> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zylon.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Custom tools

> Define your own tools using JSON Schema.

Custom tools let you define a tool interface (name + JSON schema) and let the assistant choose when to call it. When it calls your tool, it returns a `tool_use` content block with structured `input`.

## Prerequisites

* An API token. See [Backoffice Developer Console](/en/developer-manual/get-started/developer-console).

## Requirements

In the `tools` array, provide:

* `name`: unique tool name
* `description`: what the tool does
* `inputSchema`: JSON Schema describing the tool input

## Basic request and response

This example validates a request that defines a tiny calculator tool and forces its use.

```bash theme={null}
curl -X POST "https://{BASE_URL}/api/gpt/v1/messages/validate" \
  -H "Authorization: Bearer {API_TOKEN}" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "default",
    "max_tokens": 64,
    "messages": [
      { "role": "user", "content": "Add 5 and 7." }
    ],
    "tools": [
      {
        "name": "simple_calculator",
        "description": "Add two numbers.",
        "inputSchema": {
          "type": "object",
          "additionalProperties": false,
          "properties": {
            "a": { "type": "number" },
            "b": { "type": "number" }
          },
          "required": ["a", "b"]
        }
      }
    ],
    "tool_choice": { "type": "tool", "name": "simple_calculator" }
  }'
```

Example response:

```json theme={null}
{ "valid": true, "errors": null }
```

To run the request, send the same body to `POST /messages`. When the assistant returns a `tool_use` block, execute your tool and include a `tool_result` block in the next message.