Quick answer
An XML sitemap should be served with an XML content type such as application/xml or text/xml. NOTABIS reports sitemap-invalid-content-type when a sitemap body parses but its response does not advertise an XML content type. The file may still be readable, but correcting the header removes ambiguity for clients and monitoring tools.
What the issue means
The Content-Type response header describes how a client should interpret a body:
Content-Type: application/xml; charset=UTF-8
A web server that serves the same XML as text/html, or omits the header, can make the resource look like a normal page even when its markup happens to parse. Content type is separate from XML validity, the sitemap namespace, and the quality of listed URLs.
Why it matters for SEO
Search engines can often parse well-formed XML despite a weak header, but a clear type makes the sitemap’s purpose unambiguous and avoids failures in intermediaries, validators, or deployment checks. This is a crawlability hygiene issue, not proof that pages are unindexed or that rankings are being reduced.
How NOTABIS detects it
For each sitemap candidate it can fetch and parse, NOTABIS lowercases the response Content-Type value and checks whether it contains xml. If not, it records sitemap-invalid-content-type at sitemap scope. The candidate must first return a non-error response and valid sitemap XML; a fetch failure or malformed body is reported through another rule. Declared sitemaps receive the corresponding issue in the error path as well.
How to check it yourself
Request headers without relying on browser formatting:
curl -sS -D - https://example.com/sitemap.xml -o sitemap.xml
Confirm the final response, Content-Type, XML root, and namespace. Inspect CDN, object-storage, framework, and compression settings if the origin and public response differ.
How to fix it
Configure the route, web server, or storage object to send an XML content type and preserve it through redirects and a CDN. Do not “fix” the issue by changing the body to HTML or by returning a generic page from the sitemap URL. Re-fetch the public URL and validate the XML and listed URLs after deployment.
When this is intentional or does not need fixing
An internal export that is not submitted or declared as a sitemap may use a different type, but a public sitemap should be explicit. A valid sitemap index and a URL-set document both use XML content. Keep any alternate download or debugging endpoint separate from the production sitemap URL.
Related issues
Review sitemap access, sitemap unavailability, and invalid sitemap namespaces. Google’s sitemap overview describes supported sitemap formats.
Run a NOTABIS audit
NOTABIS records the sitemap response headers and flags candidates whose advertised content type does not include XML.