diff --git a/internal/encoding/wire/wire.go b/internal/encoding/wire/wire.go
index 6c4fd5a..32622b1 100644
--- a/internal/encoding/wire/wire.go
+++ b/internal/encoding/wire/wire.go
@@ -13,7 +13,6 @@
 	"math/bits"
 
 	"google.golang.org/protobuf/internal/errors"
-	"google.golang.org/protobuf/internal/flags"
 )
 
 // Number represents the field number.
@@ -490,18 +489,12 @@
 }
 
 // DecodeTag decodes the field Number and wire Type from its unified form.
-// The Number is -1 if the decoded field number overflows.
+// The Number is -1 if the decoded field number overflows int32.
 // Other than overflow, this does not check for field number validity.
 func DecodeTag(x uint64) (Number, Type) {
 	// NOTE: MessageSet allows for larger field numbers than normal.
-	if flags.ProtoLegacy {
-		if x>>3 > uint64(math.MaxInt32) {
-			return -1, 0
-		}
-	} else {
-		if x>>3 > uint64(MaxValidNumber) {
-			return -1, 0
-		}
+	if x>>3 > uint64(math.MaxInt32) {
+		return -1, 0
 	}
 	return Number(x >> 3), Type(x & 7)
 }
diff --git a/internal/impl/decode.go b/internal/impl/decode.go
index 85cc6b0..9ccfa7f 100644
--- a/internal/impl/decode.go
+++ b/internal/impl/decode.go
@@ -85,6 +85,9 @@
 		if n < 0 {
 			return 0, wire.ParseError(n)
 		}
+		if num > wire.MaxValidNumber {
+			return 0, errors.New("invalid field number")
+		}
 		b = b[n:]
 
 		var f *coderFieldInfo
diff --git a/internal/testprotos/messageset/msetextpb/msetextpb.pb.go b/internal/testprotos/messageset/msetextpb/msetextpb.pb.go
index 9a49c69..411e24c 100644
--- a/internal/testprotos/messageset/msetextpb/msetextpb.pb.go
+++ b/internal/testprotos/messageset/msetextpb/msetextpb.pb.go
@@ -117,6 +117,44 @@
 	return 0
 }
 
+type ExtLargeNumber struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+}
+
+func (x *ExtLargeNumber) Reset() {
+	*x = ExtLargeNumber{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_messageset_msetextpb_msetextpb_proto_msgTypes[2]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *ExtLargeNumber) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*ExtLargeNumber) ProtoMessage() {}
+
+func (x *ExtLargeNumber) ProtoReflect() protoreflect.Message {
+	mi := &file_messageset_msetextpb_msetextpb_proto_msgTypes[2]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use ExtLargeNumber.ProtoReflect.Descriptor instead.
+func (*ExtLargeNumber) Descriptor() ([]byte, []int) {
+	return file_messageset_msetextpb_msetextpb_proto_rawDescGZIP(), []int{2}
+}
+
 var file_messageset_msetextpb_msetextpb_proto_extTypes = []protoimpl.ExtensionInfo{
 	{
 		ExtendedType:  (*messagesetpb.MessageSet)(nil),
@@ -134,6 +172,14 @@
 		Tag:           "bytes,1001,opt,name=message_set_extension",
 		Filename:      "messageset/msetextpb/msetextpb.proto",
 	},
+	{
+		ExtendedType:  (*messagesetpb.MessageSet)(nil),
+		ExtensionType: (*ExtLargeNumber)(nil),
+		Field:         536870912,
+		Name:          "goproto.proto.messageset.ExtLargeNumber",
+		Tag:           "bytes,536870912,opt,name=message_set_extension",
+		Filename:      "messageset/msetextpb/msetextpb.proto",
+	},
 }
 
 // Extension fields to messagesetpb.MessageSet.
@@ -142,6 +188,8 @@
 	E_Ext1_MessageSetExtension = &file_messageset_msetextpb_msetextpb_proto_extTypes[0]
 	// optional goproto.proto.messageset.Ext2 message_set_extension = 1001;
 	E_Ext2_MessageSetExtension = &file_messageset_msetextpb_msetextpb_proto_extTypes[1]
+	// optional goproto.proto.messageset.ExtLargeNumber message_set_extension = 536870912;
+	E_ExtLargeNumber_MessageSetExtension = &file_messageset_msetextpb_msetextpb_proto_extTypes[2] // 1<<29
 )
 
 var File_messageset_msetextpb_msetextpb_proto protoreflect.FileDescriptor
@@ -176,11 +224,21 @@
 	0x0b, 0x32, 0x1e, 0x2e, 0x67, 0x6f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2e, 0x70, 0x72, 0x6f, 0x74,
 	0x6f, 0x2e, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x73, 0x65, 0x74, 0x2e, 0x45, 0x78, 0x74,
 	0x32, 0x52, 0x13, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x53, 0x65, 0x74, 0x45, 0x78, 0x74,
-	0x65, 0x6e, 0x73, 0x69, 0x6f, 0x6e, 0x42, 0x45, 0x5a, 0x43, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65,
-	0x2e, 0x67, 0x6f, 0x6c, 0x61, 0x6e, 0x67, 0x2e, 0x6f, 0x72, 0x67, 0x2f, 0x70, 0x72, 0x6f, 0x74,
-	0x6f, 0x62, 0x75, 0x66, 0x2f, 0x69, 0x6e, 0x74, 0x65, 0x72, 0x6e, 0x61, 0x6c, 0x2f, 0x74, 0x65,
-	0x73, 0x74, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x73, 0x2f, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65,
-	0x73, 0x65, 0x74, 0x2f, 0x6d, 0x73, 0x65, 0x74, 0x65, 0x78, 0x74, 0x70, 0x62,
+	0x65, 0x6e, 0x73, 0x69, 0x6f, 0x6e, 0x22, 0x99, 0x01, 0x0a, 0x0e, 0x45, 0x78, 0x74, 0x4c, 0x61,
+	0x72, 0x67, 0x65, 0x4e, 0x75, 0x6d, 0x62, 0x65, 0x72, 0x32, 0x86, 0x01, 0x0a, 0x15, 0x6d, 0x65,
+	0x73, 0x73, 0x61, 0x67, 0x65, 0x5f, 0x73, 0x65, 0x74, 0x5f, 0x65, 0x78, 0x74, 0x65, 0x6e, 0x73,
+	0x69, 0x6f, 0x6e, 0x12, 0x24, 0x2e, 0x67, 0x6f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2e, 0x70, 0x72,
+	0x6f, 0x74, 0x6f, 0x2e, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x73, 0x65, 0x74, 0x2e, 0x4d,
+	0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x53, 0x65, 0x74, 0x18, 0x80, 0x80, 0x80, 0x80, 0x02, 0x20,
+	0x01, 0x28, 0x0b, 0x32, 0x28, 0x2e, 0x67, 0x6f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2e, 0x70, 0x72,
+	0x6f, 0x74, 0x6f, 0x2e, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x73, 0x65, 0x74, 0x2e, 0x45,
+	0x78, 0x74, 0x4c, 0x61, 0x72, 0x67, 0x65, 0x4e, 0x75, 0x6d, 0x62, 0x65, 0x72, 0x52, 0x13, 0x6d,
+	0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x53, 0x65, 0x74, 0x45, 0x78, 0x74, 0x65, 0x6e, 0x73, 0x69,
+	0x6f, 0x6e, 0x42, 0x45, 0x5a, 0x43, 0x67, 0x6f, 0x6f, 0x67, 0x6c, 0x65, 0x2e, 0x67, 0x6f, 0x6c,
+	0x61, 0x6e, 0x67, 0x2e, 0x6f, 0x72, 0x67, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x62, 0x75, 0x66,
+	0x2f, 0x69, 0x6e, 0x74, 0x65, 0x72, 0x6e, 0x61, 0x6c, 0x2f, 0x74, 0x65, 0x73, 0x74, 0x70, 0x72,
+	0x6f, 0x74, 0x6f, 0x73, 0x2f, 0x6d, 0x65, 0x73, 0x73, 0x61, 0x67, 0x65, 0x73, 0x65, 0x74, 0x2f,
+	0x6d, 0x73, 0x65, 0x74, 0x65, 0x78, 0x74, 0x70, 0x62,
 }
 
 var (
@@ -195,21 +253,24 @@
 	return file_messageset_msetextpb_msetextpb_proto_rawDescData
 }
 
-var file_messageset_msetextpb_msetextpb_proto_msgTypes = make([]protoimpl.MessageInfo, 2)
+var file_messageset_msetextpb_msetextpb_proto_msgTypes = make([]protoimpl.MessageInfo, 3)
 var file_messageset_msetextpb_msetextpb_proto_goTypes = []interface{}{
 	(*Ext1)(nil),                    // 0: goproto.proto.messageset.Ext1
 	(*Ext2)(nil),                    // 1: goproto.proto.messageset.Ext2
-	(*messagesetpb.MessageSet)(nil), // 2: goproto.proto.messageset.MessageSet
+	(*ExtLargeNumber)(nil),          // 2: goproto.proto.messageset.ExtLargeNumber
+	(*messagesetpb.MessageSet)(nil), // 3: goproto.proto.messageset.MessageSet
 }
 var file_messageset_msetextpb_msetextpb_proto_depIdxs = []int32{
-	2, // 0: goproto.proto.messageset.Ext1.message_set_extension:extendee -> goproto.proto.messageset.MessageSet
-	2, // 1: goproto.proto.messageset.Ext2.message_set_extension:extendee -> goproto.proto.messageset.MessageSet
-	0, // 2: goproto.proto.messageset.Ext1.message_set_extension:type_name -> goproto.proto.messageset.Ext1
-	1, // 3: goproto.proto.messageset.Ext2.message_set_extension:type_name -> goproto.proto.messageset.Ext2
-	4, // [4:4] is the sub-list for method output_type
-	4, // [4:4] is the sub-list for method input_type
-	2, // [2:4] is the sub-list for extension type_name
-	0, // [0:2] is the sub-list for extension extendee
+	3, // 0: goproto.proto.messageset.Ext1.message_set_extension:extendee -> goproto.proto.messageset.MessageSet
+	3, // 1: goproto.proto.messageset.Ext2.message_set_extension:extendee -> goproto.proto.messageset.MessageSet
+	3, // 2: goproto.proto.messageset.ExtLargeNumber.message_set_extension:extendee -> goproto.proto.messageset.MessageSet
+	0, // 3: goproto.proto.messageset.Ext1.message_set_extension:type_name -> goproto.proto.messageset.Ext1
+	1, // 4: goproto.proto.messageset.Ext2.message_set_extension:type_name -> goproto.proto.messageset.Ext2
+	2, // 5: goproto.proto.messageset.ExtLargeNumber.message_set_extension:type_name -> goproto.proto.messageset.ExtLargeNumber
+	6, // [6:6] is the sub-list for method output_type
+	6, // [6:6] is the sub-list for method input_type
+	3, // [3:6] is the sub-list for extension type_name
+	0, // [0:3] is the sub-list for extension extendee
 	0, // [0:0] is the sub-list for field type_name
 }
 
@@ -243,6 +304,18 @@
 				return nil
 			}
 		}
+		file_messageset_msetextpb_msetextpb_proto_msgTypes[2].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*ExtLargeNumber); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
 	}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
@@ -250,8 +323,8 @@
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: file_messageset_msetextpb_msetextpb_proto_rawDesc,
 			NumEnums:      0,
-			NumMessages:   2,
-			NumExtensions: 2,
+			NumMessages:   3,
+			NumExtensions: 3,
 			NumServices:   0,
 		},
 		GoTypes:           file_messageset_msetextpb_msetextpb_proto_goTypes,
diff --git a/internal/testprotos/messageset/msetextpb/msetextpb.proto b/internal/testprotos/messageset/msetextpb/msetextpb.proto
index 5d1bf08..b5b50d8 100644
--- a/internal/testprotos/messageset/msetextpb/msetextpb.proto
+++ b/internal/testprotos/messageset/msetextpb/msetextpb.proto
@@ -24,3 +24,9 @@
   }
   optional int32 ext2_field1 = 1;
 }
+
+message ExtLargeNumber {
+  extend MessageSet {
+    optional ExtLargeNumber message_set_extension = 536870912; // 1<<29
+  }
+}
diff --git a/proto/decode.go b/proto/decode.go
index f64f887..07ae467 100644
--- a/proto/decode.go
+++ b/proto/decode.go
@@ -88,6 +88,9 @@
 		if tagLen < 0 {
 			return wire.ParseError(tagLen)
 		}
+		if num > wire.MaxValidNumber {
+			return errors.New("invalid field number")
+		}
 
 		// Find the field descriptor for this field number.
 		fd := fields.ByNumber(num)
diff --git a/proto/decode_test.go b/proto/decode_test.go
index 2c50dde..13db09d 100644
--- a/proto/decode_test.go
+++ b/proto/decode_test.go
@@ -1762,14 +1762,12 @@
 			pack.Tag{pack.MaxValidNumber, pack.VarintType}, pack.Varint(1006),
 			pack.Tag{pack.MaxValidNumber + 1, pack.VarintType}, pack.Varint(1007),
 		}.Marshal(),
-		allowed: flags.ProtoLegacy,
 	},
 	{
 		desc: "max+1",
 		wire: pack.Message{
 			pack.Tag{pack.MaxValidNumber + 1, pack.VarintType}, pack.Varint(1008),
 		}.Marshal(),
-		allowed: flags.ProtoLegacy,
 	},
 }
 
diff --git a/proto/messageset_test.go b/proto/messageset_test.go
index b7c4c72..3ef47a4 100644
--- a/proto/messageset_test.go
+++ b/proto/messageset_test.go
@@ -6,6 +6,7 @@
 
 import (
 	"google.golang.org/protobuf/internal/encoding/pack"
+	"google.golang.org/protobuf/internal/encoding/wire"
 	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/proto"
 
@@ -180,4 +181,40 @@
 			pack.Tag{1, pack.EndGroupType},
 		}.Marshal(),
 	},
+	{
+		desc: "MessageSet with type id out of valid field number range",
+		decodeTo: []proto.Message{func() proto.Message {
+			m := &messagesetpb.MessageSetContainer{MessageSet: &messagesetpb.MessageSet{}}
+			proto.SetExtension(m.MessageSet, msetextpb.E_ExtLargeNumber_MessageSetExtension, &msetextpb.ExtLargeNumber{})
+			return m
+		}()},
+		wire: pack.Message{
+			pack.Tag{1, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.StartGroupType},
+				pack.Tag{2, pack.VarintType}, pack.Varint(wire.MaxValidNumber + 1),
+				pack.Tag{3, pack.BytesType}, pack.LengthPrefix(pack.Message{}),
+				pack.Tag{1, pack.EndGroupType},
+			}),
+		}.Marshal(),
+	},
+	{
+		desc: "MessageSet with unknown type id out of valid field number range",
+		decodeTo: []proto.Message{func() proto.Message {
+			m := &messagesetpb.MessageSetContainer{MessageSet: &messagesetpb.MessageSet{}}
+			m.MessageSet.ProtoReflect().SetUnknown(
+				pack.Message{
+					pack.Tag{wire.MaxValidNumber + 2, pack.BytesType}, pack.LengthPrefix{},
+				}.Marshal(),
+			)
+			return m
+		}()},
+		wire: pack.Message{
+			pack.Tag{1, pack.BytesType}, pack.LengthPrefix(pack.Message{
+				pack.Tag{1, pack.StartGroupType},
+				pack.Tag{2, pack.VarintType}, pack.Varint(wire.MaxValidNumber + 2),
+				pack.Tag{3, pack.BytesType}, pack.LengthPrefix(pack.Message{}),
+				pack.Tag{1, pack.EndGroupType},
+			}),
+		}.Marshal(),
+	},
 }