Struct2JSONSchema.jl is a Julia package that converts Julia structs into JSON Schema documents compliant with JSON Schema draft 2020-12.

• Non-invasive: Generate JSON Schema without modifying existing struct definitions.
• Extensibility: Allow customization of schema generation for user-defined types and constraints.
• Robustness: Never raises an error; unrepresentable types are conservatively handled as Any.
• Long-term maintainability: Simple implementation with minimal dependencies.

For the design principles behind this package, see https://abap34.github.io/Struct2JSONSchema.jl/dev/#Concepts

using Struct2JSONSchema
using JSON

struct User
    id::Int
    name::String
    email::Union{String, Nothing}
end

doc, unknowns = generate_schema(User)
println(JSON.json(doc, 4))
# {
#    "$schema": "https://json-schema.org/draft/2020-12/schema",
#    "$ref": "#/$defs/User__2fe39a6325a38198",
#  ...

We provide both a hook mechanism for expressive schema generation and a set of utility functions designed to make common customizations straightforward.

This allows users to define desirable constraints and formats for any type definition.

using UUIDs
using JSON

struct User
    id::UUID
    name::String
end

ctx = SchemaContext()

# Hooking into the generation process
register_override!(ctx) do ctx
    if ctx.current_parent == User && ctx.current_field == :id
        return Dict("type" => "string", "format" => "uuid")
    else
        return nothing
    end
end

The same behavior can be expressed more concisely using a utility function:

struct User
    id::Int
    email::String
end

ctx = SchemaContext()

# Using a convenience utility function
register_field_override!(ctx, User, :email) do ctx
    Dict("type" => "string", "format" => "email")
end

Additional utilities are provided for tasks such as:

• Marking specific fields as optional.
• Treating Union{T, Nothing} and Union{T, Missing} as optional fields.
• Registering custom expansion strategies for abstract types.

For detailed customization options, see https://abap34.github.io/Struct2JSONSchema.jl/dev/#Customization

See the examples directory for additional usage examples.