Content Negotiation

Content Negotiation is performed by an application in order :

  • To match the requested representation as specified by the client via the Accept header with a representation the application can deliver.
  • To determine the Content-Type of incoming data and deserialize it so the application can utilize it.

Essentially, content negotiation is the client telling the server what it is sending and what it wants in return, and the server determining if it can do what the client requests.

Content negotiation validation in DotKernel API happens through middleware, and it ensures that the incoming request and the outgoing response conform to the content types specified in the config file for all routes or for a specific route.

It performs validation on the Accept and Content-Type headers of the request and response and returning appropriate errors responses when necessary.


In DotKernel API the configuration file for content negotiation is held in config/autoload/ and the array looks like this:

return [
    'content-negotiation' => [
        'default'         => [
            'Accept'       => [
            'Content-Type' => [
        '' => [
            'Accept'       => [],
            'Content-Type' => [],

Except the default key, all your keys must match the route name, for example in DotKernel API we have the route to list all admins, which name is admin.list.

If you did not specify a route name to configure your specifications about content negotiation, the default one will be in place. The default key is mandatory.

Every route configuration must come with Accept and Content-Type keys, basically this will be the keys that the request headers will be validated against.

Accept Negotiation

This specifies that your server can return that representation, or at least one of the representation sent by the client.

GET /admin HTTP/1.1
Accept: application/json

This request indicates the client wants application/json in return. Now the server, through the config file will try to validate if that representation can be returned, basically if application/json is presented in the Accept key.

If the representation cannot be returned, a status code 406 - Not Acceptable will be returned.

If the representation can be returned, the server should report the media type through Content-Type header of the response.

Due to how these validations are made, for a json media type, the server can return a more generic media type, for example, if the clients send Accept: application/vnd.api+json and you configured your Accept key as application/json the representation will still be returned as json.

If the Accept header of the request contains */* it means that whatever the server can return it is OK, so it can return anything.

Content-Type Negotiation

The second aspect of content negotiation is the Content-Type header and determine the server can deserialize the data.

POST /admin/1 HTTP/1.1
Accept: application/json
Content-Type: application/json
    "foo": "bar"

The server will try to validate the Content-Type header against your configured Content-Type key from the config file, and if the format is not supported, a status code 415 - Unsupported Media Type will be returned.

For example, if you have a route that needs a file to be uploaded , normally you will configure the Content-Type of that route to be multipart/form-data. The above request will fail as the client send application/json as Content-Type.

If the request does not contain "Content-Type" header, that means that the server will try to deserialize the data as it can.

The Request <-> Response validation

In addition to the validation described above, a third one is happening and is the last one: the server will check if the request Accept header can really be returned by the response.

Through the way DotKernel API is returning a response in handler, a content type is always set.

This cannot be the case in any custom response but in any case the server will check what Content-Type the response is returning and will try to validate that against the Accept header of the request. If the validation fails, a status code 406 - Not Acceptable will be returned.