Non-Authoritative Information

2xx · Success

Overview

203 Non-Authoritative Information signals that the returned metadata (headers, etc.) came from a proxy or local copy rather than the origin server. The status is still a success (2xx), but it warns that the response may differ from the original.

Typically a transforming proxy — image compression, ad stripping, content filtering — alters an origin 200 response and swaps the status to 203 to disclose that.

When it happens

Used when a mobile-optimizing proxy or corporate gateway modifies the origin body/headers before passing the response to the client.
It can also appear when a cache or mirror serves a representation it cannot guarantee is identical to the origin.

Request / Response example

Request

GET /page.html HTTP/1.1
Host: example.com

Response

HTTP/1.1 203 Non-Authoritative Information
Content-Type: text/html
Warning: 214 proxy.example "Transformation applied"

<html>...(proxy-modified copy)...</html>

In code

HTTP/1.1 203 Non-Authoritative Information
Via: 1.1 transforming-proxy
Warning: 214 - "Transformation applied"

Notes

It is rarely seen in practice; most proxies keep a plain 200 even after transforming a response.
The transformation is often also flagged with a Warning: 214 header, so reading both signals helps you judge origin integrity.

Related status codes

200OK

Related headers

WarningAge

Specification

RFC 9110MDN docs

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