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

# Validation

> Zod schemas to validate inputs and outputs of your functions: types, coercion in GET, error format, and output behavior.

## Input validation

When you define an `input` schema, each request is validated before executing the handler. If validation fails, the handler **does not execute** and a `400` is returned:

```json theme={null}
{
  "error": "Validation failed",
  "details": [
    {
      "path": ["email"],
      "message": "Invalid email",
      "code": "invalid_string"
    }
  ]
}
```

### Example with full schema

```typescript index.ts theme={null}
import { define, z } from "@jelou/functions";

export default define({
  name: "create-ticket",
  description: "Creates a support ticket from WhatsApp",
  input: z.object({
    name: z.string().min(1).describe("Customer name"),
    email: z.string().email().describe("Contact email"),
    subject: z.string().max(200).describe("Ticket subject"),
    priority: z.enum(["low", "medium", "high"]).default("medium"),
  }),
  output: z.object({
    ticketId: z.string(),
    status: z.string(),
  }),
  handler: async (input, ctx) => {
    ctx.log("Creating ticket", { customer: input.name, priority: input.priority });
    return { ticketId: "TKT-2024-0042", status: "open" };
  },
});
```

### Supported types

You can use any Zod type inside `z.object()`:

```typescript theme={null}
input: z.object({
  name: z.string().min(1),
  age: z.number().int().positive(),
  email: z.string().email(),
  active: z.boolean().default(true),
  role: z.enum(["admin", "user", "guest"]),
  tags: z.array(z.string()).optional(),
  metadata: z.record(z.string(), z.unknown()).optional(),
})
```

### Coercion in GET requests

For GET requests, query parameters are strings. Use `z.coerce` to convert types automatically:

```typescript index.ts theme={null}
import { define, z } from "@jelou/functions";

export default define({
  name: "search-orders",
  description: "Search orders by status",
  input: z.object({
    q: z.string(),
    limit: z.coerce.number().default(10),
    page: z.coerce.number().default(1),
    active: z.coerce.boolean().default(true),
  }),
  handler: async (input, ctx) => {
    ctx.log("Searching", { q: input.q, limit: input.limit });
    return { results: [], total: 0 };
  },
});
```

```bash theme={null}
curl "https://search-orders.fn.jelou.ai/?q=pending&limit=5&page=2"
```

### `.describe()` annotations

Use `.describe()` on each field to document the parameters. These descriptions appear automatically in the MCP schema, which helps AI agents understand how to use your function:

```typescript theme={null}
input: z.object({
  phone: z.string().min(10).describe("Phone number with country code, e.g.: 593987654321"),
  includeHistory: z.boolean().default(false).describe("Whether to include the conversation history"),
})
```

## Output validation

When you define an `output` schema, the value returned by the handler is validated **after** execution. If it does not match:

* A warning is logged
* The response is sent normally with status `200`

<Note>
  Output validation never blocks the response. It is a development tool to detect inconsistencies.
</Note>

```typescript theme={null}
output: z.object({
  name: z.string(),
  balance: z.number(),
})
```

If the handler returns `{ name: "Maria", balance: "150" }` (balance as string), you will see a warning in the logs but the client receives the response unchanged.

## Validation error format

Each error in the `details` array contains:

| Field     | Type       | Description                                                                  |
| --------- | ---------- | ---------------------------------------------------------------------------- |
| `path`    | `string[]` | Path to the field with the error, e.g.: `["email"]` or `["address", "city"]` |
| `message` | `string`   | Human-readable error message                                                 |
| `code`    | `string`   | Zod error code (e.g.: `invalid_string`, `too_small`, `invalid_enum_value`)   |

```json theme={null}
{
  "error": "Validation failed",
  "details": [
    {
      "path": ["name"],
      "message": "String must contain at least 1 character(s)",
      "code": "too_small"
    },
    {
      "path": ["priority"],
      "message": "Invalid enum value. Expected 'low' | 'medium' | 'high', received 'urgent'",
      "code": "invalid_enum_value"
    }
  ]
}
```