HTTP · 1xx Informational

100 Continue

Client should continue with request body; server acknowledged partial request.

When you see this status

Client sends Expect: 100-continue header with large request body
Server is ready to accept the request body after headers
Client waits for 100 response before sending body

Common causes

Client uses Expect header but server supports 100-continue
Server processes headers but needs confirmation before body
Network proxy or load balancer handling Expect header

How to debug it

Verify client sends Expect: 100-continue header correctly
Check server logs for 100 response before request body
Ensure no middleware strips or misinterprets Expect header

Return this code in your framework

Express (Node.js)

res.status(100).send('');

FastAPI (Python)

from fastapi import Response
return Response(status_code=100)

NestJS

@Get() @HttpCode(100) getContinue() { return ''; }

Go (net/http)

w.WriteHeader(http.StatusContinue)

About HTTP 100

HTTP status code 100 Continue is part of the 1xx Informational family. Client should continue with request body; server acknowledged partial request. Understanding it is essential for backend engineers, API designers, and anyone debugging integrations between services.

The Qaynaq Labs HTTP Status Code Explainer covers all standard codes with practical guidance: what triggers them, how to debug them in production, and exactly how to return them from common frameworks. Each code page is hand-curated and verified.

Frequently asked

Why use 100 Continue instead of sending the body directly?

Saves bandwidth and processing by letting the server reject early if headers are invalid. Avoids sending large bodies unnecessarily.

Can a server ignore the Expect header and send 100 Continue?

No. If the server does not support 100-continue, it must respond with 417 Expectation Failed or ignore the header entirely.

Does 100 Continue guarantee the request will succeed?

No. 100 Continue only acknowledges the headers. The final response (e.g., 200) depends on the full request processing.