HTTP 100 Continue

What It Means

The HTTP 100 Continue status code is an informational response indicating that the initial part of a request has been received and has not been rejected by the server. The client should continue sending the request body.

How It Works

The 100 Continue mechanism optimizes large POST/PUT requests:

1. Client sends headers with Expect: 100-continue
2. Server validates headers (authentication, content-length limits, etc.)
3. If valid, server responds with 100 Continue
4. Client sends the request body
5. Server processes and sends final response (200, 201, etc.)

When It's Used

• Large file uploads: Check if server will accept before sending megabytes
• Authentication: Verify credentials before sending body
• Content validation: Check Content-Type is acceptable

Example Request/Response

# Client request
POST /upload HTTP/1.1
Host: api.example.com
Content-Type: application/octet-stream
Content-Length: 104857600
Expect: 100-continue

# Server response (if OK to proceed)
HTTP/1.1 100 Continue

# Client then sends body...
[104857600 bytes of data]

# Server final response
HTTP/1.1 201 Created

Server Alternatives

Instead of 100 Continue, the server might respond with:

• 417 Expectation Failed: The Expect header cannot be met
• 401 Unauthorized: Authentication required before proceeding
• 413 Payload Too Large: Content-Length exceeds server limits