Anthropic MCP Connector: model intermittently emits tool_use instead of mcp_tool_use

What this error means

MCP connector: model intermittently emits tool_use instead of mcp_tool_use for tools declared in mcp_toolset with configs.<tool>.enabled: true is a Anthropic API failure pattern reported for developers trying to fix anthropic mcp connector beta returning wrong content block type (tool_use vs mcp_tool_use) causing tool invocation failures in llm agent tool declarations. Based on the imported evidence, treat this as a tool-specific troubleshooting page rather than a generic API error.

Why this happens

GitHub Issue #1581 in anthropics/anthropic-sdk-python (open, May 21 2026). Under mcp-client-2025-11-20 MCP connector beta, intermittent behavioral regression in content block type emission. Impacts LLM-driven agent workflows using MCP tool protocols. Category: Anthropic API per exact mapping — this is a direct Anthropic SDK/MCP protocol issue.

Common causes

• GitHub Issue #1581 in anthropics/anthropic-sdk-python (open, May 21 2026). Under mcp-client-2025-11-20 MCP connector beta, intermittent behavioral regression in content block type emission. Impacts LLM-driven agent workflows using MCP tool protocols. Category: Anthropic API per exact mapping — this is a direct Anthropic SDK/MCP protocol issue.

Quick fixes

1. Confirm the exact error signature matches MCP connector: model intermittently emits tool_use instead of mcp_tool_use for tools declared in mcp_toolset with configs.<tool>.enabled: true.
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/anthropics/anthropic-sdk-python/issues/1581

Evidence note: GitHub Issue #1581 in anthropics/anthropic-sdk-python (open, May 21 2026). Under mcp-client-2025-11-20 MCP connector beta, intermittent behavioral regression in content block type emission. Impacts LLM-driven agent workflows using MCP tool protocols. Category: Anthropic API per exact mapping — this is a direct Anthropic SDK/MCP protocol issue.

Related errors

• Anthropic API

FAQ

What should I check first?

Start with the exact MCP connector: model intermittently emits tool_use instead of mcp_tool_use for tools declared in mcp_toolset with configs.<tool>.enabled: true 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 MCP connector: model intermittently emits tool_use instead of mcp_tool_use for tools declared in mcp_toolset with configs.<tool>.enabled: true.