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Expect Header

Signals that the client expects a particular server behavior before continuing the request.

Header TypeRequest Header
SpecificationStandard RFC compliant
Associated CodesHTTP 100,HTTP 417

What is the Expect header?

The Expect request header lets a client state a requirement the server must meet before the request can proceed. Its only standardized value is 100-continue, used when a client wants confirmation before sending a large request body.

API Usage & Best Practices

  • Avoid wasted uploads: With Expect: 100-continue, the client sends headers first and waits for a 100 Continue before transmitting a large body, avoiding wasted bandwidth if the request would be rejected.
  • Handle rejection: If the server cannot meet the expectation, it must respond with 417 Expectation Failed.
  • Set a timeout: Clients should not wait indefinitely; if no 100 Continue arrives, they should send the body anyway after a reasonable delay.

Examples

POST /v1/uploads HTTP/1.1
Host: api.example.com
Content-Length: 10485760
Expect: 100-continue

Associated HTTP Status Codes

HTTP 100 — Continue

The server has received the request headers and the client should proceed to send the request body.

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HTTP 417 — Expectation Failed

The server cannot meet the requirements of the Expect request header.

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