diff --git a/encoding/protojson/decode.go b/encoding/protojson/decode.go
index aa4202c..9bf4e8c 100644
--- a/encoding/protojson/decode.go
+++ b/encoding/protojson/decode.go
@@ -112,8 +112,8 @@
 
 // unmarshalMessage unmarshals a message into the given protoreflect.Message.
 func (d decoder) unmarshalMessage(m pref.Message, skipTypeURL bool) error {
-	if isCustomType(m.Descriptor().FullName()) {
-		return d.unmarshalCustomType(m)
+	if unmarshal := wellKnownTypeUnmarshaler(m.Descriptor().FullName()); unmarshal != nil {
+		return unmarshal(d, m)
 	}
 
 	tok, err := d.Read()
diff --git a/encoding/protojson/encode.go b/encoding/protojson/encode.go
index 873254d..7d61933 100644
--- a/encoding/protojson/encode.go
+++ b/encoding/protojson/encode.go
@@ -147,8 +147,8 @@
 
 // marshalMessage marshals the given protoreflect.Message.
 func (e encoder) marshalMessage(m pref.Message) error {
-	if isCustomType(m.Descriptor().FullName()) {
-		return e.marshalCustomType(m)
+	if marshal := wellKnownTypeMarshaler(m.Descriptor().FullName()); marshal != nil {
+		return marshal(e, m)
 	}
 
 	e.StartObject()
diff --git a/encoding/protojson/well_known_types.go b/encoding/protojson/well_known_types.go
index 392c7fa..d26cd39 100644
--- a/encoding/protojson/well_known_types.go
+++ b/encoding/protojson/well_known_types.go
@@ -16,75 +16,84 @@
 	"google.golang.org/protobuf/internal/genid"
 	"google.golang.org/protobuf/internal/strs"
 	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/reflect/protoreflect"
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 )
 
-// isCustomType returns true if type name has special JSON conversion rules.
-// The list of custom types here has to match the ones in marshalCustomType and
-// unmarshalCustomType.
-func isCustomType(name pref.FullName) bool {
-	switch genid.WhichFile(name) {
-	case genid.Any_file:
-	case genid.Timestamp_file:
-	case genid.Duration_file:
-	case genid.Wrappers_file:
-	case genid.Struct_file:
-	case genid.FieldMask_file:
-	case genid.Empty_file:
-	default:
-		return false
+type marshalFunc func(encoder, pref.Message) error
+
+// wellKnownTypeMarshaler returns a marshal function if the message type
+// has specialized serialization behavior. It returns nil otherwise.
+func wellKnownTypeMarshaler(name protoreflect.FullName) marshalFunc {
+	if name.Parent() == genid.GoogleProtobuf_package {
+		switch name.Name() {
+		case genid.Any_message_name:
+			return encoder.marshalAny
+		case genid.Timestamp_message_name:
+			return encoder.marshalTimestamp
+		case genid.Duration_message_name:
+			return encoder.marshalDuration
+		case genid.BoolValue_message_name,
+			genid.Int32Value_message_name,
+			genid.Int64Value_message_name,
+			genid.UInt32Value_message_name,
+			genid.UInt64Value_message_name,
+			genid.FloatValue_message_name,
+			genid.DoubleValue_message_name,
+			genid.StringValue_message_name,
+			genid.BytesValue_message_name:
+			return encoder.marshalWrapperType
+		case genid.Struct_message_name:
+			return encoder.marshalStruct
+		case genid.ListValue_message_name:
+			return encoder.marshalListValue
+		case genid.Value_message_name:
+			return encoder.marshalKnownValue
+		case genid.FieldMask_message_name:
+			return encoder.marshalFieldMask
+		case genid.Empty_message_name:
+			return encoder.marshalEmpty
+		}
 	}
-	return true
+	return nil
 }
 
-// marshalCustomType marshals given well-known type message that have special
-// JSON conversion rules. It needs to be a message type where isCustomType
-// returns true, else it will panic.
-func (e encoder) marshalCustomType(m pref.Message) error {
-	name := m.Descriptor().FullName()
-	switch genid.WhichFile(name) {
-	case genid.Any_file:
-		return e.marshalAny(m)
-	case genid.Timestamp_file:
-		return e.marshalTimestamp(m)
-	case genid.Duration_file:
-		return e.marshalDuration(m)
-	case genid.Wrappers_file:
-		return e.marshalWrapperType(m)
-	case genid.Struct_file:
-		return e.marshalStructType(m)
-	case genid.FieldMask_file:
-		return e.marshalFieldMask(m)
-	case genid.Empty_file:
-		return e.marshalEmpty(m)
-	default:
-		panic(fmt.Sprintf("%s does not have a custom marshaler", name))
-	}
-}
+type unmarshalFunc func(decoder, pref.Message) error
 
-// unmarshalCustomType unmarshals given well-known type message that have
-// special JSON conversion rules. It needs to be a message type where
-// isCustomType returns true, else it will panic.
-func (d decoder) unmarshalCustomType(m pref.Message) error {
-	name := m.Descriptor().FullName()
-	switch genid.WhichFile(name) {
-	case genid.Any_file:
-		return d.unmarshalAny(m)
-	case genid.Timestamp_file:
-		return d.unmarshalTimestamp(m)
-	case genid.Duration_file:
-		return d.unmarshalDuration(m)
-	case genid.Wrappers_file:
-		return d.unmarshalWrapperType(m)
-	case genid.Struct_file:
-		return d.unmarshalStructType(m)
-	case genid.FieldMask_file:
-		return d.unmarshalFieldMask(m)
-	case genid.Empty_file:
-		return d.unmarshalEmpty(m)
-	default:
-		panic(fmt.Sprintf("%s does not have a custom unmarshaler", name))
+// wellKnownTypeUnmarshaler returns a unmarshal function if the message type
+// has specialized serialization behavior. It returns nil otherwise.
+func wellKnownTypeUnmarshaler(name protoreflect.FullName) unmarshalFunc {
+	if name.Parent() == genid.GoogleProtobuf_package {
+		switch name.Name() {
+		case genid.Any_message_name:
+			return decoder.unmarshalAny
+		case genid.Timestamp_message_name:
+			return decoder.unmarshalTimestamp
+		case genid.Duration_message_name:
+			return decoder.unmarshalDuration
+		case genid.BoolValue_message_name,
+			genid.Int32Value_message_name,
+			genid.Int64Value_message_name,
+			genid.UInt32Value_message_name,
+			genid.UInt64Value_message_name,
+			genid.FloatValue_message_name,
+			genid.DoubleValue_message_name,
+			genid.StringValue_message_name,
+			genid.BytesValue_message_name:
+			return decoder.unmarshalWrapperType
+		case genid.Struct_message_name:
+			return decoder.unmarshalStruct
+		case genid.ListValue_message_name:
+			return decoder.unmarshalListValue
+		case genid.Value_message_name:
+			return decoder.unmarshalKnownValue
+		case genid.FieldMask_message_name:
+			return decoder.unmarshalFieldMask
+		case genid.Empty_message_name:
+			return decoder.unmarshalEmpty
+		}
 	}
+	return nil
 }
 
 // The JSON representation of an Any message uses the regular representation of
@@ -140,9 +149,9 @@
 	// If type of value has custom JSON encoding, marshal out a field "value"
 	// with corresponding custom JSON encoding of the embedded message as a
 	// field.
-	if isCustomType(emt.Descriptor().FullName()) {
+	if marshal := wellKnownTypeMarshaler(emt.Descriptor().FullName()); marshal != nil {
 		e.WriteName("value")
-		return e.marshalCustomType(em)
+		return marshal(e, em)
 	}
 
 	// Else, marshal out the embedded message's fields in this Any object.
@@ -197,10 +206,10 @@
 
 	// Create new message for the embedded message type and unmarshal into it.
 	em := emt.New()
-	if isCustomType(emt.Descriptor().FullName()) {
+	if unmarshal := wellKnownTypeUnmarshaler(emt.Descriptor().FullName()); unmarshal != nil {
 		// If embedded message is a custom type,
 		// unmarshal the JSON "value" field into it.
-		if err := d.unmarshalAnyValue(em); err != nil {
+		if err := d.unmarshalAnyValue(unmarshal, em); err != nil {
 			return err
 		}
 	} else {
@@ -344,7 +353,7 @@
 
 // unmarshalAnyValue unmarshals the given custom-type message from the JSON
 // object's "value" field.
-func (d decoder) unmarshalAnyValue(m pref.Message) error {
+func (d decoder) unmarshalAnyValue(unmarshal unmarshalFunc, m pref.Message) error {
 	// Skip ObjectOpen, and start reading the fields.
 	d.Read()
 
@@ -372,7 +381,7 @@
 					return d.newError(tok.Pos(), `duplicate "value" field`)
 				}
 				// Unmarshal the field value into the given message.
-				if err := d.unmarshalCustomType(m); err != nil {
+				if err := unmarshal(d, m); err != nil {
 					return err
 				}
 				found = true
@@ -449,32 +458,6 @@
 	}
 }
 
-func (e encoder) marshalStructType(m pref.Message) error {
-	switch m.Descriptor().Name() {
-	case genid.Struct_message_name:
-		return e.marshalStruct(m)
-	case genid.ListValue_message_name:
-		return e.marshalListValue(m)
-	case genid.Value_message_name:
-		return e.marshalKnownValue(m)
-	default:
-		panic(fmt.Sprintf("invalid struct type: %v", m.Descriptor().FullName()))
-	}
-}
-
-func (d decoder) unmarshalStructType(m pref.Message) error {
-	switch m.Descriptor().Name() {
-	case genid.Struct_message_name:
-		return d.unmarshalStruct(m)
-	case genid.ListValue_message_name:
-		return d.unmarshalListValue(m)
-	case genid.Value_message_name:
-		return d.unmarshalKnownValue(m)
-	default:
-		panic(fmt.Sprintf("invalid struct type: %v", m.Descriptor().FullName()))
-	}
-}
-
 // The JSON representation for Struct is a JSON object that contains the encoded
 // Struct.fields map and follows the serialization rules for a map.
 
diff --git a/internal/genid/detect.go b/internal/genid/detect.go
deleted file mode 100644
index 0e22783..0000000
--- a/internal/genid/detect.go
+++ /dev/null
@@ -1,62 +0,0 @@
-// Copyright 2020 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 file.
-
-package genid
-
-import "google.golang.org/protobuf/reflect/protoreflect"
-
-type ProtoFile int
-
-const (
-	Unknown_file ProtoFile = iota
-	Any_file
-	Timestamp_file
-	Duration_file
-	Wrappers_file
-	Struct_file
-	FieldMask_file
-	Api_file
-	Type_file
-	SourceContext_file
-	Empty_file
-)
-
-var wellKnownTypes = map[protoreflect.FullName]ProtoFile{
-	Any_message_fullname:            Any_file,
-	Timestamp_message_fullname:      Timestamp_file,
-	Duration_message_fullname:       Duration_file,
-	BoolValue_message_fullname:      Wrappers_file,
-	Int32Value_message_fullname:     Wrappers_file,
-	Int64Value_message_fullname:     Wrappers_file,
-	UInt32Value_message_fullname:    Wrappers_file,
-	UInt64Value_message_fullname:    Wrappers_file,
-	FloatValue_message_fullname:     Wrappers_file,
-	DoubleValue_message_fullname:    Wrappers_file,
-	BytesValue_message_fullname:     Wrappers_file,
-	StringValue_message_fullname:    Wrappers_file,
-	Struct_message_fullname:         Struct_file,
-	ListValue_message_fullname:      Struct_file,
-	Value_message_fullname:          Struct_file,
-	NullValue_enum_fullname:         Struct_file,
-	FieldMask_message_fullname:      FieldMask_file,
-	Api_message_fullname:            Api_file,
-	Method_message_fullname:         Api_file,
-	Mixin_message_fullname:          Api_file,
-	Syntax_enum_fullname:            Type_file,
-	Type_message_fullname:           Type_file,
-	Field_message_fullname:          Type_file,
-	Field_Kind_enum_fullname:        Type_file,
-	Field_Cardinality_enum_fullname: Type_file,
-	Enum_message_fullname:           Type_file,
-	EnumValue_message_fullname:      Type_file,
-	Option_message_fullname:         Type_file,
-	SourceContext_message_fullname:  SourceContext_file,
-	Empty_message_fullname:          Empty_file,
-}
-
-// WhichFile identifies the proto file that an enum or message belongs to.
-// It currently only identifies well-known types.
-func WhichFile(s protoreflect.FullName) ProtoFile {
-	return wellKnownTypes[s]
-}
diff --git a/internal/genid/detect_test.go b/internal/genid/detect_test.go
deleted file mode 100644
index e9f656c..0000000
--- a/internal/genid/detect_test.go
+++ /dev/null
@@ -1,72 +0,0 @@
-// Copyright 2020 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 file.
-
-package genid_test
-
-import (
-	"testing"
-
-	"google.golang.org/protobuf/internal/genid"
-	"google.golang.org/protobuf/reflect/protoreflect"
-
-	"google.golang.org/protobuf/types/descriptorpb"
-	"google.golang.org/protobuf/types/known/anypb"
-	"google.golang.org/protobuf/types/known/apipb"
-	"google.golang.org/protobuf/types/known/durationpb"
-	"google.golang.org/protobuf/types/known/emptypb"
-	"google.golang.org/protobuf/types/known/fieldmaskpb"
-	"google.golang.org/protobuf/types/known/sourcecontextpb"
-	"google.golang.org/protobuf/types/known/structpb"
-	"google.golang.org/protobuf/types/known/timestamppb"
-	"google.golang.org/protobuf/types/known/typepb"
-	"google.golang.org/protobuf/types/known/wrapperspb"
-	"google.golang.org/protobuf/types/pluginpb"
-)
-
-func TestWhich(t *testing.T) {
-	tests := []struct {
-		in   protoreflect.FileDescriptor
-		want genid.ProtoFile
-	}{
-		{descriptorpb.File_google_protobuf_descriptor_proto, genid.Unknown_file},
-		{pluginpb.File_google_protobuf_compiler_plugin_proto, genid.Unknown_file},
-		{anypb.File_google_protobuf_any_proto, genid.Any_file},
-		{timestamppb.File_google_protobuf_timestamp_proto, genid.Timestamp_file},
-		{durationpb.File_google_protobuf_duration_proto, genid.Duration_file},
-		{wrapperspb.File_google_protobuf_wrappers_proto, genid.Wrappers_file},
-		{structpb.File_google_protobuf_struct_proto, genid.Struct_file},
-		{fieldmaskpb.File_google_protobuf_field_mask_proto, genid.FieldMask_file},
-		{apipb.File_google_protobuf_api_proto, genid.Api_file},
-		{typepb.File_google_protobuf_type_proto, genid.Type_file},
-		{sourcecontextpb.File_google_protobuf_source_context_proto, genid.SourceContext_file},
-		{emptypb.File_google_protobuf_empty_proto, genid.Empty_file},
-	}
-
-	for _, tt := range tests {
-		rangeDescriptors(tt.in, func(d protoreflect.Descriptor) {
-			got := genid.WhichFile(d.FullName())
-			if got != tt.want {
-				t.Errorf("Which(%s) = %v, want %v", d.FullName(), got, tt.want)
-			}
-		})
-	}
-}
-
-func rangeDescriptors(d interface {
-	Enums() protoreflect.EnumDescriptors
-	Messages() protoreflect.MessageDescriptors
-}, f func(protoreflect.Descriptor)) {
-	for i := 0; i < d.Enums().Len(); i++ {
-		ed := d.Enums().Get(i)
-		f(ed)
-	}
-	for i := 0; i < d.Messages().Len(); i++ {
-		md := d.Messages().Get(i)
-		if md.IsMapEntry() {
-			continue
-		}
-		f(md)
-		rangeDescriptors(md, f)
-	}
-}
diff --git a/internal/genid/doc.go b/internal/genid/doc.go
index a6f63d9..45ccd01 100644
--- a/internal/genid/doc.go
+++ b/internal/genid/doc.go
@@ -5,3 +5,7 @@
 // Package genid contains constants for declarations in descriptor.proto
 // and the well-known types.
 package genid
+
+import protoreflect "google.golang.org/protobuf/reflect/protoreflect"
+
+const GoogleProtobuf_package protoreflect.FullName = "google.protobuf"
diff --git a/internal/msgfmt/format.go b/internal/msgfmt/format.go
index 7a7fe71..9547a53 100644
--- a/internal/msgfmt/format.go
+++ b/internal/msgfmt/format.go
@@ -161,9 +161,16 @@
 		x = strings.TrimSuffix(x, "000")
 		x = strings.TrimSuffix(x, ".000")
 		return append(b, x+"s"...)
-	}
 
-	if genid.WhichFile(md.FullName()) == genid.Wrappers_file {
+	case genid.BoolValue_message_fullname,
+		genid.Int32Value_message_fullname,
+		genid.Int64Value_message_fullname,
+		genid.UInt32Value_message_fullname,
+		genid.UInt64Value_message_fullname,
+		genid.FloatValue_message_fullname,
+		genid.DoubleValue_message_fullname,
+		genid.StringValue_message_fullname,
+		genid.BytesValue_message_fullname:
 		fd := fds.ByNumber(genid.WrapperValue_Value_field_number)
 		return appendValue(b, m.Get(fd), fd)
 	}