diff --git a/encoding/protojson/decode.go b/encoding/protojson/decode.go
index d97a60c..7c0da23 100644
--- a/encoding/protojson/decode.go
+++ b/encoding/protojson/decode.go
@@ -12,7 +12,9 @@
 	"strings"
 
 	"google.golang.org/protobuf/internal/encoding/json"
+	"google.golang.org/protobuf/internal/encoding/messageset"
 	"google.golang.org/protobuf/internal/errors"
+	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/internal/pragma"
 	"google.golang.org/protobuf/internal/set"
 	"google.golang.org/protobuf/proto"
@@ -136,13 +138,14 @@
 
 // unmarshalFields unmarshals the fields into the given protoreflect.Message.
 func (o UnmarshalOptions) unmarshalFields(m pref.Message, skipTypeURL bool) error {
+	messageDesc := m.Descriptor()
+	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+		return errors.New("no support for proto1 MessageSets")
+	}
+
 	var seenNums set.Ints
 	var seenOneofs set.Ints
-
-	messageDesc := m.Descriptor()
 	fieldDescs := messageDesc.Fields()
-
-Loop:
 	for {
 		// Read field name.
 		jval, err := o.decoder.Read()
@@ -153,7 +156,7 @@
 		default:
 			return unexpectedJSONError{jval}
 		case json.EndObject:
-			break Loop
+			return nil
 		case json.Name:
 			// Continue below.
 		}
@@ -243,8 +246,6 @@
 			}
 		}
 	}
-
-	return nil
 }
 
 // findExtension returns protoreflect.ExtensionType from the resolver if found.
@@ -253,13 +254,7 @@
 	if err == nil {
 		return xt, nil
 	}
-
-	// Check if this is a MessageSet extension field.
-	xt, err = o.Resolver.FindExtensionByName(xtName + ".message_set_extension")
-	if err == nil && isMessageSetExtension(xt) {
-		return xt, nil
-	}
-	return nil, protoregistry.NotFound
+	return messageset.FindMessageSetExtension(o.Resolver, xtName)
 }
 
 func isKnownValue(fd pref.FieldDescriptor) bool {
diff --git a/encoding/protojson/decode_test.go b/encoding/protojson/decode_test.go
index 03bd10a..2385dc4 100644
--- a/encoding/protojson/decode_test.go
+++ b/encoding/protojson/decode_test.go
@@ -1297,6 +1297,17 @@
 		inputText:    `{ "[pb2.invalid_message_field]": true }`,
 		wantErr:      true,
 	}, {
+		desc:         "extensions of repeated field contains null",
+		inputMessage: &pb2.Extensions{},
+		inputText: `{
+  "[pb2.ExtensionsContainer.rpt_ext_nested]": [
+    {"optString": "one"},
+	null,
+    {"optString": "three"}
+  ],
+}`,
+		wantErr: true,
+	}, {
 		desc:         "MessageSet",
 		inputMessage: &pb2.MessageSet{},
 		inputText: `{
@@ -1323,17 +1334,7 @@
 			})
 			return m
 		}(),
-	}, {
-		desc:         "extensions of repeated field contains null",
-		inputMessage: &pb2.Extensions{},
-		inputText: `{
-  "[pb2.ExtensionsContainer.rpt_ext_nested]": [
-    {"optString": "one"},
-	null,
-    {"optString": "three"}
-  ],
-}`,
-		wantErr: true,
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc:         "not real MessageSet 1",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1349,6 +1350,7 @@
 			})
 			return m
 		}(),
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc:         "not real MessageSet 2",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1358,6 +1360,7 @@
   }
 }`,
 		wantErr: true,
+		skip:    !flags.Proto1Legacy,
 	}, {
 		desc:         "not real MessageSet 3",
 		inputMessage: &pb2.MessageSet{},
@@ -1373,6 +1376,7 @@
 			})
 			return m
 		}(),
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc:         "Empty",
 		inputMessage: &emptypb.Empty{},
diff --git a/encoding/protojson/encode.go b/encoding/protojson/encode.go
index 5128ce4..9b81eca 100644
--- a/encoding/protojson/encode.go
+++ b/encoding/protojson/encode.go
@@ -10,6 +10,9 @@
 	"sort"
 
 	"google.golang.org/protobuf/internal/encoding/json"
+	"google.golang.org/protobuf/internal/encoding/messageset"
+	"google.golang.org/protobuf/internal/errors"
+	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/internal/pragma"
 	"google.golang.org/protobuf/proto"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
@@ -84,8 +87,13 @@
 
 // marshalFields marshals the fields in the given protoreflect.Message.
 func (o MarshalOptions) marshalFields(m pref.Message) error {
+	messageDesc := m.Descriptor()
+	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+		return errors.New("no support for proto1 MessageSets")
+	}
+
 	// Marshal out known fields.
-	fieldDescs := m.Descriptor().Fields()
+	fieldDescs := messageDesc.Fields()
 	for i := 0; i < fieldDescs.Len(); i++ {
 		fd := fieldDescs.Get(i)
 		if !m.Has(fd) {
@@ -257,10 +265,10 @@
 			return true
 		}
 
-		// If extended type is a MessageSet, set field name to be the message type name.
+		// For MessageSet extensions, the name used is the parent message.
 		name := fd.FullName()
-		if isMessageSetExtension(fd) {
-			name = fd.Message().FullName()
+		if messageset.IsMessageSetExtension(fd) {
+			name = name.Parent()
 		}
 
 		// Use [name] format for JSON field name.
@@ -291,19 +299,3 @@
 	}
 	return nil
 }
-
-// isMessageSetExtension reports whether extension extends a message set.
-func isMessageSetExtension(fd pref.FieldDescriptor) bool {
-	if fd.Name() != "message_set_extension" {
-		return false
-	}
-	md := fd.Message()
-	if md == nil {
-		return false
-	}
-	if fd.FullName().Parent() != md.FullName() {
-		return false
-	}
-	xmd, ok := fd.ContainingMessage().(interface{ IsMessageSet() bool })
-	return ok && xmd.IsMessageSet()
-}
diff --git a/encoding/protojson/encode_test.go b/encoding/protojson/encode_test.go
index 42980b0..2aa61dd 100644
--- a/encoding/protojson/encode_test.go
+++ b/encoding/protojson/encode_test.go
@@ -12,6 +12,7 @@
 	"github.com/google/go-cmp/cmp"
 	"google.golang.org/protobuf/encoding/protojson"
 	"google.golang.org/protobuf/internal/encoding/pack"
+	"google.golang.org/protobuf/internal/flags"
 	pimpl "google.golang.org/protobuf/internal/impl"
 	"google.golang.org/protobuf/proto"
 	preg "google.golang.org/protobuf/reflect/protoregistry"
@@ -40,6 +41,7 @@
 		input   proto.Message
 		want    string
 		wantErr bool // TODO: Verify error message substring.
+		skip    bool
 	}{{
 		desc:  "proto2 optional scalars not set",
 		input: &pb2.Scalars{},
@@ -1038,6 +1040,7 @@
     "optString": "not a messageset extension"
   }
 }`,
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc: "not real MessageSet 1",
 		input: func() proto.Message {
@@ -1052,6 +1055,7 @@
     "optString": "not a messageset extension"
   }
 }`,
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc: "not real MessageSet 2",
 		input: func() proto.Message {
@@ -1066,6 +1070,7 @@
     "optString": "another not a messageset extension"
   }
 }`,
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc:  "BoolValue empty",
 		input: &wrapperspb.BoolValue{},
@@ -1898,6 +1903,9 @@
 
 	for _, tt := range tests {
 		tt := tt
+		if tt.skip {
+			continue
+		}
 		t.Run(tt.desc, func(t *testing.T) {
 			// Use 2-space indentation on all MarshalOptions.
 			tt.mo.Indent = "  "
diff --git a/encoding/prototext/decode.go b/encoding/prototext/decode.go
index 7874410..4f384a0 100644
--- a/encoding/prototext/decode.go
+++ b/encoding/prototext/decode.go
@@ -9,9 +9,11 @@
 	"strings"
 	"unicode/utf8"
 
+	"google.golang.org/protobuf/internal/encoding/messageset"
 	"google.golang.org/protobuf/internal/encoding/text"
 	"google.golang.org/protobuf/internal/errors"
 	"google.golang.org/protobuf/internal/fieldnum"
+	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/internal/pragma"
 	"google.golang.org/protobuf/internal/set"
 	"google.golang.org/protobuf/proto"
@@ -74,17 +76,18 @@
 // unmarshalMessage unmarshals a [][2]text.Value message into the given protoreflect.Message.
 func (o UnmarshalOptions) unmarshalMessage(tmsg [][2]text.Value, m pref.Message) error {
 	messageDesc := m.Descriptor()
+	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+		return errors.New("no support for proto1 MessageSets")
+	}
 
 	// Handle expanded Any message.
 	if messageDesc.FullName() == "google.protobuf.Any" && isExpandedAny(tmsg) {
 		return o.unmarshalAny(tmsg[0], m)
 	}
 
-	fieldDescs := messageDesc.Fields()
-	reservedNames := messageDesc.ReservedNames()
 	var seenNums set.Ints
 	var seenOneofs set.Ints
-
+	fieldDescs := messageDesc.Fields()
 	for _, tfield := range tmsg {
 		tkey := tfield[0]
 		tval := tfield[1]
@@ -128,7 +131,7 @@
 
 		if fd == nil {
 			// Ignore reserved names.
-			if reservedNames.Has(name) {
+			if messageDesc.ReservedNames().Has(name) {
 				continue
 			}
 			// TODO: Can provide option to ignore unknown message fields.
@@ -193,13 +196,7 @@
 	if err == nil {
 		return xt, nil
 	}
-
-	// Check if this is a MessageSet extension field.
-	xt, err = o.Resolver.FindExtensionByName(xtName + ".message_set_extension")
-	if err == nil && isMessageSetExtension(xt) {
-		return xt, nil
-	}
-	return nil, protoregistry.NotFound
+	return messageset.FindMessageSetExtension(o.Resolver, xtName)
 }
 
 // unmarshalSingular unmarshals given text.Value into the non-repeated field.
diff --git a/encoding/prototext/decode_test.go b/encoding/prototext/decode_test.go
index 31de4d7..20ce133 100644
--- a/encoding/prototext/decode_test.go
+++ b/encoding/prototext/decode_test.go
@@ -1310,6 +1310,7 @@
 			})
 			return m
 		}(),
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc:         "not real MessageSet 1",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1325,6 +1326,7 @@
 			})
 			return m
 		}(),
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc:         "not real MessageSet 2",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1334,6 +1336,7 @@
 }
 `,
 		wantErr: true,
+		skip:    !flags.Proto1Legacy,
 	}, {
 		desc:         "not real MessageSet 3",
 		inputMessage: &pb2.MessageSet{},
@@ -1348,6 +1351,7 @@
 			})
 			return m
 		}(),
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc:         "Any not expanded",
 		inputMessage: &anypb.Any{},
diff --git a/encoding/prototext/encode.go b/encoding/prototext/encode.go
index 7d244a9..5d8cd46 100644
--- a/encoding/prototext/encode.go
+++ b/encoding/prototext/encode.go
@@ -9,10 +9,12 @@
 	"sort"
 	"unicode/utf8"
 
+	"google.golang.org/protobuf/internal/encoding/messageset"
 	"google.golang.org/protobuf/internal/encoding/text"
 	"google.golang.org/protobuf/internal/encoding/wire"
 	"google.golang.org/protobuf/internal/errors"
 	"google.golang.org/protobuf/internal/fieldnum"
+	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/internal/mapsort"
 	"google.golang.org/protobuf/internal/pragma"
 	"google.golang.org/protobuf/proto"
@@ -72,8 +74,10 @@
 
 // marshalMessage converts a protoreflect.Message to a text.Value.
 func (o MarshalOptions) marshalMessage(m pref.Message) (text.Value, error) {
-	var msgFields [][2]text.Value
 	messageDesc := m.Descriptor()
+	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+		return text.Value{}, errors.New("no support for proto1 MessageSets")
+	}
 
 	// Handle Any expansion.
 	if messageDesc.FullName() == "google.protobuf.Any" {
@@ -85,6 +89,7 @@
 	}
 
 	// Handle known fields.
+	var msgFields [][2]text.Value
 	fieldDescs := messageDesc.Fields()
 	size := fieldDescs.Len()
 	for i := 0; i < size; i++ {
@@ -253,10 +258,10 @@
 			return true
 		}
 
-		// If extended type is a MessageSet, set field name to be the message type name.
+		// For MessageSet extensions, the name used is the parent message.
 		name := fd.FullName()
-		if isMessageSetExtension(fd) {
-			name = fd.Message().FullName()
+		if messageset.IsMessageSetExtension(fd) {
+			name = name.Parent()
 		}
 
 		// Use string type to produce [name] format.
@@ -279,22 +284,6 @@
 	return append(msgFields, entries...), nil
 }
 
-// isMessageSetExtension reports whether extension extends a message set.
-func isMessageSetExtension(fd pref.FieldDescriptor) bool {
-	if fd.Name() != "message_set_extension" {
-		return false
-	}
-	md := fd.Message()
-	if md == nil {
-		return false
-	}
-	if fd.FullName().Parent() != md.FullName() {
-		return false
-	}
-	xmd, ok := fd.ContainingMessage().(interface{ IsMessageSet() bool })
-	return ok && xmd.IsMessageSet()
-}
-
 // appendUnknown parses the given []byte and appends field(s) into the given fields slice.
 // This function assumes proper encoding in the given []byte.
 func appendUnknown(fields [][2]text.Value, b []byte) [][2]text.Value {
diff --git a/encoding/prototext/encode_test.go b/encoding/prototext/encode_test.go
index 003d490..c29169b 100644
--- a/encoding/prototext/encode_test.go
+++ b/encoding/prototext/encode_test.go
@@ -12,6 +12,7 @@
 	"google.golang.org/protobuf/encoding/prototext"
 	"google.golang.org/protobuf/internal/detrand"
 	"google.golang.org/protobuf/internal/encoding/pack"
+	"google.golang.org/protobuf/internal/flags"
 	pimpl "google.golang.org/protobuf/internal/impl"
 	"google.golang.org/protobuf/proto"
 	preg "google.golang.org/protobuf/reflect/protoregistry"
@@ -39,6 +40,7 @@
 		input   proto.Message
 		want    string
 		wantErr bool // TODO: Verify error message content.
+		skip    bool
 	}{{
 		desc:  "proto2 optional scalars not set",
 		input: &pb2.Scalars{},
@@ -1082,6 +1084,7 @@
   opt_string: "not a messageset extension"
 }
 `,
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc: "not real MessageSet 1",
 		input: func() proto.Message {
@@ -1095,6 +1098,7 @@
   opt_string: "not a messageset extension"
 }
 `,
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc: "not real MessageSet 2",
 		input: func() proto.Message {
@@ -1108,6 +1112,7 @@
   opt_string: "another not a messageset extension"
 }
 `,
+		skip: !flags.Proto1Legacy,
 	}, {
 		desc: "Any not expanded",
 		mo: prototext.MarshalOptions{
@@ -1201,6 +1206,9 @@
 
 	for _, tt := range tests {
 		tt := tt
+		if tt.skip {
+			continue
+		}
 		t.Run(tt.desc, func(t *testing.T) {
 			// Use 2-space indentation on all MarshalOptions.
 			tt.mo.Indent = "  "
diff --git a/internal/encoding/messageset/messageset.go b/internal/encoding/messageset/messageset.go
index a8b6c0d..7d05dee 100644
--- a/internal/encoding/messageset/messageset.go
+++ b/internal/encoding/messageset/messageset.go
@@ -9,6 +9,7 @@
 	"google.golang.org/protobuf/internal/encoding/wire"
 	"google.golang.org/protobuf/internal/errors"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
+	preg "google.golang.org/protobuf/reflect/protoregistry"
 )
 
 // The MessageSet wire format is equivalent to a message defiend as follows,
@@ -28,12 +29,48 @@
 	FieldMessage = wire.Number(3)
 )
 
+// ExtensionName is the field name for extensions of MessageSet.
+//
+// A valid MessageSet extension must be of the form:
+//	message MyMessage {
+//		extend proto2.bridge.MessageSet {
+//			optional MyMessage message_set_extension = 1234;
+//		}
+//		...
+//	}
+const ExtensionName = "message_set_extension"
+
 // IsMessageSet returns whether the message uses the MessageSet wire format.
 func IsMessageSet(md pref.MessageDescriptor) bool {
 	xmd, ok := md.(interface{ IsMessageSet() bool })
 	return ok && xmd.IsMessageSet()
 }
 
+// IsMessageSetExtension reports this field extends a MessageSet.
+func IsMessageSetExtension(fd pref.FieldDescriptor) bool {
+	if fd.Name() != ExtensionName {
+		return false
+	}
+	if fd.FullName().Parent() != fd.Message().FullName() {
+		return false
+	}
+	return IsMessageSet(fd.ContainingMessage())
+}
+
+// FindMessageSetExtension locates a MessageSet extension field by name.
+// In text and JSON formats, the extension name used is the message itself.
+// The extension field name is derived by appending ExtensionName.
+func FindMessageSetExtension(r preg.ExtensionTypeResolver, s pref.FullName) (pref.ExtensionType, error) {
+	xt, err := r.FindExtensionByName(s.Append(ExtensionName))
+	if err != nil {
+		return nil, err
+	}
+	if !IsMessageSetExtension(xt) {
+		return nil, preg.NotFound
+	}
+	return xt, nil
+}
+
 // SizeField returns the size of a MessageSet item field containing an extension
 // with the given field number, not counting the contents of the message subfield.
 func SizeField(num wire.Number) int {