Anthropic API / Anthropic API

Anthropic API 400 — Duplicate tool_result Blocks for Same tool_use

Fix duplicate tool_result blocks error in Anthropic Claude API agent loops Includes evidence for Anthropic API troubleshooting demand.

Source-backed Last updated May 13, 2026 1 source Needs local verification

Category: Anthropic API
Error signature: HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id
Quick fix: Compare the failing environment with a known working setup, then change one configuration value at a time.
Updated: May 13, 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

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

Confirm the exact error signature matches HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id.
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 HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id 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 HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id mean in Anthropic API?

HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id indicates the workflow described by "Anthropic API 400 — Duplicate tool_result Blocks for Same tool_use" failed in a Anthropic API context. Start by confirming the exact signature and the component that emitted it.

What should I check first for HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id?

Check whether this cause matches your environment: 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.

Is the HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id guidance source-backed?

The "Anthropic API 400 — Duplicate tool_result Blocks for Same tool_use" 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 HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id?

For "Anthropic API 400 — Duplicate tool_result Blocks for Same tool_use", 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 HTTP 400: each tool_use must have a single result. Found multiple tool_result blocks with id?

For "Anthropic API 400 — Duplicate tool_result Blocks for Same tool_use", 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.