Custom Tools

Custom Tools let you define your own callable functions that Agent blocks can invoke at runtime. Each tool is a pair: a JSON config that declares the tool's name, description, and parameters, plus a JavaScript function body that runs when the tool is called.

Tool schema

Every custom tool follows the OpenAI function-calling JSON format. The editor validates the schema on every change and requires all of the fields shown below.

{
  "type": "function",
  "function": {
    "name": "addItemToOrder",
    "description": "Add one quantity of a food item to the order.",
    "parameters": {
      "type": "object",
      "properties": {
        "itemName": {
          "type": "string",
          "description": "The name of the food item to add to order"
        }
      },
      "required": ["itemName"]
    }
  }
}

type must be "function".
function.name is the identifier the agent sees and calls. Must be unique within the workspace.
function.description tells the agent when to use the tool.
function.parameters is a standard JSON Schema object describing the inputs.

Tool code

The code tab contains the body of an async function(params, environmentVariables). Two referencing conventions are available inside the editor:

Parameters — use the bare parameter name (e.g. itemName). The editor provides autocomplete from the schema.
Environment variables — use double-brace syntax: {{API_KEY}}.

The function should return a value when the tool is expected to produce output. External imports are not supported; use fetch or built-in APIs.

Creating and editing tools

Open the Custom Tool Editor widget in your workspace dashboard.

Select an existing tool from the dropdown or create a new one from the Custom Tool List widget.

Switch between the Config and Code tabs to edit the JSON schema and JavaScript body respectively. Both tabs include an AI wand that can generate content from a natural-language prompt.

Click the Save button in the widget header. The editor validates the schema before saving and rejects duplicate tool names.

How agents invoke custom tools

When you add a custom tool to an Agent block's tool list, the tool is registered with type: "custom-tool". At runtime the agent block reads function.name as the tool identifier, function.description for the tool description, and function.parameters for the input schema. The agent decides whether and when to call the tool based on these fields and the conversation context, then executes the JavaScript function body with the resolved parameters.

Agent blocks cannot use core blocks (API, Webhook, Function, Workflow, Memory) as tools. Only integrations, MCP tools, and custom tools are allowed in the tool list.

Custom Tools vs MCP

Feature	Custom Tools	MCP
Definition	JSON schema and JS function body	External server implementing the Model Context Protocol
Scope	One function per tool	A single server can expose many tools
Code execution	Inline JavaScript in the editor	Runs on the remote MCP server
Best for	Quick, self-contained logic or API calls	Connecting to standardized third-party tool servers

For external tool integrations using MCP, see the MCP utility page.

Custom tools are workspace-scoped. All workspace members can view, edit, and delete them. Deleting a custom tool permanently removes its code and configuration.

Custom Tool Editor

Widget for editing the schema and code of a custom tool.

MCP

Connect to external tool servers using the Model Context Protocol.