diff --git a/encoding/protojson/decode.go b/encoding/protojson/decode.go
index 1d22d3a..5e32d8b 100644
--- a/encoding/protojson/decode.go
+++ b/encoding/protojson/decode.go
@@ -139,7 +139,7 @@
 // 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) {
+	if !flags.ProtoLegacy && messageset.IsMessageSet(messageDesc) {
 		return errors.New("no support for proto1 MessageSets")
 	}
 
diff --git a/encoding/protojson/decode_test.go b/encoding/protojson/decode_test.go
index 9c4f93e..c343104 100644
--- a/encoding/protojson/decode_test.go
+++ b/encoding/protojson/decode_test.go
@@ -1334,7 +1334,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 1",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1350,7 +1350,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 2",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1360,7 +1360,7 @@
   }
 }`,
 		wantErr: true,
-		skip:    !flags.Proto1Legacy,
+		skip:    !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 3",
 		inputMessage: &pb2.MessageSet{},
@@ -1376,7 +1376,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "Empty",
 		inputMessage: &emptypb.Empty{},
@@ -2457,13 +2457,13 @@
 			m.SetWeakMessage1(&weakpb.WeakImportMessage1{A: proto.Int32(1)})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "weak fields; unknown field",
 		inputMessage: &testpb.TestWeak{},
 		inputText:    `{"weak_message1":{"a":1}, "weak_message2":{"a":1}}`,
 		wantErr:      true, // weak_message2 is unknown since the package containing it is not imported
-		skip:         !flags.Proto1Legacy,
+		skip:         !flags.ProtoLegacy,
 	}}
 
 	for _, tt := range tests {
diff --git a/encoding/protojson/encode.go b/encoding/protojson/encode.go
index 9b81eca..839c7d1 100644
--- a/encoding/protojson/encode.go
+++ b/encoding/protojson/encode.go
@@ -88,7 +88,7 @@
 // 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) {
+	if !flags.ProtoLegacy && messageset.IsMessageSet(messageDesc) {
 		return errors.New("no support for proto1 MessageSets")
 	}
 
diff --git a/encoding/protojson/encode_test.go b/encoding/protojson/encode_test.go
index 5cacf9e..9d7a8ae 100644
--- a/encoding/protojson/encode_test.go
+++ b/encoding/protojson/encode_test.go
@@ -1034,7 +1034,7 @@
     "optString": "not a messageset extension"
   }
 }`,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "not real MessageSet 1",
 		input: func() proto.Message {
@@ -1049,7 +1049,7 @@
     "optString": "not a messageset extension"
   }
 }`,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "not real MessageSet 2",
 		input: func() proto.Message {
@@ -1064,7 +1064,7 @@
     "optString": "another not a messageset extension"
   }
 }`,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:  "BoolValue empty",
 		input: &wrapperspb.BoolValue{},
diff --git a/encoding/prototext/decode.go b/encoding/prototext/decode.go
index 06388c8..7207dde 100644
--- a/encoding/prototext/decode.go
+++ b/encoding/prototext/decode.go
@@ -76,7 +76,7 @@
 // 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) {
+	if !flags.ProtoLegacy && messageset.IsMessageSet(messageDesc) {
 		return errors.New("no support for proto1 MessageSets")
 	}
 
diff --git a/encoding/prototext/decode_test.go b/encoding/prototext/decode_test.go
index 588d9ee..636e21b 100644
--- a/encoding/prototext/decode_test.go
+++ b/encoding/prototext/decode_test.go
@@ -1310,7 +1310,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 1",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1326,7 +1326,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 2",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1336,7 +1336,7 @@
 }
 `,
 		wantErr: true,
-		skip:    !flags.Proto1Legacy,
+		skip:    !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 3",
 		inputMessage: &pb2.MessageSet{},
@@ -1351,7 +1351,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "Any not expanded",
 		inputMessage: &anypb.Any{},
@@ -1501,13 +1501,13 @@
 			m.SetWeakMessage1(&weakpb.WeakImportMessage1{A: proto.Int32(1)})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "weak fields; unknown field",
 		inputMessage: &testpb.TestWeak{},
 		inputText:    `weak_message1:{a:1} weak_message2:{a:1}`,
 		wantErr:      true, // weak_message2 is unknown since the package containing it is not imported
-		skip:         !flags.Proto1Legacy,
+		skip:         !flags.ProtoLegacy,
 	}}
 
 	for _, tt := range tests {
diff --git a/encoding/prototext/encode.go b/encoding/prototext/encode.go
index 5d8cd46..b732f02 100644
--- a/encoding/prototext/encode.go
+++ b/encoding/prototext/encode.go
@@ -75,7 +75,7 @@
 // marshalMessage converts a protoreflect.Message to a text.Value.
 func (o MarshalOptions) marshalMessage(m pref.Message) (text.Value, error) {
 	messageDesc := m.Descriptor()
-	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+	if !flags.ProtoLegacy && messageset.IsMessageSet(messageDesc) {
 		return text.Value{}, errors.New("no support for proto1 MessageSets")
 	}
 
diff --git a/encoding/prototext/encode_test.go b/encoding/prototext/encode_test.go
index 493a229..0c480e6 100644
--- a/encoding/prototext/encode_test.go
+++ b/encoding/prototext/encode_test.go
@@ -1078,7 +1078,7 @@
   opt_string: "not a messageset extension"
 }
 `,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "not real MessageSet 1",
 		input: func() proto.Message {
@@ -1092,7 +1092,7 @@
   opt_string: "not a messageset extension"
 }
 `,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "not real MessageSet 2",
 		input: func() proto.Message {
@@ -1106,7 +1106,7 @@
   opt_string: "another not a messageset extension"
 }
 `,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "Any not expanded",
 		mo: prototext.MarshalOptions{
diff --git a/integration_test.go b/integration_test.go
index 3f72213..cc06bd8 100644
--- a/integration_test.go
+++ b/integration_test.go
@@ -48,8 +48,8 @@
 
 	if *regenerate {
 		t.Run("Generate", func(t *testing.T) {
-			fmt.Print(mustRunCommand(t, "go", "run", "-tags", "proto1_legacy", "./internal/cmd/generate-types", "-execute"))
-			fmt.Print(mustRunCommand(t, "go", "run", "-tags", "proto1_legacy", "./internal/cmd/generate-protos", "-execute"))
+			fmt.Print(mustRunCommand(t, "go", "run", "-tags", "protolegacy", "./internal/cmd/generate-types", "-execute"))
+			fmt.Print(mustRunCommand(t, "go", "run", "-tags", "protolegacy", "./internal/cmd/generate-protos", "-execute"))
 			files := strings.Split(strings.TrimSpace(mustRunCommand(t, "git", "ls-files", "*.go")), "\n")
 			mustRunCommand(t, append([]string{"gofmt", "-w"}, files...)...)
 		})
@@ -79,7 +79,7 @@
 		runGo("PureGo", workDir, "go", "test", "-race", "-tags", "purego", "./...")
 		runGo("Reflect", workDir, "go", "test", "-race", "-tags", "protoreflect", "./...")
 		if goVersion == golangLatest {
-			runGo("Proto1Legacy", workDir, "go", "test", "-race", "-tags", "proto1_legacy", "./...")
+			runGo("ProtoLegacy", workDir, "go", "test", "-race", "-tags", "protolegacy", "./...")
 			runGo("ProtocGenGo", "cmd/protoc-gen-go/testdata", "go", "test")
 			runGo("ProtocGenGoGRPC", "cmd/protoc-gen-go-grpc/testdata", "go", "test")
 		}
@@ -94,11 +94,11 @@
 		mustRunCommand(t, runner, "--failure_list", failureList, "--enforce_recommended", driver)
 	})
 	t.Run("GeneratedGoFiles", func(t *testing.T) {
-		diff := mustRunCommand(t, "go", "run", "-tags", "proto1_legacy", "./internal/cmd/generate-types")
+		diff := mustRunCommand(t, "go", "run", "-tags", "protolegacy", "./internal/cmd/generate-types")
 		if strings.TrimSpace(diff) != "" {
 			t.Fatalf("stale generated files:\n%v", diff)
 		}
-		diff = mustRunCommand(t, "go", "run", "-tags", "proto1_legacy", "./internal/cmd/generate-protos")
+		diff = mustRunCommand(t, "go", "run", "-tags", "protolegacy", "./internal/cmd/generate-protos")
 		if strings.TrimSpace(diff) != "" {
 			t.Fatalf("stale generated files:\n%v", diff)
 		}
diff --git a/internal/encoding/text/text_test.go b/internal/encoding/text/text_test.go
index 0e40fa8..64771f3 100644
--- a/internal/encoding/text/text_test.go
+++ b/internal/encoding/text/text_test.go
@@ -615,7 +615,7 @@
 	}, {
 		in: `nums: [0xbeefbeef,0xbeefbeefbeefbeef]`,
 		wantVal: V(Msg{{ID("nums"), func() Value {
-			if flags.Proto1Legacy {
+			if flags.ProtoLegacy {
 				return V(Lst{V(int32(-1091584273)), V(int64(-4688318750159552785))})
 			} else {
 				return V(Lst{V(uint32(0xbeefbeef)), V(uint64(0xbeefbeefbeefbeef))})
diff --git a/internal/encoding/text/value.go b/internal/encoding/text/value.go
index b092598..f98dbcb 100644
--- a/internal/encoding/text/value.go
+++ b/internal/encoding/text/value.go
@@ -192,7 +192,7 @@
 		}
 		// C++ accepts large positive hex numbers as negative values.
 		// This feature is here for proto1 backwards compatibility purposes.
-		if flags.Proto1Legacy && len(v.raw) > 1 && v.raw[0] == '0' && v.raw[1] == 'x' {
+		if flags.ProtoLegacy && len(v.raw) > 1 && v.raw[0] == '0' && v.raw[1] == 'x' {
 			if !b64 {
 				return int64(int32(n)), n <= math.MaxUint32
 			}
diff --git a/internal/encoding/wire/wire.go b/internal/encoding/wire/wire.go
index 224ddf7..6c4fd5a 100644
--- a/internal/encoding/wire/wire.go
+++ b/internal/encoding/wire/wire.go
@@ -494,7 +494,7 @@
 // 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.Proto1Legacy {
+	if flags.ProtoLegacy {
 		if x>>3 > uint64(math.MaxInt32) {
 			return -1, 0
 		}
diff --git a/internal/flags/flags.go b/internal/flags/flags.go
index 8208a6d..89b0aff 100644
--- a/internal/flags/flags.go
+++ b/internal/flags/flags.go
@@ -5,12 +5,13 @@
 // Package flags provides a set of flags controlled by build tags.
 package flags
 
-// Proto1Legacy specifies whether to enable support for legacy proto1
-// functionality such as MessageSets, weak fields, and various other obscure
-// behavior that is necessary to maintain backwards compatibility with proto1.
+// ProtoLegacy specifies whether to enable support for legacy functionality
+// such as MessageSets, weak fields, and various other obscure behavior
+// that is necessary to maintain backwards compatibility with proto1 or
+// the pre-release variants of proto2 and proto3.
 //
-// This is disabled by default unless built with the "proto1_legacy" tag.
+// This is disabled by default unless built with the "protolegacy" tag.
 //
 // WARNING: The compatibility agreement covers nothing provided by this flag.
 // As such, functionality may suddenly be removed or changed at our discretion.
-const Proto1Legacy = proto1Legacy
+const ProtoLegacy = protoLegacy
diff --git a/internal/flags/proto1_legacy_disable.go b/internal/flags/proto1_legacy_disable.go
deleted file mode 100644
index 6bd2e89..0000000
--- a/internal/flags/proto1_legacy_disable.go
+++ /dev/null
@@ -1,9 +0,0 @@
-// Copyright 2018 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE.md file.
-
-// +build !proto1_legacy
-
-package flags
-
-const proto1Legacy = false
diff --git a/internal/flags/proto1_legacy_enable.go b/internal/flags/proto_legacy_disable.go
similarity index 78%
copy from internal/flags/proto1_legacy_enable.go
copy to internal/flags/proto_legacy_disable.go
index 520d537..de110ca 100644
--- a/internal/flags/proto1_legacy_enable.go
+++ b/internal/flags/proto_legacy_disable.go
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE.md file.
 
-// +build proto1_legacy
+// +build !protolegacy
 
 package flags
 
-const proto1Legacy = true
+const protoLegacy = false
diff --git a/internal/flags/proto1_legacy_enable.go b/internal/flags/proto_legacy_enable.go
similarity index 78%
rename from internal/flags/proto1_legacy_enable.go
rename to internal/flags/proto_legacy_enable.go
index 520d537..dbd2cd5 100644
--- a/internal/flags/proto1_legacy_enable.go
+++ b/internal/flags/proto_legacy_enable.go
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE.md file.
 
-// +build proto1_legacy
+// +build protolegacy
 
 package flags
 
-const proto1Legacy = true
+const protoLegacy = true
diff --git a/internal/impl/codec_messageset.go b/internal/impl/codec_messageset.go
index 073c4dc..d5fb444 100644
--- a/internal/impl/codec_messageset.go
+++ b/internal/impl/codec_messageset.go
@@ -45,7 +45,7 @@
 }
 
 func marshalMessageSet(mi *MessageInfo, b []byte, p pointer, wiretag uint64, opts marshalOptions) ([]byte, error) {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		return b, errors.New("no support for message_set_wire_format")
 	}
 	ext := *p.Extensions()
@@ -97,7 +97,7 @@
 }
 
 func unmarshalMessageSet(mi *MessageInfo, b []byte, p pointer, wtyp wire.Type, opts unmarshalOptions) (int, error) {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		return 0, errors.New("no support for message_set_wire_format")
 	}
 	if wtyp != wire.StartGroupType {
diff --git a/internal/impl/message_field.go b/internal/impl/message_field.go
index 7eb6199..10cacc7 100644
--- a/internal/impl/message_field.go
+++ b/internal/impl/message_field.go
@@ -288,7 +288,7 @@
 }
 
 func fieldInfoForWeakMessage(fd pref.FieldDescriptor, weakOffset offset) fieldInfo {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		panic("no support for proto1 weak fields")
 	}
 
diff --git a/internal/strs/strings.go b/internal/strs/strings.go
index af5f197..2208ff2 100644
--- a/internal/strs/strings.go
+++ b/internal/strs/strings.go
@@ -15,7 +15,7 @@
 
 // EnforceUTF8 reports whether to enforce strict UTF-8 validation.
 func EnforceUTF8(fd protoreflect.FieldDescriptor) bool {
-	if flags.Proto1Legacy {
+	if flags.ProtoLegacy {
 		if fd, ok := fd.(interface{ EnforceUTF8() bool }); ok {
 			return fd.EnforceUTF8()
 		}
diff --git a/proto/decode_test.go b/proto/decode_test.go
index 89fc303..c2b5ce9 100644
--- a/proto/decode_test.go
+++ b/proto/decode_test.go
@@ -98,9 +98,9 @@
 				got := reflect.New(reflect.TypeOf(want).Elem()).Interface().(proto.Message)
 				err := proto.Unmarshal(test.wire, got)
 				switch {
-				case flags.Proto1Legacy && err != nil:
+				case flags.ProtoLegacy && err != nil:
 					t.Errorf("Unmarshal returned unexpected error: %v\nMessage:\n%v", err, marshalText(want))
-				case !flags.Proto1Legacy && err == nil:
+				case !flags.ProtoLegacy && err == nil:
 					t.Errorf("Unmarshal did not return expected error for invalid UTF8: %v\nMessage:\n%v", err, marshalText(want))
 				}
 			})
diff --git a/proto/encode_test.go b/proto/encode_test.go
index 573a197..466d3d1 100644
--- a/proto/encode_test.go
+++ b/proto/encode_test.go
@@ -104,9 +104,9 @@
 			t.Run(fmt.Sprintf("%s (%T)", test.desc, want), func(t *testing.T) {
 				_, err := proto.Marshal(want)
 				switch {
-				case flags.Proto1Legacy && err != nil:
+				case flags.ProtoLegacy && err != nil:
 					t.Errorf("Marshal returned unexpected error: %v\nMessage:\n%v", err, marshalText(want))
-				case !flags.Proto1Legacy && err == nil:
+				case !flags.ProtoLegacy && err == nil:
 					t.Errorf("Marshal did not return expected error for invalid UTF8: %v\nMessage:\n%v", err, marshalText(want))
 				}
 			})
diff --git a/proto/messageset.go b/proto/messageset.go
index e5d4bd5..14ed199 100644
--- a/proto/messageset.go
+++ b/proto/messageset.go
@@ -25,7 +25,7 @@
 }
 
 func marshalMessageSet(b []byte, m protoreflect.Message, o MarshalOptions) ([]byte, error) {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		return b, errors.New("no support for message_set_wire_format")
 	}
 	var err error
@@ -53,7 +53,7 @@
 }
 
 func unmarshalMessageSet(b []byte, m protoreflect.Message, o UnmarshalOptions) error {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		return errors.New("no support for message_set_wire_format")
 	}
 	md := m.Descriptor()
diff --git a/proto/messageset_test.go b/proto/messageset_test.go
index 07b2d70..c1ef6c9 100644
--- a/proto/messageset_test.go
+++ b/proto/messageset_test.go
@@ -14,7 +14,7 @@
 )
 
 func init() {
-	if flags.Proto1Legacy {
+	if flags.ProtoLegacy {
 		testProtos = append(testProtos, messageSetTestProtos...)
 	}
 }
diff --git a/reflect/protodesc/desc_validate.go b/reflect/protodesc/desc_validate.go
index 042950c..cf172a2 100644
--- a/reflect/protodesc/desc_validate.go
+++ b/reflect/protodesc/desc_validate.go
@@ -103,7 +103,7 @@
 				return errors.New("message %q has conflicting fields: %q with %q", m.FullName(), f1.Name(), f2.Name())
 			}
 		}
-		if isMessageSet && !flags.Proto1Legacy {
+		if isMessageSet && !flags.ProtoLegacy {
 			return errors.New("message %q is a MessageSet, which is a legacy proto1 feature that is no longer supported", m.FullName())
 		}
 		if isMessageSet && (m.Syntax() != protoreflect.Proto2 || m.Fields().Len() > 0 || m.ExtensionRanges().Len() == 0) {
@@ -147,7 +147,7 @@
 			if fd.Extendee != nil {
 				return errors.New("message field %q may not have extendee: %q", f.FullName(), fd.GetExtendee())
 			}
-			if f.IsWeak() && !flags.Proto1Legacy {
+			if f.IsWeak() && !flags.ProtoLegacy {
 				return errors.New("message field %q is a weak field, which is a legacy proto1 feature that is no longer supported", f.FullName())
 			}
 			if f.IsWeak() && (f.Syntax() != protoreflect.Proto2 || !isOptionalMessage(f) || f.ContainingOneof() != nil) {
diff --git a/reflect/protodesc/file_test.go b/reflect/protodesc/file_test.go
index b20be3f..5319b52 100644
--- a/reflect/protodesc/file_test.go
+++ b/reflect/protodesc/file_test.go
@@ -769,7 +769,7 @@
 			}]}]
 		`),
 		wantErr: func() string {
-			if flags.Proto1Legacy {
+			if flags.ProtoLegacy {
 				return `message "M.M" is an invalid proto1 MessageSet`
 			} else {
 				return `message "M.M" is a MessageSet, which is a legacy proto1 feature that is no longer supported`
@@ -787,7 +787,7 @@
 			}]}]
 		`),
 		wantErr: func() string {
-			if flags.Proto1Legacy {
+			if flags.ProtoLegacy {
 				return ""
 			} else {
 				return `message "M.M" is a MessageSet, which is a legacy proto1 feature that is no longer supported`
diff --git a/testing/prototest/prototest_test.go b/testing/prototest/prototest_test.go
index 00df3a6..5eec961 100644
--- a/testing/prototest/prototest_test.go
+++ b/testing/prototest/prototest_test.go
@@ -32,7 +32,7 @@
 		(*legacypb.Legacy)(nil),
 		protoimpl.X.MessageOf((*legacy1pb.Message)(nil)).Interface(),
 	}
-	if flags.Proto1Legacy {
+	if flags.ProtoLegacy {
 		ms = append(ms, (*testpb.TestWeak)(nil))
 	}