flask-smorest automatically generates an OpenAPI documentation (formerly known as Swagger) for the API.

That documentation can be made accessible as a JSON file, along with a nice web interface such as ReDoc, Swagger UI or RapiDoc.

API parameters

The following API and OpenAPI parameters must be passed either as application configuration parameter or at initialization. If both are used, the application configuration parameter takes precedence.


Title of the API. Human friendly string describing the API.

API title must be passed either as application parameter or as title at Api initialization in spec_kwargs parameters.


Version of the API. It is copied verbatim in the documentation. It should be a string, even if the version is a number.

API version must be passed either as application parameter or as version at Api initialization in spec_kwargs parameters.


Version of the OpenAPI standard used to describe the API. It should be provided as a string.

OpenAPI version must be passed either as application parameter or as openapi_version at Api initialization in spec_kwargs parameters.

Add Documentation Information to Resources

Add Summary and Description

flask-smorest uses view functions docstrings to fill the summary and description attributes of an operation object.

def get(pet_id):
    """Find pets by ID

    Return pets based on ID.
    Internal comment not meant to be exposed.

The part of the docstring following the '---' line is ignored.

The part before the '---' line is used as summary and description. The first lines are used as summary. If an empty line is met, all following lines are used as description.

The example above produces the following documentation attributes:

    "get": {
        "summary": "Find pets by ID",
        "description": "Return pets based on ID",

The delimiter line is the line starting with the delimiter string defined in Blueprint.DOCSTRING_INFO_DELIMITER. This string defaults to "---" and can be customized in a subclass. None means “no delimiter”: the whole docstring is included in the docs.

Document Operations Parameters and Responses

Schemas passed in Blueprint.arguments to deserialize arguments are parsed automatically to generate corresponding documentation. Additional example and examples parameters can be used to provide examples (those are only valid for OpenAPI v3).

Likewise, schemas passed in Blueprint.response to serialize responses are parsed automatically to generate corresponding documentation. Additional example and examples parameters can be used to provide examples (examples is only valid for OpenAPI v3). Additional headers parameters can be used to document response headers.

A default error response is added automatically to the documentation of all resources. It is defined as a response component in the spec named "DEFAULT_ERROR". The name can be customized by setting Api.DEFAULT_ERROR_RESPONSE_NAME. The feature can be opted out by setting it to None.

Document Path Parameters

Path parameters are automatically documented. The type in the documentation is inferred from the path parameter converter used in the URL rule. Custom path parameters should be registered for their type to be correctly determined (see below).

The Blueprint.route method takes a parameters argument to pass documentation for parameters that are shared by all operations of a path. It can be used to pass extra documentation, such as examples, for path parameters.

Likewise, path parameters in url_prefix are also automatically documented, and their automatic documentation can be completed by passing a parameters argument to the Api.register_blueprint() method.

Pass Extra Documentation Information

flask-smorest tries to document the API as automatically as possible and to provide explicit means to pass extra-information that can’t be inferred from the code, such as descriptions, examples, etc.

The Blueprint.doc decorator provides a means to pass extra documentation information. It comes in handy if an OpenAPI feature is not supported, but it suffers from a few limitations, and it should be considered a last resort solution until flask-smorest is improved to fit the need.

Known issues and alternatives are discussed in issue #71.

Populate the Root Document Object

Additional root document attributes can be passed either in the code, in Api parameter spec_kwargs, or as Flask app configuration parameters.

app.config["API_SPEC_OPTIONS"] = {"x-internal-id": "2"}

api = Api(app, spec_kwargs={"host": "", "x-internal-id": "1"})

Note that app.config overrides spec_kwargs. The example above produces

{"host": "", "x-internal-id": "2"}

Document Top-level Components

Documentation components can be passed by accessing the internal apispec Components object.

api = Api(app)
    "Pet name", "query", {"description": "Item ID", "required": True}

Register Nested Blueprints

Blueprint objects can be nested just like standard flask nested blueprints. Endpoints from nested blueprints are automatically documented, but only the top level blueprints generate new “tag” entries in the OpenAPI spec. The tag name and description are taken from the top level blueprints.

Register Custom Fields

Standard marshmallow Field classes are documented with the correct type and format.

When using custom fields, the type and format must be passed, either explicitly or by specifying a parent field class, using Api.register_field():

# Map to ('string', 'ObjectId') passing type and format
api.register_field(ObjectId, "string", "ObjectId")

# Map to ('string', ) passing type
api.register_field(CustomString, "string", None)

# Map to ('string, 'date-time') passing a marshmallow Field
api.register_field(CustomDateTime, ma.fields.DateTime)

Register Custom Path Parameter Converters

Likewise, standard types used as path parameter converters in the flask routes are correctly documented, but custom path converters must be registered.

The Api.register_converter() allows to register a converter in the Api object to generate an accurate documentation.

# Register MongoDB's ObjectId converter in Flask application
app.url_map.converters["objectid"] = ObjectIdConverter

# Define custom converter to schema function
def objectidconverter2paramschema(converter):
    return {"type": "string", "format": "ObjectID"}

# Register converter in Api
api.register_converter(ObjectIdConverter, objectidconverter2paramschema)

def get_pet(pet_id):
    """Get pet by ID"""

Enforce Order in OpenAPI Specification File

When a Blueprint is registered, a tag is created with the Blueprint name. The display order in the interface is the Blueprint registration order. And the display order inside a tag is the order in which the resources are defined in the Blueprint.

In the OpenAPI specification file, the fields of a Schema are documented as schema properties. Although objects are not ordered in JSON, OpenAPI graphical interfaces tend to respect the order in which the properties are defined in the properties object in the specification file.

When using an ordererd Schema, the fields definition order is preserved when generating the specification file and the properties are displayed in that order.

This is typically done in a base class:

class MyBaseSchema(ma.Schema):
    class Meta:
        ordered = True

class User(MyBaseSchema):
    name = ma.fields.String()
    surname = ma.fields.String()

Serve the OpenAPI Documentation

Now that that the documentation is generated, it should be made available to the clients. flask-smorest can define routes to provide both the documentation as a JSON file and a nice web interface to browse it interactively. This feature is accessible through Flask app parameters.


Defines the base path for both the JSON file and the UI. If None, the documentation is not served and the following parameters are ignored.

Default: None


Path to the JSON file, relative to the base path.

Default: openapi.json

ReDoc, Swagger UI and RapiDoc interfaces are available to present the API.

Their configuration logics are similar. If an application path and a script URL are set, then flask-smorest adds a route at that path to serve the interface page using the JS script from the script URL.


Path to the ReDoc page, relative to the base path.

Default: None


URL to the ReDoc script.


Default: None


Path to the Swagger UI page, relative to the base path.

Default: None


URL to the Swagger UI script. Versions prior to 3.x are not supported.


Default: None


Dictionary representing Swagger UI configuration options. See Swagger UI Configuration for available options. All JSON serializable options are supported.

  • {'deepLinking': True, 'supportedSubmitMethods': ['get', 'post']}

Default: {}


Path to the RapiDoc page, relative to the base path.

Default: None


URL to the RapiDoc script.


Default: None


Dictionary representing RapiDoc configuration options. See RapiDoc API for available options.

  • {"theme": "dark"}

Default: {}

Here’s an example application configuration using all available UIs:

class Config:
    OPENAPI_VERSION = "3.0.2"
    OPENAPI_JSON_PATH = "api-spec.json"
    OPENAPI_REDOC_PATH = "/redoc"
    OPENAPI_SWAGGER_UI_PATH = "/swagger-ui"
    OPENAPI_RAPIDOC_PATH = "/rapidoc"

Write OpenAPI Documentation File

flask-smorest provides flask commands to print the OpenAPI JSON or YAML file to the standard output

flask openapi print [--format=json|yaml]

or write it to a JSON or YAML file

flask openapi write [--format=json|yaml] openapi.json

A typical use case is to write the OpenAPI documentation to a file in a deployment script to host it on a separate server rather than serving it from the application.