That is interesting.
Which version of Libgcrypt are you using. (gpg --version shows it)

When did you create your GPG key of ed25519?
Or... did you register your SSH key by ssh-add?
If so, gnupg/agent/command-ssh.c:2147 doesn't add prefix 0x40.
That's the problem.
Sorry, that's my badness. I didn't look through this code path.

If you can binary-edit, please add
prefix @ (0x40) to the public key in the *.key file.
There is the sequence like:

...(3:ecc(5:curve7:Ed25519)(5:flags5:eddsa)(1:q32...

This shoule be changed:

      ...(3:ecc(5:curve7:Ed25519)(5:flags5:eddsa)(1:q33@...

Sorry for your inconvenience.

Which version of Libgcrypt are you using. (gpg --version shows it)

libgcrypt-config --version
1.6.4

When did you create your GPG key of ed25519?
Or... did you register your SSH key by ssh-add?

The ssh key was generated by "ssh-keygen -t ed25519" and added by ssh-add.

If you can binary-edit, please add
prefix @ (0x40) to the public key in the *.key file.

I change from:

(3:ecc(5:curve7:Ed25519)(5:flags5:eddsa)(1:q32:..

to:

  (3:ecc(5:curve7:Ed25519)(5:flags5:eddsa)(1:q33@:..

then 'ssh-add -L', and get
The agent has no identities.

It seems not working for me. By the way, I switch to 2.1.7 version which
doesn't have this problem.

This has been fixed in the repo (7d5999f). AFAICS, you need to ssh-add the key
again.

If it's Bash, it is like:

$ rm -i ~/.gnupg/private-keys-v1.d/$(gpg-connect-agent "KEYINFO --ssh-list"

/bye | awk '{print $3}').key

Thanks. I'm very happy with gnupg 2.1.