Host Key Verification Failed

Symptom

The host is unknown and strict checking is on:

No ED25519 host key is known for ssh.<INSTANCE>.git.eu and you have requested strict checking.
Host key verification failed.
fatal: Could not read from remote repository.

The host key changed since your last connection:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
...
Offending ED25519 key in ~/.ssh/known_hosts:1
  remove with:
  ssh-keygen -f '~/.ssh/known_hosts' -R 'ssh.<INSTANCE>.git.eu'
...
Host key verification failed.

Cause

• Unknown host: ~/.ssh/known_hosts holds no key for the server, and the client refuses to ask (StrictHostKeyChecking yes, common in CI).
• Changed key: the server key differs from the one you saved. The instance may have been reinstalled or its key replaced, or another machine answers in its place.

git.eu does not publish its host key fingerprints yet: ask your instance operator for the fingerprint, through a channel other than this connection.

Diagnose

Print the fingerprint the server shows now:

ssh-keyscan -t ed25519 ssh.<INSTANCE>.git.eu | ssh-keygen -lf -

Compare the SHA256: value with the one your instance operator gives you.

Fix

Option A - Unknown host: save the confirmed key (recommended)

Once the fingerprint matches, save the key:

ssh-keyscan -t ed25519 ssh.<INSTANCE>.git.eu >> ~/.ssh/known_hosts

Option B - Changed key: replace it after confirmation

Only after the operator confirms the new fingerprint, remove the old key, then save the new one as in Option A:

ssh-keygen -R 'ssh.<INSTANCE>.git.eu'

Verify

git ls-remote git@ssh.<INSTANCE>.git.eu:<USERNAME>/<REPO>.git

No Host key verification failed line appears.

Prevent

• In CI, pin the confirmed key in the job's known_hosts instead of turning host checks off.

Related

• Permission Denied (publickey)
• SSH Connection Failed
• Access over SSH
• Knowledge Base - SSH