What should I check first?

Start with the exact HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id 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 HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id.