diff --git a/imapclient/parse.go b/imapclient/parse.go
index 08bf7ac..93923ba 100644
--- a/imapclient/parse.go
+++ b/imapclient/parse.go
@@ -131,6 +131,7 @@ var knownCodes = stringMap(
 	// With parameters.
 	"BADCHARSET", "CAPABILITY", "PERMANENTFLAGS", "UIDNEXT", "UIDVALIDITY", "UNSEEN", "APPENDUID", "COPYUID",
 	"HIGHESTMODSEQ", "MODIFIED",
+	"INPROGRESS", // ../rfc/9585:104
 )
 
 func stringMap(l ...string) map[string]struct{} {
@@ -222,6 +223,30 @@ func (c *Conn) xrespCode() (string, CodeArg) {
 		c.xspace()
 		modified := c.xuidset()
 		codeArg = CodeModified(NumSet{Ranges: modified})
+	case "INPROGRESS":
+		// ../rfc/9585:238
+		var tag string
+		var current, goal *uint32
+		if c.space() {
+			c.xtake("(")
+			tag = c.xquoted()
+			c.xspace()
+			if c.peek('n') || c.peek('N') {
+				c.xtake("nil")
+			} else {
+				v := c.xuint32()
+				current = &v
+			}
+			c.xspace()
+			if c.peek('n') || c.peek('N') {
+				c.xtake("nil")
+			} else {
+				v := c.xnzuint32()
+				goal = &v
+			}
+			c.xtake(")")
+		}
+		codeArg = CodeInProgress{tag, current, goal}
 	}
 	return W, codeArg
 }
diff --git a/imapclient/protocol.go b/imapclient/protocol.go
index bd6978c..0c8f29c 100644
--- a/imapclient/protocol.go
+++ b/imapclient/protocol.go
@@ -159,6 +159,32 @@ func (c CodeHighestModSeq) CodeString() string {
 	return fmt.Sprintf("HIGHESTMODSEQ %d", c)
 }
 
+// "INPROGRESS" response code.
+type CodeInProgress struct {
+	Tag     string // Nil is empty string.
+	Current *uint32
+	Goal    *uint32
+}
+
+func (c CodeInProgress) CodeString() string {
+	// ABNF allows inprogress-tag/state with all nil values. Doesn't seem useful enough
+	// to keep track of.
+	if c.Tag == "" && c.Current == nil && c.Goal == nil {
+		return "INPROGRESS"
+	}
+
+	// todo: quote tag properly
+	current := "nil"
+	goal := "nil"
+	if c.Current != nil {
+		current = fmt.Sprintf("%d", *c.Current)
+	}
+	if c.Goal != nil {
+		goal = fmt.Sprintf("%d", *c.Goal)
+	}
+	return fmt.Sprintf("INPROGRESS (%q %s %s)", c.Tag, current, goal)
+}
+
 // RespText represents a response line minus the leading tag.
 type RespText struct {
 	Code    string  // The first word between [] after the status.
diff --git a/imapserver/search.go b/imapserver/search.go
index 08e76e2..9ee69c8 100644
--- a/imapserver/search.go
+++ b/imapserver/search.go
@@ -14,6 +14,10 @@ import (
 	"slices"
 )
 
+// If last search output was this long ago, we write an untagged inprogress
+// response. Changed during tests. ../rfc/9585:109
+var inProgressPeriod = time.Duration(10 * time.Second)
+
 // Search returns messages matching criteria specified in parameters.
 //
 // State: Selected
@@ -138,17 +142,38 @@ func (c *conn) cmdxSearch(isUID bool, tag, cmd string, p *parser) {
 	var expungeIssued bool
 	var maxModSeq store.ModSeq
 
+	// We periodically send an untagged OK with INPROGRESS code while searching, to let
+	// clients doing slow searches know we're still working.
+	inProgressLast := time.Now()
+	// Only respond with tag if it can't be confused as end of response code. ../rfc/9585:122
+	inProgressTag := "nil"
+	if !strings.Contains(tag, "]") {
+		inProgressTag = dquote(tag).pack(c)
+	}
+
 	var uids []store.UID
 	c.xdbread(func(tx *bstore.Tx) {
 		c.xmailboxID(tx, c.mailboxID) // Validate.
 		runlock()
 		runlock = func() {}
 
-		// Normal forward search when we don't have MAX only.
+		// Normal forward search when we don't have MAX only. We only send an "inprogress"
+		// goal if we know how many messages we have to check.
+		forward := eargs == nil || max == 0 || len(eargs) != 1
+		reverse := max == 1 && (len(eargs) == 1 || min+max == len(eargs))
+		goal := "nil"
+		if len(c.uids) > 0 && forward != reverse {
+			goal = fmt.Sprintf("%d", len(c.uids))
+		}
+
 		var lastIndex = -1
-		if eargs == nil || max == 0 || len(eargs) != 1 {
+		if forward {
 			for i, uid := range c.uids {
 				lastIndex = i
+				if time.Since(inProgressLast) > inProgressPeriod {
+					c.writelinef("* OK [INPROGRESS (%s %d %s)] still searching", inProgressTag, i, goal)
+					inProgressLast = time.Now()
+				}
 				if match, modseq := c.searchMatch(tx, msgseq(i+1), uid, *sk, bodySearch, textSearch, &expungeIssued); match {
 					uids = append(uids, uid)
 					if modseq > maxModSeq {
@@ -161,8 +186,12 @@ func (c *conn) cmdxSearch(isUID bool, tag, cmd string, p *parser) {
 			}
 		}
 		// And reverse search for MAX if we have only MAX or MAX combined with MIN.
-		if max == 1 && (len(eargs) == 1 || min+max == len(eargs)) {
+		if reverse {
 			for i := len(c.uids) - 1; i > lastIndex; i-- {
+				if time.Since(inProgressLast) > inProgressPeriod {
+					c.writelinef("* OK [INPROGRESS (%s %d %s)] still searching", inProgressTag, len(c.uids)-1-i, goal)
+					inProgressLast = time.Now()
+				}
 				if match, modseq := c.searchMatch(tx, msgseq(i+1), c.uids[i], *sk, bodySearch, textSearch, &expungeIssued); match {
 					uids = append(uids, c.uids[i])
 					if modseq > maxModSeq {
diff --git a/imapserver/search_test.go b/imapserver/search_test.go
index 8594512..9a4f46d 100644
--- a/imapserver/search_test.go
+++ b/imapserver/search_test.go
@@ -260,6 +260,31 @@ func TestSearch(t *testing.T) {
 	tc.transactf("ok", `search charset utf-8 text "mox"`)
 	tc.xsearch(2, 3)
 
+	// Check for properly formed INPROGRESS response code.
+	orig := inProgressPeriod
+	inProgressPeriod = 0
+	tc.cmdf("tag1", "search undraft")
+	tc.response("ok")
+
+	inprogress := func(cur, goal uint32) imapclient.UntaggedResult {
+		return imapclient.UntaggedResult{
+			Status: "OK",
+			RespText: imapclient.RespText{
+				Code:    "INPROGRESS",
+				CodeArg: imapclient.CodeInProgress{Tag: "tag1", Current: &cur, Goal: &goal},
+				More:    "still searching",
+			},
+		}
+	}
+	tc.xuntagged(
+		imapclient.UntaggedSearch([]uint32{1, 2}),
+		// Due to inProgressPeriod 0, we get an inprogress response for each message in the mailbox.
+		inprogress(0, 3),
+		inprogress(1, 3),
+		inprogress(2, 3),
+	)
+	inProgressPeriod = orig
+
 	esearchall := func(ss string) imapclient.UntaggedEsearch {
 		return imapclient.UntaggedEsearch{All: esearchall0(ss)}
 	}
diff --git a/imapserver/server.go b/imapserver/server.go
index 343db17..18e5c39 100644
--- a/imapserver/server.go
+++ b/imapserver/server.go
@@ -181,6 +181,7 @@ var serverCapabilities = strings.Join([]string{
 	"MULTIAPPEND",                     // ../rfc/3502
 	"REPLACE",                         // ../rfc/8508
 	"PREVIEW",                         // ../rfc/8970:114
+	"INPROGRESS",                      // ../rfc/9585:101
 	// "COMPRESS=DEFLATE", // ../rfc/4978, disabled for interoperability issues: The flate reader (inflate) still blocks on partial flushes, preventing progress.
 }, " ")
 
diff --git a/rfc/index.txt b/rfc/index.txt
index a559993..e6a703f 100644
--- a/rfc/index.txt
+++ b/rfc/index.txt
@@ -237,10 +237,12 @@ https://www.iana.org/assignments/message-headers/message-headers.xhtml
 8970	Yes	-	IMAP4 Extension: Message Preview Generation
 9208	Partial	-	IMAP QUOTA Extension
 9394	Roadmap	-	IMAP PARTIAL Extension for Paged SEARCH and FETCH
-9585	?	-	IMAP Response Code for Command Progress Notifications
+9585	Yes	-	IMAP Response Code for Command Progress Notifications
 9586	Roadmap	-	IMAP Extension for Using and Returning Unique Identifiers (UIDs) Only
 9590	Yes	-	IMAP Extension for Returning Mailbox METADATA in Extended LIST
 9698	?	-	The JMAPACCESS Extension for IMAP
+9738	No	-	IMAP MESSAGELIMIT Extension
+9755	Roadmap	-	IMAP Support for UTF-8
 
 5198	-?	-	Unicode Format for Network Interchange