diff --git a/internal/cmd/pbdump/pbdump.go b/internal/cmd/pbdump/pbdump.go
new file mode 100644
index 00000000..6ebc1c70
--- /dev/null
+++ b/internal/cmd/pbdump/pbdump.go
@@ -0,0 +1,302 @@
+// 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 file.
+
+// pbdump is a tool for decoding the wire format for protocol buffer messages.
+package main
+
+import (
+	"bytes"
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+
+	"google.golang.org/proto/internal/encoding/pack"
+	"google.golang.org/proto/internal/encoding/wire"
+	"google.golang.org/proto/reflect/protoreflect"
+	"google.golang.org/proto/reflect/prototype"
+)
+
+func main() {
+	log.SetFlags(0)
+	log.SetOutput(os.Stderr)
+
+	var fs fields
+	flag.Var((*boolFields)(&fs), "bools", "List of bool fields")
+	flag.Var((*intFields)(&fs), "ints", "List of int32 or int64 fields")
+	flag.Var((*sintFields)(&fs), "sints", "List of sint32 or sint64 fields")
+	flag.Var((*uintFields)(&fs), "uints", "List of enum, uint32, or uint64 fields")
+	flag.Var((*uint32Fields)(&fs), "uint32s", "List of fixed32 fields")
+	flag.Var((*int32Fields)(&fs), "int32s", "List of sfixed32 fields")
+	flag.Var((*float32Fields)(&fs), "float32s", "List of float fields")
+	flag.Var((*uint64Fields)(&fs), "uint64s", "List of fixed64 fields")
+	flag.Var((*int64Fields)(&fs), "int64s", "List of sfixed64 fields")
+	flag.Var((*float64Fields)(&fs), "float64s", "List of double fields")
+	flag.Var((*stringFields)(&fs), "strings", "List of string fields")
+	flag.Var((*bytesFields)(&fs), "bytes", "List of bytes fields")
+	flag.Var((*messageFields)(&fs), "messages", "List of message fields")
+	flag.Var((*groupFields)(&fs), "groups", "List of group fields")
+	printDesc := flag.Bool("print_descriptor", false, "Print the message descriptor")
+	printSource := flag.Bool("print_source", false, "Print the output in valid Go syntax")
+	flag.Usage = func() {
+		log.Printf("Usage: %s [OPTIONS]... [INPUTS]...\n\n%s\n", filepath.Base(os.Args[0]), strings.Join([]string{
+			"Print structured representations of encoded protocol buffer messages.",
+			"Since the protobuf wire format is not fully self-describing, type information",
+			"about the proto message can be provided using flags (e.g., -messages).",
+			"Each field list is a comma-separated list of field identifiers,",
+			"where each field identifier is a dot-separated list of field numbers,",
+			"identifying each field relative to the root message.",
+			"",
+			"For example, \"-messages 1,3,3.1 -float32s 1.2 -bools 3.1.2\" represents:",
+			"",
+			"	message M {",
+			"		optional M1 f1 = 1;           // -messages 1",
+			"		message M1 {",
+			"			repeated float f2 = 2;    // -float32s 1.2",
+			"		}",
+			"		optional M3 f3 = 3;           // -messages 3",
+			"		message M3 {",
+			"			optional M1 f1 = 1;       // -messages 3.1",
+			"			message M1 {",
+			"				repeated bool f2 = 2; // -bools 3.1.2",
+			"			}",
+			"		}",
+			"	}",
+			"",
+			"Arbitrarily complex message schemas can be represented using these flags.",
+			"Scalar field types are marked as repeated so that pbdump can decode",
+			"the packed representations of such field types.",
+			"",
+			"If no inputs are specified, the wire data is read in from stdin, otherwise",
+			"the contents of each specified input file is concatenated and",
+			"treated as one large message.",
+			"",
+			"Options:",
+			"  -bools fields      " + flag.Lookup("bools").Usage,
+			"  -ints fields       " + flag.Lookup("ints").Usage,
+			"  -sints fields      " + flag.Lookup("sints").Usage,
+			"  -uints fields      " + flag.Lookup("uints").Usage,
+			"  -int32s fields     " + flag.Lookup("int32s").Usage,
+			"  -int64s fields     " + flag.Lookup("int64s").Usage,
+			"  -uint32s fields    " + flag.Lookup("uint32s").Usage,
+			"  -uint64s fields    " + flag.Lookup("uint64s").Usage,
+			"  -float32s fields   " + flag.Lookup("float32s").Usage,
+			"  -float64s fields   " + flag.Lookup("float64s").Usage,
+			"  -strings fields    " + flag.Lookup("strings").Usage,
+			"  -bytes fields      " + flag.Lookup("bytes").Usage,
+			"  -messages fields   " + flag.Lookup("messages").Usage,
+			"  -groups fields     " + flag.Lookup("groups").Usage,
+			"  -print_descriptor  " + flag.Lookup("print_descriptor").Usage,
+			"  -print_source      " + flag.Lookup("print_source").Usage,
+		}, "\n"))
+	}
+	flag.Parse()
+
+	// Create message types.
+	var desc protoreflect.MessageDescriptor
+	if len(fs) > 0 {
+		var err error
+		desc, err = fs.Descriptor()
+		if err != nil {
+			log.Fatalf("Descriptor error: %v", err)
+		}
+		if *printDesc {
+			log.Printf("%#v\n", desc)
+		}
+	}
+
+	// Read message input.
+	var buf []byte
+	if flag.NArg() == 0 {
+		b, err := ioutil.ReadAll(os.Stdin)
+		if err != nil {
+			log.Fatalf("ReadAll error: %v", err)
+		}
+		buf = b
+	}
+	for _, f := range flag.Args() {
+		b, err := ioutil.ReadFile(f)
+		if err != nil {
+			log.Fatalf("ReadFile error: %v", err)
+		}
+		buf = append(buf, b...)
+	}
+
+	// Parse and print message structure.
+	defer log.Printf("fatal input: %q", buf) // debug printout if panic occurs
+	var m pack.Message
+	m.UnmarshalDescriptor(buf, desc)
+	if *printSource {
+		fmt.Fprintf(os.Stdout, "%#v\n", m)
+	} else {
+		fmt.Fprintf(os.Stdout, "%+v\n", m)
+	}
+	if !bytes.Equal(buf, m.Marshal()) || len(buf) != m.Size() {
+		log.Fatalf("roundtrip mismatch:\n\tgot:  %d %x\n\twant: %d %x", m.Size(), m, len(buf), buf)
+	}
+	os.Exit(0) // exit cleanly, avoid debug printout
+}
+
+// fields is a tree of fields, keyed by a field number.
+// Fields representing messages or groups have sub-fields.
+type fields map[wire.Number]*field
+type field struct {
+	kind protoreflect.Kind
+	sub  fields // only for MessageKind or GroupKind
+}
+
+// Set parses s as a comma-separated list (see the help above for the format)
+// and treats each field identifier as the specified kind.
+func (fs *fields) Set(s string, k protoreflect.Kind) error {
+	if *fs == nil {
+		*fs = make(fields)
+	}
+	for _, s := range strings.Split(s, ",") {
+		if err := fs.set("", strings.TrimSpace(s), k); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+func (fs fields) set(prefix, s string, k protoreflect.Kind) error {
+	if s == "" {
+		return nil
+	}
+
+	// Parse next field number.
+	i := strings.IndexByte(s, '.')
+	if i < 0 {
+		i = len(s)
+	}
+	prefix = strings.TrimPrefix(prefix+"."+s[:i], ".")
+	n, _ := strconv.ParseInt(s[:i], 10, 32)
+	num := wire.Number(n)
+	if num < wire.MinValidNumber || wire.MaxValidNumber < num {
+		return fmt.Errorf("invalid field: %v", prefix)
+	}
+	s = strings.TrimPrefix(s[i:], ".")
+
+	// Handle the current field.
+	if fs[num] == nil {
+		fs[num] = &field{0, make(fields)}
+	}
+	if len(s) == 0 {
+		if fs[num].kind.IsValid() {
+			return fmt.Errorf("field %v already set as %v type", prefix, fs[num].kind)
+		}
+		fs[num].kind = k
+	}
+	if err := fs[num].sub.set(prefix, s, k); err != nil {
+		return err
+	}
+
+	// Verify that only messages or groups can have sub-fields.
+	k2 := fs[num].kind
+	if k2 > 0 && k2 != protoreflect.MessageKind && k2 != protoreflect.GroupKind && len(fs[num].sub) > 0 {
+		return fmt.Errorf("field %v of %v type cannot have sub-fields", prefix, k2)
+	}
+	return nil
+}
+
+// Descriptor returns the field tree as a message descriptor.
+func (fs fields) Descriptor() (protoreflect.MessageDescriptor, error) {
+	ftyp, err := prototype.NewFile(&prototype.File{
+		Syntax:   protoreflect.Proto2,
+		Messages: []prototype.Message{fs.messageDescriptor("M")},
+	})
+	if err != nil {
+		return nil, err
+	}
+	return ftyp.Messages().Get(0), nil
+}
+func (fs fields) messageDescriptor(name protoreflect.FullName) prototype.Message {
+	m := prototype.Message{Name: name.Name()}
+	for _, n := range fs.sortedNums() {
+		f := prototype.Field{
+			Name:        protoreflect.Name(fmt.Sprintf("f%d", n)),
+			Number:      n,
+			Cardinality: protoreflect.Optional,
+			Kind:        fs[n].kind,
+		}
+		if !f.Kind.IsValid() {
+			f.Kind = protoreflect.MessageKind
+		}
+		switch f.Kind {
+		case protoreflect.BoolKind, protoreflect.EnumKind,
+			protoreflect.Int32Kind, protoreflect.Sint32Kind, protoreflect.Uint32Kind,
+			protoreflect.Int64Kind, protoreflect.Sint64Kind, protoreflect.Uint64Kind,
+			protoreflect.Sfixed32Kind, protoreflect.Fixed32Kind, protoreflect.FloatKind,
+			protoreflect.Sfixed64Kind, protoreflect.Fixed64Kind, protoreflect.DoubleKind:
+			f.Cardinality = protoreflect.Repeated
+			f.IsPacked = true
+		case protoreflect.MessageKind, protoreflect.GroupKind:
+			s := name.Append(protoreflect.Name(fmt.Sprintf("M%d", n)))
+			f.MessageType = prototype.PlaceholderMessage(s)
+			m.Messages = append(m.Messages, fs[n].sub.messageDescriptor(s))
+		}
+		m.Fields = append(m.Fields, f)
+	}
+	return m
+}
+
+func (fs fields) sortedNums() (ns []wire.Number) {
+	for n := range fs {
+		ns = append(ns, n)
+	}
+	sort.Slice(ns, func(i, j int) bool { return ns[i] < ns[j] })
+	return ns
+}
+
+type (
+	boolFields    fields
+	intFields     fields
+	sintFields    fields
+	uintFields    fields
+	uint32Fields  fields
+	int32Fields   fields
+	float32Fields fields
+	uint64Fields  fields
+	int64Fields   fields
+	float64Fields fields
+	stringFields  fields
+	bytesFields   fields
+	messageFields fields
+	groupFields   fields
+)
+
+// String and Set implement flag.Value.
+// The String method is not implemented since the flag helper never prints it.
+func (p *boolFields) String() string        { return "not implemented" }
+func (p *intFields) String() string         { return "not implemented" }
+func (p *sintFields) String() string        { return "not implemented" }
+func (p *uintFields) String() string        { return "not implemented" }
+func (p *uint32Fields) String() string      { return "not implemented" }
+func (p *int32Fields) String() string       { return "not implemented" }
+func (p *float32Fields) String() string     { return "not implemented" }
+func (p *uint64Fields) String() string      { return "not implemented" }
+func (p *int64Fields) String() string       { return "not implemented" }
+func (p *float64Fields) String() string     { return "not implemented" }
+func (p *stringFields) String() string      { return "not implemented" }
+func (p *bytesFields) String() string       { return "not implemented" }
+func (p *messageFields) String() string     { return "not implemented" }
+func (p *groupFields) String() string       { return "not implemented" }
+func (p *boolFields) Set(s string) error    { return (*fields)(p).Set(s, protoreflect.BoolKind) }
+func (p *intFields) Set(s string) error     { return (*fields)(p).Set(s, protoreflect.Int64Kind) }
+func (p *sintFields) Set(s string) error    { return (*fields)(p).Set(s, protoreflect.Sint64Kind) }
+func (p *uintFields) Set(s string) error    { return (*fields)(p).Set(s, protoreflect.Uint64Kind) }
+func (p *uint32Fields) Set(s string) error  { return (*fields)(p).Set(s, protoreflect.Fixed32Kind) }
+func (p *int32Fields) Set(s string) error   { return (*fields)(p).Set(s, protoreflect.Sfixed32Kind) }
+func (p *float32Fields) Set(s string) error { return (*fields)(p).Set(s, protoreflect.FloatKind) }
+func (p *uint64Fields) Set(s string) error  { return (*fields)(p).Set(s, protoreflect.Fixed64Kind) }
+func (p *int64Fields) Set(s string) error   { return (*fields)(p).Set(s, protoreflect.Sfixed64Kind) }
+func (p *float64Fields) Set(s string) error { return (*fields)(p).Set(s, protoreflect.DoubleKind) }
+func (p *stringFields) Set(s string) error  { return (*fields)(p).Set(s, protoreflect.StringKind) }
+func (p *bytesFields) Set(s string) error   { return (*fields)(p).Set(s, protoreflect.BytesKind) }
+func (p *messageFields) Set(s string) error { return (*fields)(p).Set(s, protoreflect.MessageKind) }
+func (p *groupFields) Set(s string) error   { return (*fields)(p).Set(s, protoreflect.GroupKind) }
diff --git a/internal/cmd/pbdump/pbdump_test.go b/internal/cmd/pbdump/pbdump_test.go
new file mode 100644
index 00000000..524a3a27
--- /dev/null
+++ b/internal/cmd/pbdump/pbdump_test.go
@@ -0,0 +1,116 @@
+// 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 file.
+
+package main
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+
+	pref "google.golang.org/proto/reflect/protoreflect"
+	ptype "google.golang.org/proto/reflect/prototype"
+)
+
+func TestFields(t *testing.T) {
+	type fieldsKind struct {
+		kind   pref.Kind
+		fields string
+	}
+	tests := []struct {
+		inFields []fieldsKind
+		wantMsg  ptype.Message
+		wantErr  string
+	}{{
+		inFields: []fieldsKind{{pref.MessageKind, ""}},
+		wantMsg:  ptype.Message{Name: "M"},
+	}, {
+		inFields: []fieldsKind{{pref.MessageKind, "987654321"}},
+		wantErr:  "invalid field: 987654321",
+	}, {
+		inFields: []fieldsKind{{pref.MessageKind, "-1"}},
+		wantErr:  "invalid field: -1",
+	}, {
+		inFields: []fieldsKind{{pref.MessageKind, "k"}},
+		wantErr:  "invalid field: k",
+	}, {
+		inFields: []fieldsKind{{pref.MessageKind, "1.2"}, {pref.Int32Kind, "1"}},
+		wantErr:  "field 1 of int32 type cannot have sub-fields",
+	}, {
+		inFields: []fieldsKind{{pref.Int32Kind, "1"}, {pref.MessageKind, "1.2"}},
+		wantErr:  "field 1 of int32 type cannot have sub-fields",
+	}, {
+		inFields: []fieldsKind{{pref.Int32Kind, "30"}, {pref.Int32Kind, "30"}},
+		wantErr:  "field 30 already set as int32 type",
+	}, {
+		inFields: []fieldsKind{
+			{pref.Int32Kind, "10.20.31"},
+			{pref.MessageKind, "  10.20.30, 10.21   "},
+			{pref.GroupKind, "10"},
+		},
+		wantMsg: ptype.Message{
+			Name: "M",
+			Fields: []ptype.Field{
+				{Name: "f10", Number: 10, Cardinality: pref.Optional, Kind: pref.GroupKind, MessageType: ptype.PlaceholderMessage("M.M10")},
+			},
+			Messages: []ptype.Message{{
+				Name: "M10",
+				Fields: []ptype.Field{
+					{Name: "f20", Number: 20, Cardinality: pref.Optional, Kind: pref.MessageKind, MessageType: ptype.PlaceholderMessage("M.M10.M20")},
+					{Name: "f21", Number: 21, Cardinality: pref.Optional, Kind: pref.MessageKind, MessageType: ptype.PlaceholderMessage("M.M10.M21")},
+				},
+				Messages: []ptype.Message{{
+					Name: "M20",
+					Fields: []ptype.Field{
+						{Name: "f30", Number: 30, Cardinality: pref.Optional, Kind: pref.MessageKind, MessageType: ptype.PlaceholderMessage("M.M10.M20.M30")},
+						{Name: "f31", Number: 31, Cardinality: pref.Repeated, IsPacked: true, Kind: pref.Int32Kind},
+					},
+					Messages: []ptype.Message{{
+						Name: "M30",
+					}},
+				}, {
+					Name: "M21",
+				}},
+			}},
+		},
+	}}
+
+	opts := cmp.Options{
+		cmp.Comparer(func(x, y pref.Descriptor) bool {
+			if x == nil || y == nil {
+				return x == nil && y == nil
+			}
+			return x.FullName() == y.FullName()
+		}),
+		cmpopts.IgnoreFields(ptype.Field{}, "Default"),
+		cmpopts.IgnoreUnexported(ptype.Message{}, ptype.Field{}),
+	}
+	for _, tt := range tests {
+		t.Run("", func(t *testing.T) {
+			var fields fields
+			for i, tc := range tt.inFields {
+				gotErr := fields.Set(tc.fields, tc.kind)
+				if gotErr != nil {
+					if tt.wantErr == "" || !strings.Contains(fmt.Sprint(gotErr), tt.wantErr) {
+						t.Fatalf("fields %d, Set(%q, %v) = %v, want %v", i, tc.fields, tc.kind, gotErr, tt.wantErr)
+					}
+					return
+				}
+			}
+			if tt.wantErr != "" {
+				t.Errorf("all Set calls succeeded, want %v error", tt.wantErr)
+			}
+			gotMsg := fields.messageDescriptor("M")
+			if diff := cmp.Diff(tt.wantMsg, gotMsg, opts); diff != "" {
+				t.Errorf("messageDescriptor() mismatch (-want +got):\n%v", diff)
+			}
+			if _, err := fields.Descriptor(); err != nil {
+				t.Errorf("Descriptor() = %v, want nil error", err)
+			}
+		})
+	}
+}