maxLength : Integer

maxLength

Integer

A string instance is valid against this keyword if its length is less than, or equal to, the value of this keyword.

Value	This keyword must be set to a zero or positive integer
Kind	Assertion
Applies To	String
Base Dialect	2020-12
Changed In	None
Introduced In	Draft 1
Vocabulary	Validation
Specification	https://json-schema.org/draft/2020-12/json-schema-validation.html#section-6.3.1
Metaschema	https://json-schema.org/draft/2020-12/meta/validation
Official Tests	draft2020-12/maxLength.json
Default	None
Annotation	None
Affected By	None
Affects	None
Also See	minLength (Vocabulary: Validation) pattern (Vocabulary: Validation) format (Vocabulary: Format-Annotation)

The maxLength keyword restricts string instances to consists of an inclusive maximum number of Unicode code-points (logical characters), which is not necessarily the same as the number of bytes in the string.

Digging Deeper

While the IETF RFC 8259 JSON standard recommends the use of UTF-8, other Unicode encodings are permitted. Therefore a JSON string may be represented in up to 4x the number of bytes as its number of code-points (assuming UTF-32 as the upper bound).

JSON Schema does not provide a mechanism to assert on the byte size of a JSON string, as this is an implementation-dependent property of the JSON parser in use.

Common Pitfall

Be careful when making use of this keyword to inadvertently assert on the byte length of JSON strings before inserting them into byte-sensitive destinations like fixed-size buffers. Always assume that the byte length of a JSON string can arbitrary larger that the number of logical characters.

Best Practice

To restrict string instances to the empty string, prefer using the const keyword instead of setting this keyword to 0.

Remember that JSON Schema is a constraint-driven language. Therefore, non-string instances successfully validate against this keyword. If needed, make use of the type keyword to constraint the accepted type accordingly.

Examples

A schema that constrains string instances to contain at most 3 code points Schema

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "maxLength": 3
}

Valid A string value that consists of 3 code-points is valid Instance

"foo"

Invalid A string value that consists of more than 3 code-points is invalid Instance

"こんにちは"

Valid A string value that consists of less than 3 code-points is valid Instance

"hi"

Valid A non-string value is valid Instance