# MCP Tool

Let an AI app call a workflow as a tool.

## Start a workflow

### When an AI app calls this tool

Publishes this workflow as an MCP tool. The AI app's arguments land on the trigger’s data, exactly like a form submission.

- **Tool name · Optional**: What the AI app calls it. Write it as an instruction: create_refund_for_customer, not workflow_3. Lowercase letters, digits and underscores; leave it blank to use the workflow's name. AI apps see it with a short suffix this workflow keeps for good (create_refund_for_customer_4f1ab9), so two workflows never share a name.
- **What it does · Required**: The AI app chooses this tool from this sentence alone. Say what it does, when to use it, and what it returns. A one-word description is a broken tool, and publishing refuses one.
- **Parameters · Optional**: What the AI app must supply. Each parameter's name becomes a key on {{trigger.body}}, the same shape a form submission produces.
- **What to return · Optional**: The name of the step whose output the AI app receives. Leave blank to return the last step's output.

For a step-by-step guide, see [Turn your workflows into tools](https://docs.gorunner.io/mcp/workflows-as-tools).

Not in your list? See [When an app isn’t here](https://docs.gorunner.io/apps/when-an-app-isnt-here#q-not-in-list).

## For developers

In the API, which calls apps pieces, MCP Tool is `pieceId: "mcptool"`. Its trigger is `mcptool` / `mcp_tool`. [Get an app](https://docs.gorunner.io/api/apps/get-app) returns every field.

Source: https://docs.gorunner.io/apps/mcp-tool