Why Do I Get 'Node Cannot Be Used as a Tool' Error When Using the Call Workflow Agent?

Based on the details you provided and the documentation, the error “Node cannot be used as a tool” typically means that the call workflow (Custom n8n Workflow Tool) node isn’t being recognized as a valid tool by the AI Agent. This can happen even if everything seems to work during testing because the test run might not enforce all validations that are active when the workflow is live.

Here are some things to double-check:

1. Tool Configuration:
   Make sure that in your call workflow node you’ve filled out all required parameters properly. In particular, the Name field must not contain any whitespace or special characters. A misconfigured name might prevent the node from being registered as a tool.

2. Input Schema:
   Since the call workflow node is used as a tool, you should enable the option to “Specify input schema” and correctly define the schema. The Tools Agent expects the node to adhere to the integration interface (LangChain tool calling) and this schema helps ensure the data passed from the agent is in the correct format.

3. Workflow Version:
   Verify that you are using the latest version of the call workflow node. In some cases, if older versions of a node or certain sub-nodes (for example, Window Buffer Memory) are being used, it may result in compatibility issues with the tool interface. Removing and re-adding the call workflow node can sometimes resolve the issue.

4. Consistency Between Test and Production:
   The testing mode may bypass or relax some validations that are strictly enforced when the workflow is active. Confirm that the live version of your workflow has all parameters set as required for a valid tool.

Check these points in your setup and make the necessary adjustments. Once the node is correctly configured—with a valid name, input schema, and up-to-date version—it should be recognized as a tool by your agent when running in active mode.

If after verifying these the issue persists, consider reviewing your workflow logs or enabling extended debug output to get more details on how the agent is validating the tool node.

Based on the documentation, the Call n8n Workflow tool is supported by the Tools Agent—but only when the target workflow is configured as a tool rather than as another AI agent workflow. In your case, the error “Node cannot be used as a tool” typically appears when the workflow you’re calling (the one managing Google Tasks) relies on an AI Agent node rather than being a standard workflow with supported nodes.

During testing, n8n can bypass certain checks (and your test run works fine), but once the workflow is active the Tools Agent enforces that only eligible tools are called. In other words, if you’re chaining an AI Agent (via the Call Workflow node) to invoke another workflow that itself uses an AI Agent node, it won’t be recognized as a valid external tool.

To resolve the issue, consider the following steps:

• Ensure that the workflow called by your Call Workflow tool is a “normal” workflow which contains only supported nodes (for example, nodes dedicated to handling Google Tasks via their native integrations) rather than another AI Agent node.

• Review the configuration of the target workflow so that its purpose is solely to perform a specific task and return a result in the expected format.

This separation is important because chaining several AI Agent workflows (i.e. having an agent call another agent) is not currently supported. Once you adjust the target workflow to ensure it’s a valid tool (without an embedded AI Agent), the error should no longer occur in production.