"OpenSSH for Windows" version
8.1.0.0

Client OperatingSystem
Windows 10 Enterprise

What is failing
ssh-add -s 'C:\Program Files\OpenSC Project\OpenSC\pkcs11\opensc-pkcs11.dll'

Using the -s "Add keys from PKCS#11 provider", i.e. using ssh-agent with smart cards does not work.

Expected output

Identity added ...

Actual output

Enter passphrase for PKCS#11:
Could not add card "C:\Program Files\OpenSC Project\OpenSC\pkcs11\opensc-pkcs11.dll": communication with agent failed

ssh-add is able to communicate with the agent. ssh-add without any options adds the default .ssh/id_rsa file, ssh-add -l is able to list it. ssh-add -s PKCS#11 does not work, although it does prompt for a PIN, so it obviously recognizes the -s option.

FWIW, ssh.exe is able to use a smart card with the option -I 'C:\Program Files\OpenSC Project\OpenSC\pkcs11\opensc-pkcs11.dll', it's just ssh-agent that is failing.