dvc add: "Invalid private key" when using agent and specifying key with public key

[weberbr]

Bug Report

Description

I use an ssh agent with dozens of keys at a time. To avoid "Too many authentication failures" I must specify which private key my agent should use by adding an Identity File <path to corresponding public key> line to each of my hosts. Somewhere in the SSH stack that DVC uses (based on traceback info I'm guessing sshfs), this key loaded as a private key, which it of course fails the format checks on.

I took a look in the dvc_ssh client.py as well, and I'm not sure this case is handled properly there either.

Traceback I grabbed by setting logging output file: dvc_ssh_debug.log

Reproduce

1. Generate an ssh keypair with ssh-keygen
2. Install the public key to an authorized_keys file on a remote server with ssh-copy-id
3. Install the private key to a local ssh-agent with ssh-add
4. Edit your ~/.ssh/config to include:

Host <remote server>
    Hostname <remote hostname>
    IdentityFile <path to key>.pub

5. In a new repo: dvc init
6. dvc remote add remotessh ssh://<remote server>/<path>
7. dvc add -r remotessh --to-remote mydata

Throws: ERROR: unexpected error - Invalid private key

Expected

dvc specifies to the agent which key to use using the provided public key

Environment information

Output of dvc doctor:

DVC version: 3.59.1 (pip)
-------------------------
Platform: Python 3.13.1 on Linux-6.8.0-52-generic-x86_64-with-glibc2.35
Subprojects:
        dvc_data = 3.16.9
        dvc_objects = 5.1.0
        dvc_render = 1.0.2
        dvc_task = 0.40.2
        scmrepo = 3.3.10
Supports:
        http (aiohttp = 3.11.12, aiohttp-retry = 2.9.1),
        https (aiohttp = 3.11.12, aiohttp-retry = 2.9.1),
        ssh (sshfs = 2025.2.0)
Config:
        Global: /home/myuser/.config/dvc
        System: /etc/xdg/dvc
Cache types: hardlink, symlink
Cache directory: ext4 on /dev/mapper/vgubuntu-root
Caches: local
Remotes: ssh, ssh
Workspace directory: ext4 on /dev/mapper/vgubuntu-root
Repo: dvc, git
Repo.site_cache_dir: /var/tmp/dvc/repo/6cf41b69515915320785cfda6839dd61

Additional Information (if any):

[weberbr]

It appears the issue is in either sshfs or asyncssh: I've filed another issue with sshfs here fsspec/sshfs#59