My Agent Throws an Error After I Add or Replace a Workflow Tool

Last updated: April 14, 2026

This usually happens when the combined workflow tool name exceeds 64 characters, or when one of the workflow's inputs has no name. Shorten your workflow and subflow names so the combined tool name stays under 64 characters, and remove any unnamed "ghost" inputs from your workflow.

Symptoms

  • Your agent was working fine, but starts failing immediately after you add or swap a workflow tool.

  • You see a vague error referencing Anthropic or a 400 BadRequestError when the agent tries to respond.

  • The same workflow runs successfully on its own — it only fails when used as a tool inside an agent.

  • Copying the agent and retrying produces the same error.

Cause

When you add a workflow as a tool to an agent, Gumloop creates a tool name for the AI model using the format:

Workflow_Name_-_Subflow_Name

Spaces and special characters are replaced with underscores. The AI model (Anthropic Claude) requires every tool name to be 64 characters or fewer and match the pattern ^[a-zA-Z0-9_-]{1,64}$. If the combined name exceeds this limit, the model rejects the entire request and your agent fails before it can even start responding.

A second common cause is an unnamed input on the workflow. If you added an input node but left its name blank, this creates an empty property key in the tool schema — which the model also rejects.

How to Fix It

Fix 1: Shorten workflow and subflow names

The tool name sent to the model combines both the workflow name and the subflow name. For example, a workflow called "MVP Churn Assistant - V2 for Agent" with a subflow called "Get SF Data and Clarify Next Steps" becomes:

MVP_Churn_Assistant_-_V2_for_Agent_-_Get_SF_Data_and_Clarify_Next_Steps (71 characters — over the limit)

  1. Open your workflow in the pipeline editor.

  2. Count the total characters: workflow name + " - " + subflow name (spaces count as 1 character each since they become underscores).

  3. If the total exceeds 64 characters, rename the workflow, the subflow, or both to bring it under the limit.

  4. Go back to your agent, open a new chat session, and test again.

Tip: Keep names descriptive but concise. "PR Review Brief" works just as well as "Generate Partnership Review Brief (Test - Gamma)" and saves 40+ characters.

Fix 2: Remove unnamed "ghost" inputs

If shortening names didn't fully resolve the error, check your workflow for inputs that have no name.

  1. Open the workflow that your agent uses as a tool.

  2. Look at the input nodes at the top of the workflow. Each input should have a visible name.

  3. If you see an input with a blank or empty name (a "ghost" input), click the ✕ button to the right of that input's header to remove it.

  4. Save the workflow, then open a new chat session on your agent and test again.

How Do I Know It Worked?

Open a new chat session with your agent (don't retry in the old chat that had the error). Send a message that triggers the workflow tool. If the agent responds normally and the workflow executes, the issue is resolved.

Good to Know

  • The 64-character limit applies to each individual tool name — not the total across all tools. If you have multiple workflows, each one's combined name must be under 64 characters independently.

  • This limit comes from the AI model provider (Anthropic), not from Gumloop. It applies to all tool names, including MCP tools and workflow tools.

  • If your agent has many workflow tools, check all of them — a single oversized name will cause the entire agent to fail, not just that one tool.

Still Need Help?

If this didn't resolve your issue, reach out to support at support@gumloop.com.

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