How to debug Jenkins error message "could not find a suitable ssh-agent provider"?

Check the version of your ssh-agent used by Jenkins.
This bug (for linux, but could apply to Windows too) reports (10 days ago, January 2014) this very same error message:
"JENKINS-20276: Native Library Error after upgrading ssh-agent from 1.3 to 1.4".

Downgrading to 1.3 resolves the issue.

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Update 2019, five years later: as commented, this should be fixed now.

ssh-agent.exe is part of a Git for Windows distribution

D:\git\git>where ssh-agent.exe
D:\prgs\gits\current\usr\bin\ssh-agent.exe

(provided path/to/git/usr/bin is first in the %PATH% used by Jenkins)