OpenAPI: what schema to accept any (complex) JSON value

An arbitrary-type schema can be defined using an empty schema {}:

# swagger: '2.0'
definitions:
  AnyValue: {}

# openapi: 3.0.0
components:
  schemas:
    AnyValue: {}

or if you want a description:

# swagger: '2.0'
definitions:
  AnyValue:
    description: 'Can be anything: string, number, array, object, etc. (except `null`)'

# openapi: 3.0.0
components:
  schemas:
    AnyValue:
      description: 'Can be anything: string, number, array, object, etc., including `null`'

Without a defined type, a schema allows any values. Note that OpenAPI 2.0 Specification does not support null values, but some tools might support nulls nevertheless.

In OpenAPI 3.0, type-less schemas allow null values unless nulls are explicitly disallowed by other constraints (such as an enum).

See this Q&A for more details on how type-less schemas work.

Here's how Swagger Editor 2.0 handles a body parameter with the AnyValue schema:

I don't know how code generators handle this though.