diff --git a/gencapdefs.py b/gencapdefs.py
index e1cfbc7f..a64ef143 100644
--- a/gencapdefs.py
+++ b/gencapdefs.py
@@ -219,6 +219,12 @@ CAPDEFS = [
         url="https://github.com/ircv3/ircv3-specifications/pull/527",
         standard="proposed IRCv3",
     ),
+   CapDef(
+        identifier="ExtendedISupport",
+        name="draft/extended-isupport",
+        url="https://github.com/ircv3/ircv3-specifications/pull/543",
+        standard="proposed IRCv3",
+    ),
 ]
 
 def validate_defs():
diff --git a/irc/caps/constants.go b/irc/caps/constants.go
index 57f088e3..38497a2b 100644
--- a/irc/caps/constants.go
+++ b/irc/caps/constants.go
@@ -64,10 +64,11 @@ const (
 	BotTagName = "bot"
 	// https://ircv3.net/specs/extensions/chathistory
 	ChathistoryTargetsBatchType = "draft/chathistory-targets"
+	ExtendedISupportBatchType   = "draft/extended-isupport"
 )
 
 func init() {
-	nameToCapability = make(map[string]Capability)
+	nameToCapability = make(map[string]Capability, numCapabs)
 	for capab, name := range capabilityNames {
 		nameToCapability[name] = Capability(capab)
 	}
diff --git a/irc/caps/defs.go b/irc/caps/defs.go
index 18ff5759..9847ae4a 100644
--- a/irc/caps/defs.go
+++ b/irc/caps/defs.go
@@ -7,7 +7,7 @@ package caps
 
 const (
 	// number of recognized capabilities:
-	numCapabs = 34
+	numCapabs = 35
 	// length of the uint32 array that represents the bitset:
 	bitsetLen = 2
 )
@@ -53,6 +53,10 @@ const (
 	// https://github.com/ircv3/ircv3-specifications/pull/362
 	EventPlayback Capability = iota
 
+	// ExtendedISupport is the proposed IRCv3 capability named "draft/extended-isupport":
+	// https://github.com/ircv3/ircv3-specifications/pull/543
+	ExtendedISupport Capability = iota
+
 	// Languages is the proposed IRCv3 capability named "draft/languages":
 	// https://gist.github.com/DanielOaks/8126122f74b26012a3de37db80e4e0c6
 	Languages Capability = iota
@@ -163,6 +167,7 @@ var (
 		"draft/channel-rename",
 		"draft/chathistory",
 		"draft/event-playback",
+		"draft/extended-isupport",
 		"draft/languages",
 		"draft/message-redaction",
 		"draft/multiline",
diff --git a/irc/client.go b/irc/client.go
index 218f4d3d..bb5badde 100644
--- a/irc/client.go
+++ b/irc/client.go
@@ -179,6 +179,8 @@ type Session struct {
 
 	batchCounter atomic.Uint32
 
+	isupportSentPrereg bool
+
 	quitMessage string
 
 	awayMessage string
diff --git a/irc/commands.go b/irc/commands.go
index ccbec383..4bd88dd7 100644
--- a/irc/commands.go
+++ b/irc/commands.go
@@ -152,6 +152,10 @@ func init() {
 			handler:   isonHandler,
 			minParams: 1,
 		},
+		"ISUPPORT": {
+			handler:      isupportHandler,
+			usablePreReg: true,
+		},
 		"JOIN": {
 			handler:   joinHandler,
 			minParams: 1,
diff --git a/irc/handlers.go b/irc/handlers.go
index ecd23dbc..17fd905f 100644
--- a/irc/handlers.go
+++ b/irc/handlers.go
@@ -1320,6 +1320,15 @@ func isonHandler(server *Server, client *Client, msg ircmsg.Message, rb *Respons
 	return false
 }
 
+// ISUPPORT
+func isupportHandler(server *Server, client *Client, msg ircmsg.Message, rb *ResponseBuffer) bool {
+	server.RplISupport(client, rb)
+	if !client.registered {
+		rb.session.isupportSentPrereg = true
+	}
+	return false
+}
+
 // JOIN <channel>{,<channel>} [<key>{,<key>}]
 func joinHandler(server *Server, client *Client, msg ircmsg.Message, rb *ResponseBuffer) bool {
 	// #1417: allow `JOIN 0` with a confirmation code
diff --git a/irc/help.go b/irc/help.go
index 343bd717..a11bb725 100644
--- a/irc/help.go
+++ b/irc/help.go
@@ -259,6 +259,11 @@ appropriate channel privs.`,
 		text: `ISON <nickname>{ <nickname>}
 
 Returns whether the given nicks exist on the network.`,
+	},
+	"isupport": {
+		text: `ISUPPORT
+
+Returns RPL_ISUPPORT lines describing the server's capabilities.`,
 	},
 	"join": {
 		text: `JOIN <channel>{,<channel>} [<key>{,<key>}]
diff --git a/irc/server.go b/irc/server.go
index eef47c2c..906633d2 100644
--- a/irc/server.go
+++ b/irc/server.go
@@ -434,7 +434,9 @@ func (server *Server) playRegistrationBurst(session *Session) {
 	session.Send(nil, server.name, RPL_MYINFO, d.nick, server.name, Ver, rplMyInfo1, rplMyInfo2, rplMyInfo3)
 
 	rb := NewResponseBuffer(session)
-	server.RplISupport(c, rb)
+	if !(rb.session.capabilities.Has(caps.ExtendedISupport) && rb.session.isupportSentPrereg) {
+		server.RplISupport(c, rb)
+	}
 	if d.account != "" && session.capabilities.Has(caps.Persistence) {
 		reportPersistenceStatus(c, rb, false)
 	}
@@ -456,10 +458,17 @@ func (server *Server) playRegistrationBurst(session *Session) {
 
 // RplISupport outputs our ISUPPORT lines to the client. This is used on connection and in VERSION responses.
 func (server *Server) RplISupport(client *Client, rb *ResponseBuffer) {
+	server.sendRplISupportLines(client, rb, server.Config().Server.isupport.CachedReply)
+}
+
+func (server *Server) sendRplISupportLines(client *Client, rb *ResponseBuffer, lines [][]string) {
+	if rb.session.capabilities.Has(caps.ExtendedISupport) {
+		batchID := rb.StartNestedBatch("chathistory", caps.ExtendedISupportBatchType)
+		defer rb.EndNestedBatch(batchID)
+	}
 	translatedISupport := client.t("are supported by this server")
 	nick := client.Nick()
-	config := server.Config()
-	for _, cachedTokenLine := range config.Server.isupport.CachedReply {
+	for _, cachedTokenLine := range lines {
 		length := len(cachedTokenLine) + 2
 		tokenline := make([]string, length)
 		tokenline[0] = nick
@@ -794,13 +803,19 @@ func (server *Server) applyConfig(config *Config) (err error) {
 	}
 
 	if !initial {
-		// push new info to all of our clients
-		for _, sClient := range server.clients.AllClients() {
-			for _, tokenline := range newISupportReplies {
-				sClient.Send(nil, server.name, RPL_ISUPPORT, append([]string{sClient.nick}, tokenline...)...)
+		// send 005 updates (somewhat rare)
+		if len(newISupportReplies) != 0 {
+			for _, sClient := range server.clients.AllClients() {
+				for _, session := range sClient.Sessions() {
+					rb := NewResponseBuffer(session)
+					server.sendRplISupportLines(sClient, rb, newISupportReplies)
+					rb.Send(false)
+				}
 			}
+		}
 
-			if sendRawOutputNotice {
+		if sendRawOutputNotice {
+			for _, sClient := range server.clients.AllClients() {
 				sClient.Notice(sClient.t("This server is in debug mode and is logging all user I/O. If you do not wish for everything you send to be readable by the server owner(s), please disconnect."))
 			}
 		}