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

# Length Validators

> Validators for checking length constraints on strings, lists, and collections

Length validators check the size of strings, lists, and other collections.

<Note>
  Length validators support [transforms](/evals/validators#transforms) when validating strings. Transforms are applied before measuring length.
</Note>

## length!

Checks that the value has exactly the specified length.

```yaml theme={"dark"}
generate_code:
  output:
    length!: 6
```

| Parameter | Type    | Description           |
| --------- | ------- | --------------------- |
| value     | integer | Exact required length |

<Accordion title="Examples">
  ```yaml theme={"dark"}
  # Exact code length
  generate_pin:
    output:
      length!: 4

  # Fixed-length ID
  create_id:
    output:
      length!: 36  # UUID length

  # Specific list size
  get_top_results:
    output:
      length!: 5
  ```
</Accordion>

## min\_length!

Checks that the value has at least the specified length.

```yaml theme={"dark"}
output:
  min_length!: 10
```

| Parameter | Type    | Description             |
| --------- | ------- | ----------------------- |
| value     | integer | Minimum required length |

<Accordion title="Examples">
  ```yaml theme={"dark"}
  # Ensure substantive response
  output:
    min_length!: 50

  # Require at least one result
  search_products:
    output:
      min_length!: 1

  # Validate input length
  validate_password:
    input:
      password:
        min_length!: 8
  ```
</Accordion>

## max\_length!

Checks that the value has at most the specified length.

```yaml theme={"dark"}
output:
  max_length!: 1000
```

| Parameter | Type    | Description            |
| --------- | ------- | ---------------------- |
| value     | integer | Maximum allowed length |

<Accordion title="Examples">
  ```yaml theme={"dark"}
  # Limit response length
  output:
    max_length!: 500

  # Limit results returned
  search_products:
    output:
      max_length!: 10

  # Username length limit
  validate_username:
    input:
      username:
        max_length!: 32
  ```
</Accordion>

## Combining Length Validators

### Length Range

```yaml theme={"dark"}
output:
  min_length!: 50
  max_length!: 500
```

### With Other Validators

```yaml theme={"dark"}
output:
  not_null!: true
  type!: "string"
  min_length!: 10
  max_length!: 1000
  contains!: "result"
```

## Common Patterns

### Response Quality Checks

```yaml theme={"dark"}
output:
  not_null!: true
  type!: "string"
  min_length!: 20   # Not too short
  max_length!: 2000 # Not too long
```

### Array Size Validation

```yaml theme={"dark"}
search_products:
  output:
    type!: "array"
    min_length!: 1
    max_length!: 50

recommend:
  output:
    length!: 3  # Exactly 3 recommendations
```

### Code Generation

```yaml theme={"dark"}
generate_pin:
  output:
    type!: "string"
    length!: 4
    pattern!: "\\d{4}"

generate_code:
  output:
    type!: "string"
    length!: 6
    pattern!: "[A-Z0-9]{6}"
```

## Using Transforms

Transforms are applied before measuring length:

```yaml theme={"dark"}
# Trim whitespace before checking length
output:
  min_length!:
    value: 10
    transform: trim

# Collapse whitespace then check length
output:
  max_length!:
    value: 100
    transform: [trim, collapse_whitespace]
```

## Notes on Length Calculation

* **Strings**: Length is the number of characters
* **Arrays**: Length is the number of items
* **Objects**: Length is the number of keys

```yaml theme={"dark"}
# String length (characters)
output:  # "Hello" has length 5
  length!: 5

# Array length (items)
items:  # [1, 2, 3] has length 3
  length!: 3

# Object length (keys)
config:  # {"a": 1, "b": 2} has length 2
  length!: 2
```