Is your sitemap telling the truth?
It is the list of pages you hand search. Nobody reads it after the day it was made.
The file everyone has and nobody checks
A sitemap is a claim: these are my pages, and this is when they changed. Search takes the claim seriously, which is exactly why a wrong one costs you.
A wrong list is worse than none
Search treats this file as your own account of your best pages. Hand it addresses that redirect or repeat and it spends its budget on those, and trusts the list less next time.
It breaks silently, and looks fine
The most common failure is a file that is perfectly valid and lists nothing, or lists the http versions of everything from before the site moved. Both load without a single error.
Nobody opens it after the first day
It is generated once and then never read again, while the site changes around it for years. Two minutes of reading it is the whole cost.
A file in, an answer out
Open yoursite.com/sitemap.xml
It is public. If an error page comes back instead, that is already the answer.
Paste the whole thing
Index files that point at other sitemaps are read too. It runs in your browser.
See what search is handed
Every problem in the list, ranked by what it costs you, with the fix for each.
What it checks
Whether it is a sitemap at all
An error page served at that address is the failure nothing warns you about. Checked before anything else, because the rest is meaningless if this is true.
http addresses and bare paths
A site that moved to https and never rebuilt this file lists every page at an address that now redirects. Paths with no domain are invalid outright.
Pages listed more than once
Usually two systems writing to one file, or pagination looping. It wastes crawl budget and it means the file is not built the way you think.
Dates that are all the build time
Stamping every page with the moment of the build is not a lastmod. Search learns to ignore the signal on sites that do it.
Addresses that do not belong
Carts, search results, feeds, tag and author archives. Listing them says these are among your best pages.
Domains and limits
One sitemap may only list one domain, and www counts as a different one. Fifty thousand addresses or 50MB and the file is rejected whole.
Straight talk about what it cannot see
It reads the list you paste. It cannot open the addresses on it, and it says so rather than implying more.
It cannot open the pages
An address that 404s or redirects looks exactly like a working one in a list. Checking those needs a crawl, and this is not one.
It cannot tell you what is missing
A page you never added is invisible to a check that only sees the file. Compare the count here against the page count you expect.
Listed is not indexed
A sitemap is a request. Search decides, and a thin or duplicate page stays out however carefully it is listed.
Your file never leaves your machine
The whole check runs in your browser. Nothing is uploaded, which is also why there is no limit on it.
Two minutes on a file you have never opened
Check it alongside your robots.txt. Between them they decide what search is allowed to see and what it is told to look at.