Json Schema

??? api "API Documentation" [pydantic.json_schema][pydantic.json_schema]

Pydantic allows automatic creation and customization of JSON schemas from models. The generated JSON schemas are compliant with the following specifications:

Generating JSON Schema

Use the following functions to generate JSON schema:

[BaseModel.model_json_schema][pydantic.main.BaseModel.model_json_schema] returns a jsonable dict of a model's schema.
[TypeAdapter.json_schema][pydantic.type_adapter.TypeAdapter.json_schema] returns a jsonable dict of an adapted type's schema.

!!! note These methods are not to be confused with [BaseModel.model_dump_json][pydantic.main.BaseModel.model_dump_json] and [TypeAdapter.dump_json][pydantic.type_adapter.TypeAdapter.dump_json], which serialize instances of the model or adapted type, respectively. These methods return JSON strings. In comparison, [BaseModel.model_json_schema][pydantic.main.BaseModel.model_json_schema] and [TypeAdapter.json_schema][pydantic.type_adapter.TypeAdapter.json_schema] return a jsonable dict representing the JSON schema of the model or adapted type, respectively.

!!! note "on the "jsonable" nature of JSON schema" Regarding the "jsonable" nature of the [model_json_schema][pydantic.main.BaseModel.model_json_schema] results, calling json.dumps(m.model_json_schema())on some BaseModel m returns a valid JSON string. Similarly, for [TypeAdapter.json_schema][pydantic.type_adapter.TypeAdapter.json_schema], calling json.dumps(TypeAdapter(<some_type>).json_schema()) returns a valid JSON string.

!!! tip Pydantic offers support for both of:

1. [Customizing JSON Schema](#customizing-json-schema)
2. [Customizing the JSON Schema Generation Process](#customizing-the-json-schema-generation-process)

The first approach generally has a more narrow scope, allowing for customization of the JSON schema for
more specific cases and types. The second approach generally has a more broad scope, allowing for customization
of the JSON schema generation process overall. The same effects can be achieved with either approach, but
depending on your use case, one approach might offer a more simple solution than the other.

Here's an example of generating JSON schema from a BaseModel:

python

import json
from enum import Enum
from typing import Annotated, Union

from pydantic import BaseModel, Field
from pydantic.config import ConfigDict


class FooBar(BaseModel):
    count: int
    size: Union[float, None] = None


class Gender(str, Enum):
    male = 'male'
    female = 'female'
    other = 'other'
    not_given = 'not_given'


class MainModel(BaseModel):
    """
    This is the description of the main model
    """

    model_config = ConfigDict(title='Main')

    foo_bar: FooBar
    gender: Annotated[Union[Gender, None], Field(alias='Gender')] = None
    snap: int = Field(
        default=42,
        title='The Snap',
        description='this is the value of snap',
        gt=30,
        lt=50,
    )


main_model_schema = MainModel.model_json_schema()  # (1)!
print(json.dumps(main_model_schema, indent=2))  # (2)!
"""
{
  "$defs": {
    "FooBar": {
      "properties": {
        "count": {
          "title": "Count",
          "type": "integer"
        },
        "size": {
          "anyOf": [
            {
              "type": "number"
            },
            {
              "type": "null"
            }
          ],
          "default": null,
          "title": "Size"
        }
      },
      "required": [
        "count"
      ],
      "title": "FooBar",
      "type": "object"
    },
    "Gender": {
      "enum": [
        "male",
        "female",
        "other",
        "not_given"
      ],
      "title": "Gender",
      "type": "string"
    }
  },
  "description": "This is the description of the main model",
  "properties": {
    "foo_bar": {
      "$ref": "#/$defs/FooBar"
    },
    "Gender": {
      "anyOf": [
        {
          "$ref": "#/$defs/Gender"
        },
        {
          "type": "null"
        }
      ],
      "default": null
    },
    "snap": {
      "default": 42,
      "description": "this is the value of snap",
      "exclusiveMaximum": 50,
      "exclusiveMinimum": 30,
      "title": "The Snap",
      "type": "integer"
    }
  },
  "required": [
    "foo_bar"
  ],
  "title": "Main",
  "type": "object"
}
"""

This produces a "jsonable" dict of MainModel's schema.
Calling json.dumps on the schema dict produces a JSON string.

The [TypeAdapter][pydantic.type_adapter.TypeAdapter] class lets you create an object with methods for validating, serializing, and producing JSON schemas for arbitrary types. This serves as a complete replacement for schema_of in Pydantic V1 (which is now deprecated).

Here's an example of generating JSON schema from a [TypeAdapter][pydantic.type_adapter.TypeAdapter]:

python

from pydantic import TypeAdapter

adapter = TypeAdapter(list[int])
print(adapter.json_schema())
#> {'items': {'type': 'integer'}, 'type': 'array'}

You can also generate JSON schemas for combinations of [BaseModels][pydantic.main.BaseModel] and [TypeAdapters][pydantic.type_adapter.TypeAdapter], as shown in this example:

python

import json
from typing import Union

from pydantic import BaseModel, TypeAdapter


class Cat(BaseModel):
    name: str
    color: str


class Dog(BaseModel):
    name: str
    breed: str


ta = TypeAdapter(Union[Cat, Dog])
ta_schema = ta.json_schema()
print(json.dumps(ta_schema, indent=2))
"""
{
  "$defs": {
    "Cat": {
      "properties": {
        "name": {
          "title": "Name",
          "type": "string"
        },
        "color": {
          "title": "Color",
          "type": "string"
        }
      },
      "required": [
        "name",
        "color"
      ],
      "title": "Cat",
      "type": "object"
    },
    "Dog": {
      "properties": {
        "name": {
          "title": "Name",
          "type": "string"
        },
        "breed": {
          "title": "Breed",
          "type": "string"
        }
      },
      "required": [
        "name",
        "breed"
      ],
      "title": "Dog",
      "type": "object"
    }
  },
  "anyOf": [
    {
      "$ref": "#/$defs/Cat"
    },
    {
      "$ref": "#/$defs/Dog"
    }
  ]
}
"""

Configuring the `JsonSchemaMode`

Specify the mode of JSON schema generation via the mode parameter in the [model_json_schema][pydantic.main.BaseModel.model_json_schema] and [TypeAdapter.json_schema][pydantic.type_adapter.TypeAdapter.json_schema] methods. By default, the mode is set to 'validation', which produces a JSON schema corresponding to the model's validation schema.

The [JsonSchemaMode][pydantic.json_schema.JsonSchemaMode] is a type alias that represents the available options for the mode parameter:

'validation'
'serialization'

Here's an example of how to specify the mode parameter, and how it affects the generated JSON schema:

python

from decimal import Decimal

from pydantic import BaseModel


class Model(BaseModel):
    a: Decimal = Decimal('12.34')


print(Model.model_json_schema(mode='validation'))
"""
{
    'properties': {
        'a': {
            'anyOf': [
                {'type': 'number'},
                {
                    'pattern': '^(?!^[-+.]*$)[+-]?0*\\d*\\.?\\d*$',
                    'type': 'string',
                },
            ],
            'default': '12.34',
            'title': 'A',
        }
    },
    'title': 'Model',
    'type': 'object',
}
"""

print(Model.model_json_schema(mode='serialization'))
"""
{
    'properties': {
        'a': {
            'default': '12.34',
            'pattern': '^(?!^[-+.]*$)[+-]?0*\\d*\\.?\\d*$',
            'title': 'A',
            'type': 'string',
        }
    },
    'title': 'Model',
    'type': 'object',
}
"""

Customizing JSON Schema

The generated JSON schema can be customized at both the field level and model level.

At both the field and model levels, you can use the json_schema_extra option to add extra information to the JSON schema.

For custom types, Pydantic offers other tools for customizing JSON schema generation:

WithJsonSchema annotation
SkipJsonSchema annotation
Implementing __get_pydantic_core_schema__
Implementing __get_pydantic_json_schema__

Field-Level Customization

Fields can have their JSON Schema customized. This is usually done using the [Field()][pydantic.fields.Field] function.

Some field parameters are used exclusively to customize the generated JSON Schema:

title: The title of the field.
description: The description of the field.
examples: The examples of the field.
json_schema_extra: Extra JSON Schema properties to be added to the field (see the dedicated documentation).
field_title_generator: A function that programmatically sets the field's title, based on its name and info.

Here's an example:

python

import json
from typing import Annotated

from pydantic import BaseModel, EmailStr, Field, SecretStr


class User(BaseModel):
    age: int = Field(description='Age of the user')
    email: Annotated[EmailStr, Field(examples=['[email protected]'])]  # (1)!
    name: str = Field(title='Username')
    password: SecretStr = Field(
        json_schema_extra={
            'title': 'Password',
            'description': 'Password of the user',
            'examples': ['123456'],
        }
    )


print(json.dumps(User.model_json_schema(), indent=2))
"""
{
  "properties": {
    "age": {
      "description": "Age of the user",
      "title": "Age",
      "type": "integer"
    },
    "email": {
      "examples": [
        "[email protected]"
      ],
      "format": "email",
      "title": "Email",
      "type": "string"
    },
    "name": {
      "title": "Username",
      "type": "string"
    },
    "password": {
      "description": "Password of the user",
      "examples": [
        "123456"
      ],
      "format": "password",
      "title": "Password",
      "type": "string",
      "writeOnly": true
    }
  },
  "required": [
    "age",
    "email",
    "name",
    "password"
  ],
  "title": "User",
  "type": "object"
}
"""

The annotated pattern can also be used.

Programmatic field title generation

The field_title_generator parameter can be used to programmatically generate the title for a field based on its name and info.

See the following example:

python

import json

from pydantic import BaseModel, Field
from pydantic.fields import FieldInfo


def make_title(field_name: str, field_info: FieldInfo) -> str:
    return field_name.upper()


class Person(BaseModel):
    name: str = Field(field_title_generator=make_title)
    age: int = Field(field_title_generator=make_title)


print(json.dumps(Person.model_json_schema(), indent=2))
"""
{
  "properties": {
    "name": {
      "title": "NAME",
      "type": "string"
    },
    "age": {
      "title": "AGE",
      "type": "integer"
    }
  },
  "required": [
    "name",
    "age"
  ],
  "title": "Person",
  "type": "object"
}
"""

Model-Level Customization

You can also use [model config][pydantic.config.ConfigDict] to customize JSON schema generation on a model. Specifically, the following config options are relevant:

[title][pydantic.config.ConfigDict.title]
[json_schema_extra][pydantic.config.ConfigDict.json_schema_extra]
[json_schema_mode_override][pydantic.config.ConfigDict.json_schema_mode_override]
[field_title_generator][pydantic.config.ConfigDict.field_title_generator]
[model_title_generator][pydantic.config.ConfigDict.model_title_generator]

Using `json_schema_extra`

The json_schema_extra option can be used to add extra information to the JSON schema, either at the Field level or at the Model level. You can pass a dict or a Callable to json_schema_extra.

Using `json_schema_extra` with a `dict`

You can pass a dict to json_schema_extra to add extra information to the JSON schema:

python

import json

from pydantic import BaseModel, ConfigDict


class Model(BaseModel):
    a: str

    model_config = ConfigDict(json_schema_extra={'examples': [{'a': 'Foo'}]})


print(json.dumps(Model.model_json_schema(), indent=2))
"""
{
  "examples": [
    {
      "a": "Foo"
    }
  ],
  "properties": {
    "a": {
      "title": "A",
      "type": "string"
    }
  },
  "required": [
    "a"
  ],
  "title": "Model",
  "type": "object"
}
"""

Using `json_schema_extra` with a `Callable`

You can pass a Callable to json_schema_extra to modify the JSON schema with a function:

python

import json

from pydantic import BaseModel, Field


def pop_default(s):
    s.pop('default')


class Model(BaseModel):
    a: int = Field(default=1, json_schema_extra=pop_default)


print(json.dumps(Model.model_json_schema(), indent=2))
"""
{
  "properties": {
    "a": {
      "title": "A",
      "type": "integer"
    }
  },
  "title": "Model",
  "type": "object"
}
"""

Merging `json_schema_extra`

Starting in v2.9, Pydantic merges json_schema_extra dictionaries from annotated types. This pattern offers a more additive approach to merging rather than the previous override behavior. This can be quite helpful for cases of reusing json schema extra information across multiple types.

We viewed this change largely as a bug fix, as it resolves unintentional differences in the json_schema_extra merging behavior between BaseModel and TypeAdapter instances - see this issue for more details.

python

import json
from typing import Annotated

from typing_extensions import TypeAlias

from pydantic import Field, TypeAdapter

ExternalType: TypeAlias = Annotated[
    int, Field(json_schema_extra={'key1': 'value1'})
]

ta = TypeAdapter(
    Annotated[ExternalType, Field(json_schema_extra={'key2': 'value2'})]
)
print(json.dumps(ta.json_schema(), indent=2))
"""
{
  "key1": "value1",
  "key2": "value2",
  "type": "integer"
}
"""

!!! note We no longer (and never fully did) support composing a mix of dict and callable type json_schema_extra specifications. If this is a requirement for your use case, please open a pydantic issue and explain your situation - we'd be happy to reconsider this decision when presented with a compelling case.

`WithJsonSchema` annotation

!!! tip Using [WithJsonSchema][pydantic.json_schema.WithJsonSchema] is preferred over implementing __get_pydantic_json_schema__() for custom types, as it's more simple and less error-prone.

An annotation used to override the JSON Schema for a type.

This is useful when you want to set a JSON Schema for a type that don't produce any JSON Schemas by default (e.g. [Callable][collections.abc.Callable]). Note that this overrides the whole generated JSON Schema for the type (in the following example, the 'type' also needs to be provided).

python

import json
from typing import Annotated

from pydantic import BaseModel, WithJsonSchema

MyInt = Annotated[
    int,
    WithJsonSchema({'type': 'integer', 'examples': [1, 0, -1]}),
]


class Model(BaseModel):
    a: MyInt


print(json.dumps(Model.model_json_schema(), indent=2))
"""
{
  "properties": {
    "a": {
      "examples": [
        1,
        0,
        -1
      ],
      "title": "A",
      "type": "integer"
    }
  },
  "required": [
    "a"
  ],
  "title": "Model",
  "type": "object"
}
"""

See the [API documentation][pydantic.json_schema.WithJsonSchema] for more details.

!!! note You might be tempted to use the [WithJsonSchema][pydantic.json_schema.WithJsonSchema] annotation to fine-tune the JSON Schema of fields having validators attached. Instead, it is recommended to use the json_schema_input_type argument.

`SkipJsonSchema` annotation

??? api "API Documentation" [pydantic.json_schema.SkipJsonSchema][pydantic.json_schema.SkipJsonSchema]

The [SkipJsonSchema][pydantic.json_schema.SkipJsonSchema] annotation can be used to skip an included field (or part of a field's specifications) from the generated JSON schema. See the API docs for more details.

Implementing `__get_pydantic_core_schema__` <a name="implementing_get_pydantic_core_schema"></a>

Custom types (used as field_name: TheType or field_name: Annotated[TheType, ...]) as well as Annotated metadata (used as field_name: Annotated[int, SomeMetadata]) can modify or override the generated schema by implementing __get_pydantic_core_schema__. This method receives two positional arguments:

The type annotation that corresponds to this type (so in the case of TheType[T][int] it would be TheType[int]).
A handler/callback to call the next implementer of __get_pydantic_core_schema__.

The handler system works just like wrap field validators. In this case the input is the type and the output is a core_schema.

Here is an example of a custom type that overrides the generated core_schema:

python

from dataclasses import dataclass
from typing import Any

from pydantic_core import core_schema

from pydantic import BaseModel, GetCoreSchemaHandler


@dataclass
class CompressedString:
    dictionary: dict[int, str]
    text: list[int]

    def build(self) -> str:
        return ' '.join([self.dictionary[key] for key in self.text])

    @classmethod
    def __get_pydantic_core_schema__(
        cls, source: type[Any], handler: GetCoreSchemaHandler
    ) -> core_schema.CoreSchema:
        assert source is CompressedString
        return core_schema.no_info_after_validator_function(
            cls._validate,
            core_schema.str_schema(),
            serialization=core_schema.plain_serializer_function_ser_schema(
                cls._serialize,
                info_arg=False,
                return_schema=core_schema.str_schema(),
            ),
        )

    @staticmethod
    def _validate(value: str) -> 'CompressedString':
        inverse_dictionary: dict[str, int] = {}
        text: list[int] = []
        for word in value.split(' '):
            if word not in inverse_dictionary:
                inverse_dictionary[word] = len(inverse_dictionary)
            text.append(inverse_dictionary[word])
        return CompressedString(
            {v: k for k, v in inverse_dictionary.items()}, text
        )

    @staticmethod
    def _serialize(value: 'CompressedString') -> str:
        return value.build()


class MyModel(BaseModel):
    value: CompressedString


print(MyModel.model_json_schema())
"""
{
    'properties': {'value': {'title': 'Value', 'type': 'string'}},
    'required': ['value'],
    'title': 'MyModel',
    'type': 'object',
}
"""
print(MyModel(value='fox fox fox dog fox'))
"""
value = CompressedString(dictionary={0: 'fox', 1: 'dog'}, text=[0, 0, 0, 1, 0])
"""

print(MyModel(value='fox fox fox dog fox').model_dump(mode='json'))
#> {'value': 'fox fox fox dog fox'}

Since Pydantic would not know how to generate a schema for CompressedString, if you call handler(source) in its __get_pydantic_core_schema__ method you would get a pydantic.errors.PydanticSchemaGenerationError error. This will be the case for most custom types, so you almost never want to call into handler for custom types.

The process for Annotated metadata is much the same except that you can generally call into handler to have Pydantic handle generating the schema.

python

from collections.abc import Sequence
from dataclasses import dataclass
from typing import Annotated, Any

from pydantic_core import core_schema

from pydantic import BaseModel, GetCoreSchemaHandler, ValidationError


@dataclass
class RestrictCharacters:
    alphabet: Sequence[str]

    def __get_pydantic_core_schema__(
        self, source: type[Any], handler: GetCoreSchemaHandler
    ) -> core_schema.CoreSchema:
        if not self.alphabet:
            raise ValueError('Alphabet may not be empty')
        schema = handler(
            source
        )  # get the CoreSchema from the type / inner constraints
        if schema['type'] != 'str':
            raise TypeError('RestrictCharacters can only be applied to strings')
        return core_schema.no_info_after_validator_function(
            self.validate,
            schema,
        )

    def validate(self, value: str) -> str:
        if any(c not in self.alphabet for c in value):
            raise ValueError(
                f'{value!r} is not restricted to {self.alphabet!r}'
            )
        return value


class MyModel(BaseModel):
    value: Annotated[str, RestrictCharacters('ABC')]


print(MyModel.model_json_schema())
"""
{
    'properties': {'value': {'title': 'Value', 'type': 'string'}},
    'required': ['value'],
    'title': 'MyModel',
    'type': 'object',
}
"""
print(MyModel(value='CBA'))
#> value='CBA'

try:
    MyModel(value='XYZ')
except ValidationError as e:
    print(e)
    """
    1 validation error for MyModel
    value
      Value error, 'XYZ' is not restricted to 'ABC' [type=value_error, input_value='XYZ', input_type=str]
    """

So far we have been wrapping the schema, but if you just want to modify it or ignore it you can as well.

To modify the schema, first call the handler, then mutate the result:

python

from typing import Annotated, Any

from pydantic_core import ValidationError, core_schema

from pydantic import BaseModel, GetCoreSchemaHandler


class SmallString:
    def __get_pydantic_core_schema__(
        self,
        source: type[Any],
        handler: GetCoreSchemaHandler,
    ) -> core_schema.CoreSchema:
        schema = handler(source)
        assert schema['type'] == 'str'
        schema['max_length'] = 10  # modify in place
        return schema


class MyModel(BaseModel):
    value: Annotated[str, SmallString()]


try:
    MyModel(value='too long!!!!!')
except ValidationError as e:
    print(e)
    """
    1 validation error for MyModel
    value
      String should have at most 10 characters [type=string_too_long, input_value='too long!!!!!', input_type=str]
    """

!!! tip Note that you must return a schema, even if you are just mutating it in place.

To override the schema completely, do not call the handler and return your own CoreSchema:

python

from typing import Annotated, Any

from pydantic_core import ValidationError, core_schema

from pydantic import BaseModel, GetCoreSchemaHandler


class AllowAnySubclass:
    def __get_pydantic_core_schema__(
        self, source: type[Any], handler: GetCoreSchemaHandler
    ) -> core_schema.CoreSchema:
        # we can't call handler since it will fail for arbitrary types
        def validate(value: Any) -> Any:
            if not isinstance(value, source):
                raise ValueError(
                    f'Expected an instance of {source}, got an instance of {type(value)}'
                )
            return value

        return core_schema.no_info_plain_validator_function(validate)


class Foo:
    pass


class Model(BaseModel):
    f: Annotated[Foo, AllowAnySubclass()]


print(Model(f=Foo()))
#> f=<__main__.Foo object at 0x0123456789ab>


class NotFoo:
    pass


try:
    Model(f=NotFoo())
except ValidationError as e:
    print(e)
    """
    1 validation error for Model
    f
      Value error, Expected an instance of <class '__main__.Foo'>, got an instance of <class '__main__.NotFoo'> [type=value_error, input_value=<__main__.NotFoo object at 0x0123456789ab>, input_type=NotFoo]
    """

Implementing `__get_pydantic_json_schema__` <a name="implementing_get_pydantic_json_schema"></a>

You can also implement __get_pydantic_json_schema__ to modify or override the generated json schema. Modifying this method only affects the JSON schema - it doesn't affect the core schema, which is used for validation and serialization.

Here's an example of modifying the generated JSON schema:

python

import json
from typing import Any

from pydantic_core import core_schema as cs

from pydantic import GetCoreSchemaHandler, GetJsonSchemaHandler, TypeAdapter
from pydantic.json_schema import JsonSchemaValue


class Person:
    name: str
    age: int

    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age

    @classmethod
    def __get_pydantic_core_schema__(
        cls, source_type: Any, handler: GetCoreSchemaHandler
    ) -> cs.CoreSchema:
        return cs.typed_dict_schema(
            {
                'name': cs.typed_dict_field(cs.str_schema()),
                'age': cs.typed_dict_field(cs.int_schema()),
            },
        )

    @classmethod
    def __get_pydantic_json_schema__(
        cls, core_schema: cs.CoreSchema, handler: GetJsonSchemaHandler
    ) -> JsonSchemaValue:
        json_schema = handler(core_schema)
        json_schema = handler.resolve_ref_schema(json_schema)
        json_schema['examples'] = [
            {
                'name': 'John Doe',
                'age': 25,
            }
        ]
        json_schema['title'] = 'Person'
        return json_schema


print(json.dumps(TypeAdapter(Person).json_schema(), indent=2))
"""
{
  "examples": [
    {
      "age": 25,
      "name": "John Doe"
    }
  ],
  "properties": {
    "name": {
      "title": "Name",
      "type": "string"
    },
    "age": {
      "title": "Age",
      "type": "integer"
    }
  },
  "required": [
    "name",
    "age"
  ],
  "title": "Person",
  "type": "object"
}
"""

Using `field_title_generator`

The field_title_generator parameter can be used to programmatically generate the title for a field based on its name and info. This is similar to the field level field_title_generator, but the ConfigDict option will be applied to all fields of the class.

See the following example:

python

import json

from pydantic import BaseModel, ConfigDict


class Person(BaseModel):
    model_config = ConfigDict(
        field_title_generator=lambda field_name, field_info: field_name.upper()
    )
    name: str
    age: int


print(json.dumps(Person.model_json_schema(), indent=2))
"""
{
  "properties": {
    "name": {
      "title": "NAME",
      "type": "string"
    },
    "age": {
      "title": "AGE",
      "type": "integer"
    }
  },
  "required": [
    "name",
    "age"
  ],
  "title": "Person",
  "type": "object"
}
"""

Using `model_title_generator`

The model_title_generator config option is similar to the field_title_generator option, but it applies to the title of the model itself, and accepts the model class as input.

See the following example:

python

import json

from pydantic import BaseModel, ConfigDict


def make_title(model: type) -> str:
    return f'Title-{model.__name__}'


class Person(BaseModel):
    model_config = ConfigDict(model_title_generator=make_title)
    name: str
    age: int


print(json.dumps(Person.model_json_schema(), indent=2))
"""
{
  "properties": {
    "name": {
      "title": "Name",
      "type": "string"
    },
    "age": {
      "title": "Age",
      "type": "integer"
    }
  },
  "required": [
    "name",
    "age"
  ],
  "title": "Title-Person",
  "type": "object"
}
"""

JSON schema types

Types, custom field types, and constraints (like max_length) are mapped to the corresponding spec formats in the following priority order (when there is an equivalent available):

JSON Schema Core
JSON Schema Validation
OpenAPI Data Types
The standard format JSON field is used to define Pydantic extensions for more complex string sub-types.

The field schema mapping from Python or Pydantic to JSON schema is done as follows:

Top-level schema generation

You can also generate a top-level JSON schema that only includes a list of models and related sub-models in its $defs:

python

import json

from pydantic import BaseModel
from pydantic.json_schema import models_json_schema


class Foo(BaseModel):
    a: str = None


class Model(BaseModel):
    b: Foo


class Bar(BaseModel):
    c: int


_, top_level_schema = models_json_schema(
    [(Model, 'validation'), (Bar, 'validation')], title='My Schema'
)
print(json.dumps(top_level_schema, indent=2))
"""
{
  "$defs": {
    "Bar": {
      "properties": {
        "c": {
          "title": "C",
          "type": "integer"
        }
      },
      "required": [
        "c"
      ],
      "title": "Bar",
      "type": "object"
    },
    "Foo": {
      "properties": {
        "a": {
          "default": null,
          "title": "A",
          "type": "string"
        }
      },
      "title": "Foo",
      "type": "object"
    },
    "Model": {
      "properties": {
        "b": {
          "$ref": "#/$defs/Foo"
        }
      },
      "required": [
        "b"
      ],
      "title": "Model",
      "type": "object"
    }
  },
  "title": "My Schema"
}
"""

Customizing the JSON Schema Generation Process

??? api "API Documentation" [pydantic.json_schema][pydantic.json_schema.GenerateJsonSchema]

If you need custom schema generation, you can use a schema_generator, modifying the [GenerateJsonSchema][pydantic.json_schema.GenerateJsonSchema] class as necessary for your application.

The various methods that can be used to produce JSON schema accept a keyword argument schema_generator: type[GenerateJsonSchema] = GenerateJsonSchema, and you can pass your custom subclass to these methods in order to use your own approach to generating JSON schema.

GenerateJsonSchema implements the translation of a type's pydantic-core schema into a JSON schema. By design, this class breaks the JSON schema generation process into smaller methods that can be easily overridden in subclasses to modify the "global" approach to generating JSON schema.

python

from pydantic import BaseModel
from pydantic.json_schema import GenerateJsonSchema


class MyGenerateJsonSchema(GenerateJsonSchema):
    def generate(self, schema, mode='validation'):
        json_schema = super().generate(schema, mode=mode)
        json_schema['title'] = 'Customize title'
        json_schema['$schema'] = self.schema_dialect
        return json_schema


class MyModel(BaseModel):
    x: int


print(MyModel.model_json_schema(schema_generator=MyGenerateJsonSchema))
"""
{
    'properties': {'x': {'title': 'X', 'type': 'integer'}},
    'required': ['x'],
    'title': 'Customize title',
    'type': 'object',
    '$schema': 'https://json-schema.org/draft/2020-12/schema',
}
"""

Below is an approach you can use to exclude any fields from the schema that don't have valid json schemas:

python

from typing import Callable

from pydantic_core import PydanticOmit, core_schema

from pydantic import BaseModel
from pydantic.json_schema import GenerateJsonSchema, JsonSchemaValue


class MyGenerateJsonSchema(GenerateJsonSchema):
    def handle_invalid_for_json_schema(
        self, schema: core_schema.CoreSchema, error_info: str
    ) -> JsonSchemaValue:
        raise PydanticOmit


def example_callable():
    return 1


class Example(BaseModel):
    name: str = 'example'
    function: Callable = example_callable


instance_example = Example()

validation_schema = instance_example.model_json_schema(
    schema_generator=MyGenerateJsonSchema, mode='validation'
)
print(validation_schema)
"""
{
    'properties': {
        'name': {'default': 'example', 'title': 'Name', 'type': 'string'}
    },
    'title': 'Example',
    'type': 'object',
}
"""

JSON schema sorting

By default, Pydantic recursively sorts JSON schemas by alphabetically sorting keys. Notably, Pydantic skips sorting the values of the properties key, to preserve the order of the fields as they were defined in the model.

If you would like to customize this behavior, you can override the sort method in your custom GenerateJsonSchema subclass. The below example uses a no-op sort method to disable sorting entirely, which is reflected in the preserved order of the model fields and json_schema_extra keys:

python

import json
from typing import Optional

from pydantic import BaseModel, Field
from pydantic.json_schema import GenerateJsonSchema, JsonSchemaValue


class MyGenerateJsonSchema(GenerateJsonSchema):
    def sort(
        self, value: JsonSchemaValue, parent_key: Optional[str] = None
    ) -> JsonSchemaValue:
        """No-op, we don't want to sort schema values at all."""
        return value


class Bar(BaseModel):
    c: str
    b: str
    a: str = Field(json_schema_extra={'c': 'hi', 'b': 'hello', 'a': 'world'})


json_schema = Bar.model_json_schema(schema_generator=MyGenerateJsonSchema)
print(json.dumps(json_schema, indent=2))
"""
{
  "type": "object",
  "properties": {
    "c": {
      "type": "string",
      "title": "C"
    },
    "b": {
      "type": "string",
      "title": "B"
    },
    "a": {
      "type": "string",
      "c": "hi",
      "b": "hello",
      "a": "world",
      "title": "A"
    }
  },
  "required": [
    "c",
    "b",
    "a"
  ],
  "title": "Bar"
}
"""

Customizing the `$ref`s in JSON Schema

The format of $refs can be altered by calling [model_json_schema()][pydantic.main.BaseModel.model_json_schema] or [model_dump_json()][pydantic.main.BaseModel.model_dump_json] with the ref_template keyword argument. The definitions are always stored under the key $defs, but a specified prefix can be used for the references.

This is useful if you need to extend or modify the JSON schema default definitions location. For example, with OpenAPI:

python

import json

from pydantic import BaseModel
from pydantic.type_adapter import TypeAdapter


class Foo(BaseModel):
    a: int


class Model(BaseModel):
    a: Foo


adapter = TypeAdapter(Model)

print(
    json.dumps(
        adapter.json_schema(ref_template='#/components/schemas/{model}'),
        indent=2,
    )
)
"""
{
  "$defs": {
    "Foo": {
      "properties": {
        "a": {
          "title": "A",
          "type": "integer"
        }
      },
      "required": [
        "a"
      ],
      "title": "Foo",
      "type": "object"
    }
  },
  "properties": {
    "a": {
      "$ref": "#/components/schemas/Foo"
    }
  },
  "required": [
    "a"
  ],
  "title": "Model",
  "type": "object"
}
"""

Miscellaneous Notes on JSON Schema Generation

The JSON schema for Optional fields indicates that the value null is allowed.
The Decimal type is exposed in JSON schema (and serialized) as a string.
Since the namedtuple type doesn't exist in JSON, a model's JSON schema does not preserve namedtuples as namedtuples.
Sub-models used are added to the $defs JSON attribute and referenced, as per the spec.
Sub-models with modifications (via the Field class) like a custom title, description, or default value, are recursively included instead of referenced.
The description for models is taken from either the docstring of the class or the argument description to the Field class.
The schema is generated by default using aliases as keys, but it can be generated using model property names instead by calling [model_json_schema()][pydantic.main.BaseModel.model_json_schema] or [model_dump_json()][pydantic.main.BaseModel.model_dump_json] with the by_alias=False keyword argument.

Generating JSON Schema

Configuring the JsonSchemaMode

Customizing JSON Schema

Field-Level Customization

Programmatic field title generation

Model-Level Customization

Using json_schema_extra

Using json_schema_extra with a dict

Using json_schema_extra with a Callable

Merging json_schema_extra

WithJsonSchema annotation

SkipJsonSchema annotation

Implementing __get_pydantic_core_schema__ <a name="implementing_get_pydantic_core_schema"></a>

Implementing __get_pydantic_json_schema__ <a name="implementing_get_pydantic_json_schema"></a>

Using field_title_generator

Using model_title_generator

JSON schema types

Top-level schema generation

Customizing the JSON Schema Generation Process

JSON schema sorting

Customizing the $refs in JSON Schema

Miscellaneous Notes on JSON Schema Generation

Configuring the `JsonSchemaMode`

Using `json_schema_extra`

Using `json_schema_extra` with a `dict`

Using `json_schema_extra` with a `Callable`

Merging `json_schema_extra`

`WithJsonSchema` annotation

`SkipJsonSchema` annotation

Implementing `__get_pydantic_core_schema__` <a name="implementing_get_pydantic_core_schema"></a>

Implementing `__get_pydantic_json_schema__` <a name="implementing_get_pydantic_json_schema"></a>

Using `field_title_generator`

Using `model_title_generator`

Customizing the `$ref`s in JSON Schema