---
title: CallTool
description: JSON Schema for the CallTool action.
---

Call an MCP server tool via `app.callServerTool()`.

```json
{
  "type": "object",
  "properties": {
    "action": {
      "const": "toolCall",
      "default": "toolCall",
      "type": "string"
    },
    "tool": {
      "description": "Name of the server tool to call",
      "type": "string"
    },
    "arguments": {
      "additionalProperties": true,
      "description": "Arguments to pass. Supports {{ key }} interpolation.",
      "type": "object"
    },
    "resultKey": {
      "type": [
        "string",
        "null"
      ],
      "default": null,
      "description": "State key to store the tool result under"
    }
  },
  "required": [
    "action",
    "tool"
  ]
}
```
