How does 100 Continue save bandwidth for large file uploads?

When a client sends Expect: 100-continue with the request headers (but withholds the body), the server can inspect the headers first. If the upload would be rejected (wrong content type, too large, missing auth), the server sends a 4xx response immediately and the client never uploads the body. Without this mechanism, the client would upload the entire file (potentially gigabytes) before learning it was rejected. The savings are proportional to the file size: for a 1GB upload rejected after headers, you save 1GB of bandwidth and minutes of upload time.

What happens to the HTTP connection after a 101 Switching Protocols response?

After the server sends 101, the TCP connection is no longer an HTTP connection. It becomes whatever protocol was negotiated in the Upgrade header, most commonly WebSocket. The server and client communicate using the new protocol's framing, not HTTP request/response pairs. The connection remains open until either side closes it using the new protocol's close mechanism (e.g., WebSocket close frame). HTTP middleware, logging, and error handling no longer apply to this connection after the upgrade.

How much performance improvement does 103 Early Hints provide?

103 Early Hints typically saves 100-500ms of page load time, depending on server think time and the number of critical resources hinted. The savings equal the server processing time that would otherwise be wasted with the browser idle. If your server takes 300ms to generate HTML, and you hint 3 critical resources, the browser starts downloading them 300ms earlier. Shopify reported 0.5-1 second improvements in Largest Contentful Paint (LCP) after implementing 103. The benefit is largest for dynamic pages with server-side rendering and multiple critical CSS/JS/font resources.

Why is 102 Processing rarely used in modern APIs?

102 Processing was designed for WebDAV operations that could take minutes (copying directory trees). Modern APIs handle long-running operations differently: return 202 Accepted immediately with a job ID, then let clients poll a status endpoint or subscribe to webhooks for completion. This approach is more flexible (works across HTTP/1.1, HTTP/2, and load balancers), more resilient (survives connection drops), and more cacheable. Most HTTP client libraries and browsers don't handle 102 well, making it impractical outside legacy WebDAV systems.

Which browsers support 103 Early Hints and what Link types are allowed?

Chrome (v103+), Edge, and Firefox support 103 Early Hints. Safari added support in version 17. For security, browsers only process rel=preload and rel=preconnect in 103 responses; other rel values (prefetch, dns-prefetch, modulepreload) are ignored. The Link header must specify the 'as' attribute for preload (as=style, as=script, as=font, as=image). CDNs like Cloudflare can generate 103 from cached Link headers even if your origin doesn't send them, making adoption easier for dynamic sites.

HTTP 1xx Informational Responses