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

# Parameters

> Define the typed, validated inputs that the AI passes to your tool at call time.

## What are parameters?

Parameters are the inputs your tool receives from the AI. When the AI decides to call your tool, it constructs a JSON payload matching your parameter schema. MCPCore validates this payload before your code runs.

Your code accesses parameters through the `params` object:

```javascript theme={null}
// If you defined a parameter named "userId":
const id = params.userId;
```

***

## Adding parameters

In the tool editor, click **Add parameter** for each input your code needs.

***

## Parameter fields

| Field             | Required | Description                                                                 |
| ----------------- | -------- | --------------------------------------------------------------------------- |
| **Name**          | Yes      | The key used to access the value in `params.*`. Use camelCase.              |
| **Type**          | Yes      | The expected data type (see types below).                                   |
| **Description**   | Yes      | What the parameter represents. The AI uses this to know what value to pass. |
| **Required**      | —        | If checked, the AI must always provide this parameter.                      |
| **Default value** | —        | Used when the parameter is not required and the AI doesn't provide it.      |

***

## Supported types

### `string`

A plain text value.

```javascript theme={null}
// Parameter: "repo", type: string
const repoName = params.repo;      // "anthropics/anthropic-sdk-python"
```

### `number`

An integer or floating-point number.

```javascript theme={null}
// Parameter: "limit", type: number, default: 10
const limit = params.limit;        // 25
```

### `boolean`

`true` or `false`.

```javascript theme={null}
// Parameter: "includeArchived", type: boolean, default: false
const includeArchived = params.includeArchived;   // false
```

### `array`

A JSON array. Use the **Items type** field to specify the type of each element.

```javascript theme={null}
// Parameter: "userIds", type: array of strings
const ids = params.userIds;    // ["user_1", "user_2", "user_3"]
```

### `object`

A JSON object with sub-properties. Define each sub-property with its own type and description.

```javascript theme={null}
// Parameter: "filter" of type object with sub-fields: status (string), since (string)
const { status, since } = params.filter;
```

***

## Best practices for parameter descriptions

The AI reads the parameter description to understand what value to pass. Good descriptions reduce tool call errors.

| Parameter | Weak description | Strong description                                                                            |
| --------- | ---------------- | --------------------------------------------------------------------------------------------- |
| `repo`    | "The repo"       | "GitHub repository in `owner/repo` format, e.g. `torvalds/linux`"                             |
| `since`   | "Start date"     | "ISO 8601 date string. Filters results to records created after this date, e.g. `2024-01-01`" |
| `limit`   | "How many"       | "Maximum number of results to return. Min 1, max 100. Defaults to 20."                        |

***

## Parameter validation

MCPCore validates every tool call against your parameter schema before running the code. If validation fails:

* Missing required parameters → `400` error returned to the AI
* Wrong type → `400` error with a clear message
* Your code is never executed

This means you do not need to validate required parameters in your code — they are guaranteed to be present and of the correct type when your code runs.

***

## Optional parameters

For optional parameters, always provide a sensible default value or handle the `undefined` case in code:

```javascript theme={null}
// Parameter "status" is optional, no default defined
const status = params.status ?? "active";   // default to "active" in code

const rows = await db.select("orders", {
  user_id: params.userId,
  status,
});
```

***

## Working with objects

Object parameters are ideal for grouping related inputs:

```javascript theme={null}
// Parameter "pagination" of type object:
//   - page (number, required)
//   - pageSize (number, default: 20)
const { page, pageSize = 20 } = params.pagination;

const offset = (page - 1) * pageSize;
const rows = await db.query(
  "SELECT * FROM products ORDER BY created_at DESC LIMIT $1 OFFSET $2",
  [pageSize, offset]
);

return { page, pageSize, total: rows.rowCount, results: rows.rows };
```

***

## Working with arrays

```javascript theme={null}
// Parameter "tags" of type array of strings
const tags = params.tags;   // ["javascript", "react", "nextjs"]

const results = await db.query(
  "SELECT * FROM posts WHERE tags && $1",
  [tags]
);

return { posts: results.rows };
```