Special Keywords

---

enum

Restricts values to a fixed set. Elements can be of any JSON type, including null.

{"enum": ["alpha", "beta", "gamma", 1, null]}

"gamma"

Strategy: picks one element from the enum array uniformly at random. The type keyword (if present) is ignored when enum is set.

---

nullable (OpenAPI)

An OpenAPI 3.0 extension. When true, the generator returns null approximately 50 % of the time; otherwise it generates a normal value from the type.

{"type": "string", "format": "email", "nullable": true}

"richard56@example.com"

# or on another call:
None

Strategy: a coin-flip at the start of generation; heads returns None immediately, tails proceeds with normal schema generation.

Limitations: nullable is not part of JSON Schema draft-07+; it is specific to OpenAPI 3.0. For JSON Schema 2019-09 and later, use {"type": ["string", "null"]} instead.

---

const

Constrains the value to a single fixed value.

{"const": "fixed-value"}

"fixed-value"

Strategy: always returns the const value unchanged. Any type keyword alongside const is ignored.

---

Type as array

type may be an array of type names. The generator picks one at random and generates a value of that type.

{"type": ["string", "integer", "null"]}

"From always activity test nearly out begin newspaper."

# or on another call:
42
# or on another call:
None

Strategy: one type is chosen uniformly at random from the array; generation then proceeds as if {"type": "<chosen>"} was specified.

---

if / then / else

Conditional schema application. The generator randomly decides whether to satisfy the if schema:

• satisfying if: generates from the merged result of the base schema and then (if present).
• not satisfying if: generates from the merged result of the base schema and else (if present).

{
  "type": "integer",
  "if":   {"minimum": 10},
  "then": {"multipleOf": 2},
  "else": {"maximum": 9}
}

-20  # satisfied "else": value < 10
# or on another call:
16  # satisfied "if" + "then": value >= 10 and even

Strategy: the if/then/else branch is chosen randomly (50/50). The chosen sub-schema is merged with the root schema and generation proceeds from the merged result.

Limitations: the generator does not guarantee which branch it satisfies on any given call.

---

$ref

References another schema defined in $defs (draft 2019-09+) or definitions (draft-07 and earlier) within the same root schema.

{
  "$defs": {
    "Point": {
      "type": "object",
      "properties": {"x": {"type": "number"}, "y": {"type": "number"}},
      "required": ["x", "y"]
    }
  },
  "$ref": "#/$defs/Point"
}

{"x": -467939042953601.44, "y": -744861589451087.2}

Strategy: resolves the JSON Pointer in $ref against the root schema stored in the generation context, then generates from the referenced schema.

Limitations:

• Only local (#/...) references are supported. External URL references are not resolved.
• Circular $ref chains are detected; when the recursion depth exceeds 3 * max_depth, a UnsatisfiableConstraintsError is raised.

---

Boolean schemas

A JSON Schema can be the literal true or false instead of an object.

Schema	Behaviour
true	Equivalent to {} — generates any random JSON value.
false	Always raises UnsatisfiableConstraintsError; no value is valid.

true

7698  # any JSON value
{"x": 1.5}  # any JSON value
[1, 2, 3]  # any JSON value