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Overview

Validation keywords in a schema impose requirements for successful validation of an instance. These keywords are all assertions without any annotation behavior.

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Any Instance Type

These keywords apply to instances of any type.

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type

The type keyword validates that an instance matches one of the specified JSON types. Value: String or array of strings Valid type values:
  • "null" - JSON null
  • "boolean" - JSON boolean
  • "object" - JSON object
  • "array" - JSON array
  • "number" - JSON number (including integers and floats)
  • "string" - JSON string
  • "integer" - Any number with a zero fractional part
{
  "type": "string"
}
If type is an array, the instance validates if its type matches any of the types in the array.

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enum

The enum keyword validates that an instance value equals one of the specified values. Value: Array (should have at least one element; elements should be unique) 
{
  "enum": ["red", "green", "blue", null, 42]
}
Elements in the array can be of any type, including null. The instance must be exactly equal to one of the enum values.

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const

The const keyword validates that an instance value equals a specific constant value. Value: Any JSON value, including null 
{
  "const": "active"
}
This is functionally equivalent to an enum with a single value.

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Numeric Instances

These keywords apply to instances of type number or integer.

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multipleOf

Validates that a number is a multiple of the specified value. Value: Number (must be strictly greater than 0) 
{
  "type": "number",
  "multipleOf": 0.5
}
A numeric instance is valid only if division by this keyword’s value results in an integer.
  • Value 10 with multipleOf: 5 → Valid (10 / 5 = 2)
  • Value 7 with multipleOf: 3 → Invalid (7 / 3 = 2.333…)
  • Value 1.5 with multipleOf: 0.5 → Valid (1.5 / 0.5 = 3)

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maximum

Validates that a number is less than or equal to the specified maximum. Value: Number 
{
  "type": "number",
  "maximum": 100
}
The instance validates if it is less than or exactly equal to maximum.

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exclusiveMaximum

Validates that a number is strictly less than the specified maximum. Value: Number 
{
  "type": "number",
  "exclusiveMaximum": 100
}
The instance validates if it is strictly less than (not equal to) exclusiveMaximum.

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minimum

Validates that a number is greater than or equal to the specified minimum. Value: Number 
{
  "type": "number",
  "minimum": 0
}
The instance validates if it is greater than or exactly equal to minimum.

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exclusiveMinimum

Validates that a number is strictly greater than the specified minimum. Value: Number 
{
  "type": "number",
  "exclusiveMinimum": 0
}
The instance validates if it is strictly greater than (not equal to) exclusiveMinimum. 
{
  "type": "number",
  "minimum": 0,
  "maximum": 100
}

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String Instances

These keywords apply to instances of type string.

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maxLength

Validates that a string’s length is less than or equal to the specified value. Value: Non-negative integer 
{
  "type": "string",
  "maxLength": 50
}
The length of a string is defined as the number of Unicode code points, not bytes or grapheme clusters.

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minLength

Validates that a string’s length is greater than or equal to the specified value. Value: Non-negative integer 
{
  "type": "string",
  "minLength": 1
}
Omitting this keyword has the same behavior as a value of 0.

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pattern

Validates that a string matches a regular expression. Value: String (should be a valid regular expression) 
{
  "type": "string",
  "pattern": "^[A-Z][a-z]+$"
}
Regular expressions are not implicitly anchored. Use ^ and $ to anchor the pattern to the start and end of the string.
{
  "type": "string",
  "pattern": "^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\\.[a-zA-Z]{2,}$"
}

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Array Instances

These keywords apply to instances of type array.

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maxItems

Validates that an array’s size is less than or equal to the specified value. Value: Non-negative integer 
{
  "type": "array",
  "maxItems": 10
}

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minItems

Validates that an array’s size is greater than or equal to the specified value. Value: Non-negative integer 
{
  "type": "array",
  "minItems": 1
}
Omitting this keyword has the same behavior as a value of 0.

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uniqueItems

Validates that all elements in an array are unique. Value: Boolean 
{
  "type": "array",
  "uniqueItems": true
}
  • If true: All array elements must be unique
  • If false (or omitted): Duplicate elements are allowed
[1, 2, 3, 4, 5]

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Object Instances

These keywords apply to instances of type object.

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maxProperties

Validates that an object’s number of properties is less than or equal to the specified value. Value: Non-negative integer 
{
  "type": "object",
  "maxProperties": 10
}

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minProperties

Validates that an object’s number of properties is greater than or equal to the specified value. Value: Non-negative integer 
{
  "type": "object",
  "minProperties": 1
}
Omitting this keyword has the same behavior as a value of 0.

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required

Validates that an object contains all specified properties. Value: Array of strings (elements must be unique) 
{
  "type": "object",
  "required": ["name", "email"]
}
An object validates if every item in the required array is the name of a property in the instance.

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dependentRequired

Specifies properties that are required if another specific property is present. Value: Object where values are arrays of strings 
{
  "type": "object",
  "properties": {
    "creditCard": { "type": "string" },
    "billingAddress": { "type": "string" }
  },
  "dependentRequired": {
    "creditCard": ["billingAddress"]
  }
}
In this example, if creditCard is present, then billingAddress must also be present.
Conditional Requirements
{
  "type": "object",
  "properties": {
    "hasLicense": { "type": "boolean" },
    "licenseNumber": { "type": "string" },
    "licenseExpiry": { "type": "string" }
  },
  "dependentRequired": {
    "licenseNumber": ["hasLicense"],
    "licenseExpiry": ["hasLicense", "licenseNumber"]
  }
}

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Complete Example

Here’s a comprehensive example combining multiple validation keywords: 
{
  "type": "object",
  "required": ["username", "email", "age"],
  "properties": {
    "username": {
      "type": "string",
      "minLength": 3,
      "maxLength": 20,
      "pattern": "^[a-zA-Z0-9_]+$"
    },
    "email": {
      "type": "string",
      "format": "email"
    },
    "age": {
      "type": "integer",
      "minimum": 0,
      "maximum": 150
    },
    "tags": {
      "type": "array",
      "minItems": 1,
      "maxItems": 5,
      "uniqueItems": true,
      "items": {
        "type": "string"
      }
    },
    "status": {
      "enum": ["active", "inactive", "pending"]
    }
  },
  "maxProperties": 10
}

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