Enum Values in JSON Schema — Restricting to Allowed Values

Restricting Values with enum

The enum keyword defines an explicit list of values that a field may hold. If the input does not exactly match one of the listed values, validation fails.

Basic String Enum

{
  "status": {
    "type": "string",
    "enum": ["active", "inactive", "pending"]
  }
}

Only the three listed strings are accepted. "Active" (capitalized) or "unknown" would be rejected.

Numeric Enum

{
  "priority": {
    "type": "integer",
    "enum": [1, 2, 3, 4, 5]
  }
}

This restricts the priority to a 1-5 scale, combining type checking with value restriction.

Mixed-Type Enum

{
  "value": {
    "enum": ["auto", 0, null]
  }
}

When using mixed types in an enum, you can omit the type keyword entirely. The enum alone is sufficient to validate that the value matches one of the listed options.

enum vs. const

If only a single value is allowed, use const instead of a one-element enum:

{ "version": { "const": "2.0" } }

This is semantically clearer and behaves identically to "enum": ["2.0"].

Generator Behavior

Generators typically do not produce enum automatically from a single sample — they have no way to know the full set of valid values from one example. To generate enums:

1. Provide multiple sample objects with different values for the same field.
2. Or add the enum constraint manually after generation.

Best Practices

• Keep enum lists exhaustive: every valid value should be listed. If new values are added later, the schema must be updated.
• Pair enum with description to document what each value means.
• For APIs, consider whether the enum should be open or closed — a closed enum rejects unknown values, which can break forward compatibility.