diff --git a/ssh/agent/keyring.go b/ssh/agent/keyring.go
index c1b4361087..41509c0d66 100644
--- a/ssh/agent/keyring.go
+++ b/ssh/agent/keyring.go
@@ -34,6 +34,8 @@ var errLocked = errors.New("agent: locked")
 
 // NewKeyring returns an Agent that holds keys in memory.  It is safe
 // for concurrent use by multiple goroutines.
+//
+// The returned Agent only supports the "lifetime" constraint.
 func NewKeyring() Agent {
 	return &keyring{}
 }
@@ -146,12 +148,16 @@ func (r *keyring) List() ([]*Key, error) {
 // Insert adds a private key to the keyring. If a certificate
 // is given, that certificate is added as public key. Note that
 // any constraints given are ignored.
+// Add returns an error if key contains ConfirmBeforeUse
 func (r *keyring) Add(key AddedKey) error {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	if r.locked {
 		return errLocked
 	}
+	if key.ConfirmBeforeUse {
+		return errors.New("agent: confirm before use constraint is not supported")
+	}
 	signer, err := ssh.NewSignerFromKey(key.PrivateKey)
 
 	if err != nil {
diff --git a/ssh/agent/keyring_test.go b/ssh/agent/keyring_test.go
index e9c90a3131..cd6aed29df 100644
--- a/ssh/agent/keyring_test.go
+++ b/ssh/agent/keyring_test.go
@@ -120,3 +120,16 @@ func TestAddDuplicateKey(t *testing.T) {
 		t.Fatal("key with the updated comment not found")
 	}
 }
+
+func TestAddKeyWithConfirmBeforeUse(t *testing.T) {
+	agent, cleanup := startKeyringAgent(t)
+	defer cleanup()
+	key := testPrivateKeys["rsa"]
+	err := agent.Add(AddedKey{
+		PrivateKey:       key,
+		ConfirmBeforeUse: true,
+	})
+	if err == nil {
+		t.Fatal("adding a key with confirm before use constraint succeeded")
+	}
+}
diff --git a/ssh/test/agent_unix_test.go b/ssh/test/agent_unix_test.go
index 9257bfe1bc..fb99c9073b 100644
--- a/ssh/test/agent_unix_test.go
+++ b/ssh/test/agent_unix_test.go
@@ -24,9 +24,8 @@ func TestAgentForward(t *testing.T) {
 		t.Fatalf("Error adding key: %s", err)
 	}
 	if err := keyring.Add(agent.AddedKey{
-		PrivateKey:       testPrivateKeys["ecdsa"],
-		ConfirmBeforeUse: true,
-		LifetimeSecs:     3600,
+		PrivateKey:   testPrivateKeys["ecdsa"],
+		LifetimeSecs: 3600,
 	}); err != nil {
 		t.Fatalf("Error adding key with constraints: %s", err)
 	}