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Preflight Profile (Mixin)

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A resource that accepts POST, PUT or PATCH requests can offer to preflight a request’s headers before the client sends the full request payload. Preflighting is valuable to calls where the payload is large or expensive to produce. If the resource rejects the client’s preflight request, the client can skip the full payload transmission.


Resources that support the Preflight profile include the Preflight profile header in its list of profile headers.

Preflight Modification

A client uses preflight modification to check if a resource accepts the content of a PUT, POST or PATCH request before actually sending the entire content. Also known as “look before you leap,” this approach takes two requests to complete but saves time overall during an unacceptable modification request. The client can skip sending the content if the preflight request fails. The client initiates the preflight request sequence by sending the requested operation with a expect: 100-continue header, the headers to be preflighted, and an empty payload. The resource may require specific headers to be preflighted and reject the preflight request with an appropriate status code.

The preflight response of 100 Continue is not a guarantee of acceptance but rather a likelihood of acceptance. The resource may still choose to reject the full request upon delivery for other reasons.

Decision Flow

If the resource accepts the preflight request, it returns the status 100 Continue and the client can send the full request with a payload. If the resource cannot perform preflight for any reason but can accept a full request, it sends back 417 Expectation Failed. The client falls back to the non-preflight version of the flow and sends the full request with a payload.

If the resource rejects the continuation request, it responds with an appropriate status code and error message.


HTTP/1.1 Semantics and Content: RFC 7231

© 2019-2023 Matt Bishop
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