JSON Schema#

msgspec provides a few utilities for generating JSON Schema specifications from msgspec-compatible types and constraints.

  • msgspec.json.schema: generates a complete JSON Schema for a single type.

  • msgspec.json.schema_components: generates JSON schemas for multiple types, along with a corresponding components mapping. This is mainly useful when generating multiple schemas to include in a larger specification like OpenAPI.

The generated schemas are compatible with JSON Schema 2020-12 and OpenAPI 3.1.


import msgspec
from msgspec import Struct, Meta
from typing import Annotated, Optional

# A float constrained to values > 0
PositiveFloat = Annotated[float, Meta(gt=0)]

class Dimensions(Struct):
    """Dimensions for a product, all measurements in centimeters"""
    length: PositiveFloat
    width: PositiveFloat
    height: PositiveFloat

class Product(Struct):
    """A product in a catalog"""
    id: int
    name: str
    price: PositiveFloat
    tags: set[str] = set()
    dimensions: Optional[Dimensions] = None

# Generate a schema for a list of products
schema = msgspec.json.schema(list[Product])

# Print out that schema as JSON
  "type": "array",
  "items": {"$ref": "#/$defs/Product"},
  "$defs": {
    "Dimensions": {
      "title": "Dimensions",
      "description": "Dimensions for a product, all measurements in centimeters",
      "type": "object",
      "properties": {
        "length": {"type": "number", "exclusiveMinimum": 0},
        "width": {"type": "number", "exclusiveMinimum": 0},
        "height": {"type": "number", "exclusiveMinimum": 0}
      "required": ["length", "width", "height"]
    "Product": {
      "title": "Product",
      "description": "A product in a catalog",
      "type": "object",
      "properties": {
        "id": {"type": "integer"},
        "name": {"type": "string"},
        "price": {"type": "number", "exclusiveMinimum": 0},
        "tags": {
          "type": "array",
          "items": {"type": "string"},
          "default": [],
        "dimensions": {
          "anyOf": [{"type": "null"}, {"$ref": "#/$defs/Dimensions"}],
          "default": null,
      "required": ["id", "name", "price"]