? HTTP 100 Continue

? Quick Overview

HTTP 100 Continue is an informational status code used in client–server communication. It tells the client that the server has received the request headers and the client can proceed to send the request body.

? Key Concepts

Belongs to the 1xx Informational HTTP status code class
Used mainly with large request bodies
Reduces unnecessary data transfer
Commonly paired with the Expect: 100-continue header

? Syntax / Theory

When a client sends a request with the header Expect: 100-continue, it waits for the server’s response before sending the body. If the server replies with 100 Continue, the client proceeds with the request body.

? Code Example(s)

? View Code Example

// HTTP request headers asking server permission before sending body
POST /upload HTTP/1.1
Host: example.com
Expect: 100-continue
Content-Length: 5242880

? Live Output / Explanation

Server Response

If the server accepts the headers, it responds with:

? View Code Example

// Server tells client to continue sending request body
HTTP/1.1 100 Continue

? Interactive Example / Visualization

Think of HTTP 100 Continue like a security check: the client asks "Should I send the big data?", and the server replies "Yes, go ahead."

Client

❓

☁️

Server

// Click "Start Simulation" to begin...

? View Code Example

// Pseudo-flow of 100 Continue interaction
Client -> Headers only
Server -> 100 Continue
Client -> Sends full request body
Server -> Final response

? Use Cases

Large file uploads
APIs receiving big JSON payloads
Reducing bandwidth waste
Early request validation

? Tips & Best Practices

Use Expect: 100-continue only for large requests
Servers should respond quickly to avoid client delays
Clients should handle servers that ignore the header

? Try It Yourself

Create a request with and without Expect: 100-continue
Test behavior using curl or Postman
Observe how servers respond to large payloads