100 Continue

What it means

HTTP 100 Continue is a provisional response. When a client sends a request with an Expect: 100-continue header — usually before uploading a large body — the server replies with 100 to signal that the request line and headers are acceptable and the client may proceed to send the body. It is never the final response to a request.

When it happens

It happens during large POST or PUT uploads where a client wants confirmation that the server will accept the request before transmitting a potentially large payload. Most browsers and HTTP libraries handle it automatically, so you rarely see it directly.

How to fix it

• Nothing to fix — 100 Continue is normal protocol behaviour, not an error.
• If an upload stalls, confirm your client and any proxies handle the Expect: 100-continue header correctly.
• For misbehaving intermediaries, you can drop the Expect header so the client sends the body immediately.

SEO impact

None. Informational responses are invisible to search engines, which only index the final response a resource returns.