Describing Parameters

In OpenAPI 3.0, parameters are defined in the parameters section of an operation or path. To describe a parameter, you specify its name, location (in), data type (defined by either schema or content) and other attributes, such as description or required. Here is an example:

paths:
  /users/{userId}:
    get:
      summary: Get a user by ID
      parameters:
        - in: path
          name: userId
          schema:
            type: integer
          required: true
          description: Numeric ID of the user to get

Note that parameters is an array, so, in YAML, each parameter definition must be listed with a dash (-) in front of it.

Parameter Types

OpenAPI 3.0 distinguishes between the following parameter types based on the parameter location. The location is determined by the parameter’s in key, for example, in: query or in: path.

Path Parameters

Path parameters are variable parts of a URL path. They are typically used to point to a specific resource within a collection, such as a user identified by ID. A URL can have several path parameters, each denoted with curly braces { }.

Each path parameter must be substituted with an actual value when the client makes an API call. In OpenAPI, a path parameter is defined using in: path. The parameter name must be the same as specified in the path. Also remember to add required: true, because path parameters are always required. For example, the /users/{id} endpoint would be described as:

Path parameters containing arrays and objects can be serialized in different ways:

  • path-style expansion (matrix) – semicolon-prefixed, such as /map/point;x=50;y=20

  • label expansion – dot-prefixed, such as /color.R=100.G=200.B=150

  • simple-style – comma-delimited, such as /users/12,34,56

The serialization method is specified by the style and explode keywords. To learn more, see Parameter Serialization.

Query Parameters

Query parameters are the most common type of parameters. They appear at the end of the request URL after a question mark (?), with different name=value pairs separated by ampersands (&). Query parameters can be required and optional.

Use in: query to denote query parameters

Note: To describe API keys passed as query parameters, use securitySchemes and security instead. See API Keys.

Query parameters can be primitive values, arrays and objects. OpenAPI 3.0 provides several ways to serialize objects and arrays in the query string.

Arrays can be serialized as:

  • form – /products?color=blue,green,red or /products?color=blue&color=green, depending on the explode keyword

  • spaceDelimited (same as collectionFormat: ssv in OpenAPI 2.0) – /products?color=blue%20green%20red

  • pipeDelimited (same as collectionFormat: pipes in OpenAPI 2.0) – /products?color=blue|green|red

Objects can be serialized as:

  • form – /points?color=R,100,G,200,B,150 or /points?R=100&G=200&B=150, depending on the explode keyword

  • deepObject – /points?color[R]=100&color[G]=200&color[B]=150

The serialization method is specified by the style and explode keywords. To learn more, see Parameter Serialization.

Reserved Characters in Query Parameters

RFC 3986 defines a set of reserved characters :/?#[]@!$&'()*+,;= that are used as URI component delimiters. When these characters need to be used literally in a query parameter value, they are usually percent-encoded. For example, / is encoded as %2F (or %2f), so that the parameter value quotes/h2g2.txt would be sent as

If you want a query parameter that is not percent-encoded, add allowReserved: true to the parameter definition:

In this case, the parameter value would be sent like so:

Header Parameters

An API call may require that custom headers be sent with an HTTP request. OpenAPI lets you define custom request headers as in: header parameters. For example, suppose, a call to GET /ping requires the X-Request-ID header:

Using OpenAPI 3.0, you would define this operation as follows:

In a similar way, you can define custom response headers. Header parameter can be primitives, arrays and objects. Arrays and objects are serialized using the simple style. For more information, see Parameter Serialization.

Note: Header parameters named Accept, Content-Type and Authorization are not allowed. To describe these headers, use the corresponding OpenAPI keywords:

Header
OpenAPI keywords
For more information, see...

Content-Type

Request content type: requestBody.content.<media-type> Response content type: responses.<code>.content.<media-type>

Describing Request Body, Describing Responses, Media Types

Accept

responses.<code>.content.<media-type>

Describing Responses, Media Types

Authorization

securitySchemes, security

Authentication

Cookie Parameters

Operations can also pass parameters in the Cookie header, as Cookie: name=value. Multiple cookie parameters are sent in the same header, separated by a semicolon and space.

Use in: cookie to define cookie parameters:

Cookie parameters can be primitive values, arrays and objects. Arrays and objects are serialized using the form style. For more information, see Parameter Serialization.

Note: To define cookie authentication, use API keys instead.

Required and Optional Parameters

By default, OpenAPI treats all request parameters as optional. You can add required: true to mark a parameter as required. Note that path parameters must have required: true, because they are always required.

schema vs content

To describe the parameter contents, you can use either the schema or content keyword. They are mutually exclusive and used in different scenarios. In most cases, you would use schema. It lets you describe primitive values, as well as simple arrays and objects serialized into a string. The serialization method for array and object parameters is defined by the style and explode keywords used in that parameter.

content is used in complex serialization scenarios that are not covered by style and explode. For example, if you need to send a JSON string in the query string like so:

In this case, you need to wrap the parameter schema into content/<media-type> as shown below. The schema defines the parameter data structure, and the media type (in this example – application/json) serves as a reference to an external specification that describes the serialization format.

Note for Swagger UI and Swagger Editor users: Parameters with content are supported in Swagger UI 3.23.7+ and Swagger Editor 3.6.34+.

Default Parameter Values

Use the default keyword in the parameter schema to specify the default value for an optional parameter. The default value is the one that the server uses if the client does not supply the parameter value in the request. The value type must be the same as the parameter’s data type. A typical example is paging parameters such as offset and limit:

Assuming offset defaults to 0 and limit defaults to 20 and ranges from 0 to 100, you would define these parameters as:

Common Mistakes

There are two common mistakes when using the default keyword:

  • Using default with required parameters or properties, for example, with path parameters. This does not make sense – if a value is required, the client must always send it, and the default value is never used.

  • Using default to specify a sample value. This is not intended use of default and can lead to unexpected behavior in some Swagger tools. Use the example or examples keyword for this purpose instead. See Adding Examples.

Enum Parameters

You can restrict a parameter to a fixed set of values by adding the enum to the parameter’s schema. The enum values must be of the same type as the parameter data type.

More info: Defining an Enum.

Constant Parameters

You can define a constant parameter as a required parameter with only one possible value:

The enum property specifies possible values. In this example, only one value can be used, and this will be the only value available in the Swagger UI for the user to choose from.

Note: A constant parameter is not the same as the default parameter value. A constant parameter is always sent by the client, whereas the default value is something that the server uses if the parameter is not sent by the client.

Empty-Valued and Nullable Parameters

Query string parameters may only have a name and no value, like so:

Use allowEmptyValue to describe such parameters:

OpenAPI 3.0 also supports nullable in schemas, allowing operation parameters to have the null value. For example, the following schema corresponds to int? in C# and java.lang.Integer in Java:

Note: nullable is not the same as an optional parameter or an empty-valued parameter. nullable means the parameter value can be null. Specific implementations may choose to map an absent or empty-valued parameter to null, but strictly speaking these are not the same thing.

Parameter Examples

You can specify an example or multiple examples for a parameter. The example value should match the parameter schema. Single example:

Multiple named examples:

For details, see Adding Examples.

Deprecated Parameters

Use deprecated: true to mark a parameter as deprecated.

Common Parameters

Common Parameters for All Methods of a Path

Parameters shared by all operations of a path can be defined on the path level instead of the operation level. Path-level parameters are inherited by all operations of that path. A typical use case are the GET/PUT/PATCH/DELETE operations that manipulate a resource accessed via a path parameter

Any extra parameters defined at the operation level are used together with path-level parameters:

Specific path-level parameters can be overridden on the operation level, but cannot be removed.

Common Parameters for Various Paths

Different API paths may have common parameters, such as pagination parameters. You can define common parameters under parameters in the global components section and reference them elsewhere via $ref.

Note that the parameters defined in components are not parameters applied to all operations — they are simply global definitions that can be easily re-used.

Parameter Dependencies

OpenAPI 3.0 does not support parameter dependencies and mutually exclusive parameters. There is an open feature request at https://github.com/OAI/OpenAPI-Specification/issues/256. What you can do is document the restrictions in the parameter description and define the logic in the 400 Bad Request response. For example, consider the /report endpoint that accepts either a relative date range (rdate) or an exact range (start_date+end_date):

You can describe this endpoint as follows:

References

Parameter Object

PreviousPaths and OperationsNextParameter Serialization

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