diff --git a/Makefile b/Makefile
index e69da6c4..2be9474a 100644
--- a/Makefile
+++ b/Makefile
@@ -32,6 +32,7 @@ test:
 	cd irc/modes && go test . && go vet .
 	cd irc/mysql && go test . && go vet .
 	cd irc/passwd && go test . && go vet .
+	cd irc/sno && go test . && go vet .
 	cd irc/utils && go test . && go vet .
 	./.check-gofmt.sh
 
diff --git a/irc/modes.go b/irc/modes.go
index e4396793..f8a1e85d 100644
--- a/irc/modes.go
+++ b/irc/modes.go
@@ -75,30 +75,28 @@ func ApplyUserModeChanges(client *Client, changes modes.ModeChanges, force bool,
 			}
 		} else {
 			// server notices are weird
-			if !client.HasMode(modes.Operator) {
+			if !client.HasMode(modes.Operator) || change.Op == modes.List {
 				continue
 			}
-			var masks []sno.Mask
-			if change.Op == modes.Add || change.Op == modes.Remove {
-				var newArg string
-				for _, char := range change.Arg {
-					mask := sno.Mask(char)
-					if sno.ValidMasks[mask] {
-						masks = append(masks, mask)
-						newArg += string(char)
-					}
-				}
-				change.Arg = newArg
-			}
-			if change.Op == modes.Add {
+
+			currentMasks := client.server.snomasks.MasksEnabled(client)
+			addMasks, removeMasks, newArg := sno.EvaluateSnomaskChanges(change.Op == modes.Add, change.Arg, currentMasks)
+
+			success := false
+			if len(addMasks) != 0 {
 				oper := client.Oper()
 				// #1176: require special operator privileges to subscribe to snomasks
 				if oper.HasRoleCapab("snomasks") || oper.HasRoleCapab("ban") {
-					client.server.snomasks.AddMasks(client, masks...)
-					applied = append(applied, change)
+					success = true
+					client.server.snomasks.AddMasks(client, addMasks...)
 				}
-			} else if change.Op == modes.Remove {
-				client.server.snomasks.RemoveMasks(client, masks...)
+			}
+			if len(removeMasks) != 0 {
+				success = true
+				client.server.snomasks.RemoveMasks(client, removeMasks...)
+			}
+			if success {
+				change.Arg = newArg
 				applied = append(applied, change)
 			}
 		}
diff --git a/irc/modes/modes.go b/irc/modes/modes.go
index f7efedf5..f91cc06b 100644
--- a/irc/modes/modes.go
+++ b/irc/modes/modes.go
@@ -212,12 +212,10 @@ func ParseUserModeChanges(params ...string) (ModeChanges, map[rune]bool) {
 			// put arg into modechange if needed
 			switch Mode(mode) {
 			case ServerNotice:
-				// always require arg
+				// arg is optional for ServerNotice (we accept bare `-s`)
 				if len(params) > skipArgs {
 					change.Arg = params[skipArgs]
 					skipArgs++
-				} else {
-					continue
 				}
 			}
 
diff --git a/irc/modes/modes_test.go b/irc/modes/modes_test.go
index 27f04888..67d28c2f 100644
--- a/irc/modes/modes_test.go
+++ b/irc/modes/modes_test.go
@@ -15,6 +15,38 @@ func assertEqual(supplied, expected interface{}, t *testing.T) {
 	}
 }
 
+func TestParseUserModeChanges(t *testing.T) {
+	emptyUnknown := make(map[rune]bool)
+	changes, unknown := ParseUserModeChanges("+i")
+	assertEqual(unknown, emptyUnknown, t)
+	assertEqual(changes, ModeChanges{ModeChange{Op: Add, Mode: Invisible}}, t)
+
+	// no-op change to sno
+	changes, unknown = ParseUserModeChanges("+is")
+	assertEqual(unknown, emptyUnknown, t)
+	assertEqual(changes, ModeChanges{ModeChange{Op: Add, Mode: Invisible}, ModeChange{Op: Add, Mode: ServerNotice}}, t)
+
+	// add snomasks
+	changes, unknown = ParseUserModeChanges("+is", "ac")
+	assertEqual(unknown, emptyUnknown, t)
+	assertEqual(changes, ModeChanges{ModeChange{Op: Add, Mode: Invisible}, ModeChange{Op: Add, Mode: ServerNotice, Arg: "ac"}}, t)
+
+	// remove snomasks
+	changes, unknown = ParseUserModeChanges("+s", "-cx")
+	assertEqual(unknown, emptyUnknown, t)
+	assertEqual(changes, ModeChanges{ModeChange{Op: Add, Mode: ServerNotice, Arg: "-cx"}}, t)
+
+	// remove all snomasks (arg is parsed but has no meaning)
+	changes, unknown = ParseUserModeChanges("-is", "ac")
+	assertEqual(unknown, emptyUnknown, t)
+	assertEqual(changes, ModeChanges{ModeChange{Op: Remove, Mode: Invisible}, ModeChange{Op: Remove, Mode: ServerNotice, Arg: "ac"}}, t)
+
+	// remove all snomasks
+	changes, unknown = ParseUserModeChanges("-is")
+	assertEqual(unknown, emptyUnknown, t)
+	assertEqual(changes, ModeChanges{ModeChange{Op: Remove, Mode: Invisible}, ModeChange{Op: Remove, Mode: ServerNotice}}, t)
+}
+
 func TestIssue874(t *testing.T) {
 	emptyUnknown := make(map[rune]bool)
 	modes, unknown := ParseChannelModeChanges("+k")
diff --git a/irc/sno/constants.go b/irc/sno/constants.go
index 1eb1bc5a..542ab57f 100644
--- a/irc/sno/constants.go
+++ b/irc/sno/constants.go
@@ -7,6 +7,8 @@ package sno
 // Mask is a type of server notice mask.
 type Mask rune
 
+type Masks []Mask
+
 // Notice mask types
 const (
 	LocalAnnouncements Mask = 'a'
@@ -18,8 +20,8 @@ const (
 	LocalQuits         Mask = 'q'
 	Stats              Mask = 't'
 	LocalAccounts      Mask = 'u'
-	LocalXline         Mask = 'x'
 	LocalVhosts        Mask = 'v'
+	LocalXline         Mask = 'x'
 )
 
 var (
@@ -39,17 +41,17 @@ var (
 	}
 
 	// ValidMasks contains the snomasks that we support.
-	ValidMasks = map[Mask]bool{
-		LocalAnnouncements: true,
-		LocalConnects:      true,
-		LocalChannels:      true,
-		LocalKills:         true,
-		LocalNicks:         true,
-		LocalOpers:         true,
-		LocalQuits:         true,
-		Stats:              true,
-		LocalAccounts:      true,
-		LocalXline:         true,
-		LocalVhosts:        true,
+	ValidMasks = []Mask{
+		LocalAnnouncements,
+		LocalConnects,
+		LocalChannels,
+		LocalKills,
+		LocalNicks,
+		LocalOpers,
+		LocalQuits,
+		Stats,
+		LocalAccounts,
+		LocalVhosts,
+		LocalXline,
 	}
 )
diff --git a/irc/sno/utils.go b/irc/sno/utils.go
new file mode 100644
index 00000000..572ab394
--- /dev/null
+++ b/irc/sno/utils.go
@@ -0,0 +1,87 @@
+// Copyright (c) 2020 Shivaram Lingamneni
+// released under the MIT license
+
+package sno
+
+import (
+	"strings"
+)
+
+func IsValidMask(r rune) bool {
+	for _, m := range ValidMasks {
+		if m == Mask(r) {
+			return true
+		}
+	}
+	return false
+}
+
+func (masks Masks) String() string {
+	var buf strings.Builder
+	buf.Grow(len(masks))
+	for _, m := range masks {
+		buf.WriteRune(rune(m))
+	}
+	return buf.String()
+}
+
+func (masks Masks) Contains(mask Mask) bool {
+	for _, m := range masks {
+		if mask == m {
+			return true
+		}
+	}
+	return false
+}
+
+// Evaluate changes to snomasks made with MODE. There are several cases:
+// adding snomasks with `/mode +s a` or `/mode +s +a`, removing them with `/mode +s -a`,
+// adding all with `/mode +s *` or `/mode +s +*`, removing all with `/mode +s -*` or `/mode -s`
+func EvaluateSnomaskChanges(add bool, arg string, currentMasks Masks) (addMasks, removeMasks Masks, newArg string) {
+	if add {
+		if len(arg) == 0 {
+			return
+		}
+		add := true
+		switch arg[0] {
+		case '+':
+			arg = arg[1:]
+		case '-':
+			add = false
+			arg = arg[1:]
+		default:
+			// add
+		}
+		if strings.IndexByte(arg, '*') != -1 {
+			if add {
+				for _, mask := range ValidMasks {
+					if !currentMasks.Contains(mask) {
+						addMasks = append(addMasks, mask)
+					}
+				}
+			} else {
+				removeMasks = currentMasks
+			}
+		} else {
+			for _, r := range arg {
+				if IsValidMask(r) {
+					m := Mask(r)
+					if add && !currentMasks.Contains(m) {
+						addMasks = append(addMasks, m)
+					} else if !add && currentMasks.Contains(m) {
+						removeMasks = append(removeMasks, m)
+					}
+				}
+			}
+		}
+		if len(addMasks) != 0 {
+			newArg = "+" + addMasks.String()
+		} else if len(removeMasks) != 0 {
+			newArg = "-" + removeMasks.String()
+		}
+	} else {
+		removeMasks = currentMasks
+		newArg = ""
+	}
+	return
+}
diff --git a/irc/sno/utils_test.go b/irc/sno/utils_test.go
new file mode 100644
index 00000000..46ce30bf
--- /dev/null
+++ b/irc/sno/utils_test.go
@@ -0,0 +1,53 @@
+// Copyright (c) 2020 Shivaram Lingamneni
+// released under the MIT license
+
+package sno
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+)
+
+func assertEqual(supplied, expected interface{}, t *testing.T) {
+	if !reflect.DeepEqual(supplied, expected) {
+		panic(fmt.Sprintf("expected %#v but got %#v", expected, supplied))
+	}
+}
+
+func TestEvaluateSnomaskChanges(t *testing.T) {
+	add, remove, newArg := EvaluateSnomaskChanges(true, "*", nil)
+	assertEqual(add, Masks{'a', 'c', 'j', 'k', 'n', 'o', 'q', 't', 'u', 'v', 'x'}, t)
+	assertEqual(len(remove), 0, t)
+	assertEqual(newArg, "+acjknoqtuvx", t)
+
+	add, remove, newArg = EvaluateSnomaskChanges(true, "*", Masks{'a', 'u'})
+	assertEqual(add, Masks{'c', 'j', 'k', 'n', 'o', 'q', 't', 'v', 'x'}, t)
+	assertEqual(len(remove), 0, t)
+	assertEqual(newArg, "+cjknoqtvx", t)
+
+	add, remove, newArg = EvaluateSnomaskChanges(true, "-a", Masks{'a', 'u'})
+	assertEqual(len(add), 0, t)
+	assertEqual(remove, Masks{'a'}, t)
+	assertEqual(newArg, "-a", t)
+
+	add, remove, newArg = EvaluateSnomaskChanges(true, "-*", Masks{'a', 'u'})
+	assertEqual(len(add), 0, t)
+	assertEqual(remove, Masks{'a', 'u'}, t)
+	assertEqual(newArg, "-au", t)
+
+	add, remove, newArg = EvaluateSnomaskChanges(true, "+c", Masks{'a', 'u'})
+	assertEqual(add, Masks{'c'}, t)
+	assertEqual(len(remove), 0, t)
+	assertEqual(newArg, "+c", t)
+
+	add, remove, newArg = EvaluateSnomaskChanges(false, "", Masks{'a', 'u'})
+	assertEqual(len(add), 0, t)
+	assertEqual(remove, Masks{'a', 'u'}, t)
+	assertEqual(newArg, "", t)
+
+	add, remove, newArg = EvaluateSnomaskChanges(false, "*", Masks{'a', 'u'})
+	assertEqual(len(add), 0, t)
+	assertEqual(remove, Masks{'a', 'u'}, t)
+	assertEqual(newArg, "", t)
+}
diff --git a/irc/snomanager.go b/irc/snomanager.go
index b7e48107..0bd394bc 100644
--- a/irc/snomanager.go
+++ b/irc/snomanager.go
@@ -24,11 +24,6 @@ func (m *SnoManager) AddMasks(client *Client, masks ...sno.Mask) {
 	defer m.sendListMutex.Unlock()
 
 	for _, mask := range masks {
-		// confirm mask is valid
-		if !sno.ValidMasks[mask] {
-			continue
-		}
-
 		currentClientList := m.sendLists[mask]
 
 		if currentClientList == nil {
@@ -101,19 +96,23 @@ func (m *SnoManager) Send(mask sno.Mask, content string) {
 	}
 }
 
-// String returns the snomasks currently enabled.
-func (m *SnoManager) String(client *Client) string {
+// MasksEnabled returns the snomasks currently enabled.
+func (m *SnoManager) MasksEnabled(client *Client) (result sno.Masks) {
 	m.sendListMutex.RLock()
 	defer m.sendListMutex.RUnlock()
 
-	var masks string
 	for mask, clients := range m.sendLists {
 		for c := range clients {
 			if c == client {
-				masks += string(mask)
+				result = append(result, mask)
 				break
 			}
 		}
 	}
-	return masks
+	return
+}
+
+func (m *SnoManager) String(client *Client) string {
+	masks := m.MasksEnabled(client)
+	return masks.String()
 }