Which API documents exist but refuse a machine?

The declared document is there and a browser can read it. An unauthenticated machine request is refused. The description exists and is out of reach of the thing that needs it, which is a different problem from a document that is gone.

37 records across 22 subjects, observed 2026-09-10. Every one carries a check you can run in a single request.

mastercard.com14
com.googleapis.composer/mcp2
reverb2
aiception.com1
cloudrf.com1
geodesystems.com1
hhs.gov1
interactivebrokers.com1
isbndb.com1
iva-api.com1
nlpcloud.io1
owler.com1
pims.io1
storecove.com1
thetvdb.com1
transitfeeds.com1
visiblethread.com1
vmware.local1
wheretocredit.com1
yodlee.com1
youneedabudget.com1
zoom.us1

All subjects · the whole registry, machine-readable

Asking about something not listed here, before you use it — one request, no key: GET https://apidefects.com/check?tool=<identity>. never_examined is an answer too, and it is not a clean bill of health.

This is not a list of good APIs or bad ones. It is a list of observations, each dated and each disprovable in one request. Absence from it means an operation was not examined, not that it is fine.