diff --git a/connect.go b/connect.go
index d55b211..cbe5ee0 100644
--- a/connect.go
+++ b/connect.go
@@ -11,6 +11,7 @@ import (
 	"net"
 	"os"
 	"os/signal"
+	"sync"
 	"syscall"
 	"time"
 
@@ -53,6 +54,9 @@ type Connect struct {
 	// Set the TTY to be used as the input and output for the Session/Cmd.
 	PtyRelayTty *os.File
 
+	// StdoutMutex is a mutex for use Stdout.
+	StdoutMutex *sync.Mutex
+
 	// CheckKnownHosts if true, check knownhosts.
 	// Ignored if HostKeyCallback is set.
 	// Set it before CraeteClient.
@@ -139,7 +143,7 @@ func (c *Connect) CreateClient(host, port, user string, authMethods []ssh.AuthMe
 				// append default files
 				c.KnownHostsFiles = append(c.KnownHostsFiles, "~/.ssh/known_hosts")
 			}
-			config.HostKeyCallback = c.verifyAndAppendNew
+			config.HostKeyCallback = c.VerifyAndAppendNew
 		} else {
 			config.HostKeyCallback = ssh.InsecureIgnoreHostKey()
 		}
diff --git a/knownhosts.go b/knownhosts.go
index cf7fff6..7973ce1 100644
--- a/knownhosts.go
+++ b/knownhosts.go
@@ -10,6 +10,7 @@ import (
 	"net"
 	"os"
 	"os/signal"
+	"slices"
 	"strings"
 	"syscall"
 	"text/template"
@@ -36,19 +37,19 @@ type OverwriteInventory struct {
 // If is no problem, error returns Nil.
 //
 // 【参考】: https://github.com/tatsushid/minssh/blob/57eae8c5bcf5d94639891f3267f05251f05face4/pkg/minssh/minssh.go#L190-L237
-func (c *Connect) verifyAndAppendNew(hostname string, remote net.Addr, key ssh.PublicKey) (err error) {
+func (c *Connect) VerifyAndAppendNew(hostname string, remote net.Addr, key ssh.PublicKey) (err error) {
 	// set TextAskWriteKnownHosts default text
 	if len(c.TextAskWriteKnownHosts) == 0 {
 		c.TextAskWriteKnownHosts += "The authenticity of host '{{.Address}} ({{.RemoteAddr}})' can't be established.\n"
 		c.TextAskWriteKnownHosts += "RSA key fingerprint is {{.Fingerprint}}\n"
-		c.TextAskWriteKnownHosts += "Are you sure you want to continue connecting (yes/no)?"
+		c.TextAskWriteKnownHosts += "Are you sure you want to continue connecting ((yes|y)/(no|n))? "
 	}
 
 	// set TextAskOverwriteKnownHosts default text
 	if len(c.TextAskOverwriteKnownHosts) == 0 {
 		c.TextAskOverwriteKnownHosts += "The authenticity of host '{{.Address}} ({{.RemoteAddr}})' can't be established.\n"
 		c.TextAskOverwriteKnownHosts += "Old key: {{.OldKeyText}}\n"
-		c.TextAskOverwriteKnownHosts += "Are you sure you want to overwrite {{.Fingerprint}}, continue connecting (yes/no)?"
+		c.TextAskOverwriteKnownHosts += "Are you sure you want to overwrite {{.Fingerprint}}, continue connecting ((yes|y)/(no|n))? "
 	}
 
 	// check count KnownHostsFiles
@@ -79,6 +80,12 @@ func (c *Connect) verifyAndAppendNew(hostname string, remote net.Addr, key ssh.P
 	filepath := knownHostsFiles[0]
 	var line int
 
+	// check mutex
+	if c.StdoutMutex != nil {
+		c.StdoutMutex.Lock()
+		defer c.StdoutMutex.Unlock()
+	}
+
 	// check error
 	keyErr, ok := err.(*knownhosts.KeyError)
 	if !ok || len(keyErr.Want) > 0 {
@@ -107,7 +114,7 @@ func (c *Connect) verifyAndAppendNew(hostname string, remote net.Addr, key ssh.P
 
 	err = writeKnownHostsKey(filepath, line, hostname, remote, key)
 
-	return nil
+	return err
 }
 
 // askAddingUnknownHostKey
@@ -150,9 +157,9 @@ func askAddingUnknownHostKey(text string, address string, remote net.Addr, key s
 			return false, fmt.Errorf("failed to read answer: %s", err)
 		}
 		answer = string(strings.ToLower(strings.TrimSpace(answer)))
-		if answer == "yes" {
+		if slices.Contains([]string{"yes", "y"}, answer) {
 			return true, nil
-		} else if answer == "no" {
+		} else if slices.Contains([]string{"no", "n"}, answer) {
 			return false, nil
 		}
 		fmt.Print("Please type 'yes' or 'no': ")
@@ -204,7 +211,7 @@ func askOverwriteKnownHostKey(text string, address string, remote net.Addr, key
 		} else if answer == "no" {
 			return false, nil
 		}
-		fmt.Print("Please type 'yes' or 'no': ")
+		fmt.Print("Please type 'yes|y' or 'no|n': ")
 	}
 }