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How the schema is inferred

A JSON object becomes an object schema with additionalProperties set to false and required set to the keys the sample actually contained. A nested object is moved into $defs and referenced, so the document stays readable when the same shape would otherwise be inlined several times. Arrays use items. If every element is an object, the item schema is the merge of those objects: a key is required only when every element has it, and a key present on only some elements stays a property but drops out of required. An empty array has no items schema at all, because there is nothing to infer. Null becomes type null. A value that is null in one element and a string in another becomes a type array of string and null. Integers and fractional numbers collapse to number when both appear.

Draft 2020-12 on purpose

The $schema keyword points at the 2020-12 meta-schema. $defs is used instead of the older definitions keyword. type arrays express nullable primitives, which 2020-12 allows. The page does not emit OpenAPI's nullable: true, and it does not wrap the document in a components section. If you need an OpenAPI schema, copy the definitions across and adjust the dialect. format is never guessed. An email-shaped string is still type string. Guessing format: date-time from a single ISO string produces schemas that reject values the API accepts. Leave format to a human who has read the contract.

Worked example

The sample user is a root object titled Root. id is integer, name and the address fields are string, active is boolean, and tags is an array of strings. address is a $ref to $defs/Address, whose required list is city and zip, with additionalProperties false. Nothing is optional, because the sample is one complete object. Paste an array of two users and omit zip on the second if you want zip to disappear from required. The schema will still list zip under properties. That is the difference between 'the key may be absent' and 'the key cannot exist'. additionalProperties false means a later field such as country fails validation until you add it or set the flag to true.

Closed objects are a choice

additionalProperties false is useful when you want a fixture to catch typos, and it is hostile when an API adds fields without a version bump. The flag is set because a sample cannot name the keys it did not include, and a silent additionalProperties true would hide that. Change it if your clients should ignore unknown fields. The schema is not a substitute for an authored contract. One payload cannot reveal a numeric string, a discriminated union, or a field that is optional but happened to be filled in. Compare the result with the TypeScript inference if you want the same rules in a type system. Both pages run in the browser. The document is not uploaded.

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