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HTTP Header
Expect
RequestIndicates that the client expects the server to acknowledge a condition before sending the request body.
HTTP header reference, syntax, examples, and developer usage.
What is the Expect header?
The Expect HTTP header is used to transmit metadata between a client and server as part of HTTP requests or responses.
HTTP headers define how content should be interpreted, cached, authenticated, secured, or processed by browsers and APIs.
Direction
This is a request header sent by the client to the server.
Syntax
Expect: 100-continue
Example
Expect: 100-continue
Common use cases
- Avoiding sending large request bodies to servers that will reject them
- Optimizing upload flows by checking server availability first
- Used by HTTP clients when uploading large payloads
Common mistakes
- Using the header in the wrong request or response context
- Sending invalid header values
- Incorrect header syntax
- Assuming the header automatically changes server behaviour
Practical developer insight
100-continue is the only defined value. The client sends the headers first and waits for a 100 Continue response before sending the body. If the server responds with 4xx instead, the client avoids transmitting a potentially large payload unnecessarily.