diff --git a/irc/client.go b/irc/client.go
index d4d1813b..70f75194 100644
--- a/irc/client.go
+++ b/irc/client.go
@@ -235,7 +235,10 @@ func (client *Client) ChangeNickname(nickname string) {
 	}
 }
 
-func (client *Client) Reply(reply string) {
+func (client *Client) Reply(reply string, args ...interface{}) {
+	if len(args) > 0 {
+		reply = fmt.Sprintf(reply, args...)
+	}
 	client.socket.Write(reply)
 }
 
diff --git a/irc/commands.go b/irc/commands.go
index 34f39c95..40431d49 100644
--- a/irc/commands.go
+++ b/irc/commands.go
@@ -708,21 +708,13 @@ func NewOperCommand(args []string) (editableCommand, error) {
 
 type CapCommand struct {
 	BaseCommand
-	subCommand CapSubCommand
-	args       []string
+	subCommand   CapSubCommand
+	capabilities CapabilitySet
 }
 
 func (msg *CapCommand) String() string {
-	return fmt.Sprintf("CAP(subCommand=%s, args=%s)", msg.subCommand, msg.args)
-}
-
-func (msg *CapCommand) Capabilities() []Capability {
-	strs := strings.Split(msg.args[0], " ")
-	caps := make([]Capability, len(strs))
-	for index, str := range strs {
-		caps[index] = Capability(str)
-	}
-	return caps
+	return fmt.Sprintf("CAP(subCommand=%s, capabilities=%s)",
+		msg.subCommand, msg.capabilities)
 }
 
 func NewCapCommand(args []string) (editableCommand, error) {
@@ -731,8 +723,15 @@ func NewCapCommand(args []string) (editableCommand, error) {
 	}
 
 	cmd := &CapCommand{
-		subCommand: CapSubCommand(args[0]),
-		args:       args[1:],
+		subCommand:   CapSubCommand(strings.ToUpper(args[0])),
+		capabilities: make(CapabilitySet),
+	}
+
+	if len(args) > 1 {
+		strs := spacesExpr.Split(args[1], -1)
+		for _, str := range strs {
+			cmd.capabilities[Capability(str)] = true
+		}
 	}
 	return cmd, nil
 }
diff --git a/irc/server.go b/irc/server.go
index 245dcac2..d3ac14b5 100644
--- a/irc/server.go
+++ b/irc/server.go
@@ -301,28 +301,36 @@ func (msg *CapCommand) HandleRegServer(server *Server) {
 	switch msg.subCommand {
 	case CAP_LS:
 		client.capState = CapNegotiating
-		client.Reply(fmt.Sprintf("CAP LS :%d", MultiPrefix))
+		client.Reply("CAP LS * :%s", SupportedCapabilities)
 
 	case CAP_LIST:
-		client.Reply(fmt.Sprintf("CAP LIST :%s", client.capabilities))
+		client.Reply("CAP LIST * :%s", client.capabilities)
 
 	case CAP_REQ:
 		client.capState = CapNegotiating
-		caps := msg.Capabilities()
-		if (len(caps) != 1) && (caps[0] != MultiPrefix) {
-			client.Reply("CAP NAK :" + msg.args[0])
-			return
+		for capability := range msg.capabilities {
+			if !SupportedCapabilities[capability] {
+				client.Reply("CAP NAK * :%s", msg.capabilities)
+				return
+			}
 		}
-		for _, capability := range caps {
+		for capability := range msg.capabilities {
 			client.capabilities[capability] = true
 		}
-		client.Reply("CAP ACK :" + msg.args[0])
+		client.Reply("CAP ACK * :%s", msg.capabilities)
 
 	case CAP_CLEAR:
+		format := strings.TrimRight(
+			strings.Repeat("%s%s ", len(client.capabilities)), " ")
+		args := make([]interface{}, len(client.capabilities))
+		index := 0
 		for capability := range client.capabilities {
+			args[index] = Disable
+			args[index+1] = capability
+			index += 2
 			delete(client.capabilities, capability)
 		}
-		client.Reply("CAP ACK :")
+		client.Reply("CAP ACK * :"+format, args...)
 
 	case CAP_END:
 		client.capState = CapNegotiated
diff --git a/irc/types.go b/irc/types.go
index ec92a6a4..fc628bdd 100644
--- a/irc/types.go
+++ b/irc/types.go
@@ -16,6 +16,10 @@ type Capability string
 
 type CapModifier rune
 
+func (mod CapModifier) String() string {
+	return string(mod)
+}
+
 type CapState uint
 
 type CapabilitySet map[Capability]bool
@@ -25,6 +29,7 @@ func (set CapabilitySet) String() string {
 	index := 0
 	for capability := range set {
 		strs[index] = string(capability)
+		index += 1
 	}
 	return strings.Join(strs, " ")
 }