diff --git a/src/cmd/go/internal/fsys/fsys_test.go b/src/cmd/go/internal/fsys/fsys_test.go
index 7f175c70311..c080c14987c 100644
--- a/src/cmd/go/internal/fsys/fsys_test.go
+++ b/src/cmd/go/internal/fsys/fsys_test.go
@@ -1,7 +1,6 @@
 package fsys
 
 import (
-	"cmd/go/internal/txtar"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -12,6 +11,8 @@ import (
 	"path/filepath"
 	"reflect"
 	"testing"
+
+	"golang.org/x/tools/txtar"
 )
 
 // initOverlay resets the overlay state to reflect the config.
diff --git a/src/cmd/go/internal/txtar/archive_test.go b/src/cmd/go/internal/txtar/archive_test.go
deleted file mode 100644
index 3f734f67625..00000000000
--- a/src/cmd/go/internal/txtar/archive_test.go
+++ /dev/null
@@ -1,67 +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 file.
-
-package txtar
-
-import (
-	"bytes"
-	"fmt"
-	"reflect"
-	"testing"
-)
-
-var tests = []struct {
-	name   string
-	text   string
-	parsed *Archive
-}{
-	{
-		name: "basic",
-		text: `comment1
-comment2
--- file1 --
-File 1 text.
--- foo ---
-More file 1 text.
--- file 2 --
-File 2 text.
--- empty --
--- noNL --
-hello world`,
-		parsed: &Archive{
-			Comment: []byte("comment1\ncomment2\n"),
-			Files: []File{
-				{"file1", []byte("File 1 text.\n-- foo ---\nMore file 1 text.\n")},
-				{"file 2", []byte("File 2 text.\n")},
-				{"empty", []byte{}},
-				{"noNL", []byte("hello world\n")},
-			},
-		},
-	},
-}
-
-func Test(t *testing.T) {
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			a := Parse([]byte(tt.text))
-			if !reflect.DeepEqual(a, tt.parsed) {
-				t.Fatalf("Parse: wrong output:\nhave:\n%s\nwant:\n%s", shortArchive(a), shortArchive(tt.parsed))
-			}
-			text := Format(a)
-			a = Parse(text)
-			if !reflect.DeepEqual(a, tt.parsed) {
-				t.Fatalf("Parse after Format: wrong output:\nhave:\n%s\nwant:\n%s", shortArchive(a), shortArchive(tt.parsed))
-			}
-		})
-	}
-}
-
-func shortArchive(a *Archive) string {
-	var buf bytes.Buffer
-	fmt.Fprintf(&buf, "comment: %q\n", a.Comment)
-	for _, f := range a.Files {
-		fmt.Fprintf(&buf, "file %q: %q\n", f.Name, f.Data)
-	}
-	return buf.String()
-}
diff --git a/src/cmd/go/proxy_test.go b/src/cmd/go/proxy_test.go
index 74bfecc08db..a387fe67dbf 100644
--- a/src/cmd/go/proxy_test.go
+++ b/src/cmd/go/proxy_test.go
@@ -25,12 +25,12 @@ import (
 
 	"cmd/go/internal/modfetch/codehost"
 	"cmd/go/internal/par"
-	"cmd/go/internal/txtar"
 
 	"golang.org/x/mod/module"
 	"golang.org/x/mod/semver"
 	"golang.org/x/mod/sumdb"
 	"golang.org/x/mod/sumdb/dirhash"
+	"golang.org/x/tools/txtar"
 )
 
 var (
diff --git a/src/cmd/go/script_test.go b/src/cmd/go/script_test.go
index 8a7c77a46fa..3c5855bd6f2 100644
--- a/src/cmd/go/script_test.go
+++ b/src/cmd/go/script_test.go
@@ -31,9 +31,10 @@ import (
 	"cmd/go/internal/imports"
 	"cmd/go/internal/par"
 	"cmd/go/internal/robustio"
-	"cmd/go/internal/txtar"
 	"cmd/go/internal/work"
 	"cmd/internal/sys"
+
+	"golang.org/x/tools/txtar"
 )
 
 var testSum = flag.String("testsum", "", `may be tidy, listm, or listall. If set, TestScript generates a go.sum file at the beginning of each test and updates test files if they pass.`)
diff --git a/src/cmd/go/testdata/addmod.go b/src/cmd/go/testdata/addmod.go
index 03869e68def..a1ace4ce590 100644
--- a/src/cmd/go/testdata/addmod.go
+++ b/src/cmd/go/testdata/addmod.go
@@ -29,7 +29,7 @@ import (
 	"path/filepath"
 	"strings"
 
-	"cmd/go/internal/txtar"
+	"golang.org/x/tools/txtar"
 )
 
 func usage() {
diff --git a/src/cmd/go/testdata/savedir.go b/src/cmd/go/testdata/savedir.go
index d469c31a919..6a8a232702f 100644
--- a/src/cmd/go/testdata/savedir.go
+++ b/src/cmd/go/testdata/savedir.go
@@ -24,7 +24,7 @@ import (
 	"strings"
 	"unicode/utf8"
 
-	"../internal/txtar"
+	"golang.org/x/tools/txtar"
 )
 
 func usage() {
diff --git a/src/cmd/go/internal/txtar/archive.go b/src/cmd/vendor/golang.org/x/tools/txtar/archive.go
similarity index 96%
rename from src/cmd/go/internal/txtar/archive.go
rename to src/cmd/vendor/golang.org/x/tools/txtar/archive.go
index 17966848771..214256617b5 100644
--- a/src/cmd/go/internal/txtar/archive.go
+++ b/src/cmd/vendor/golang.org/x/tools/txtar/archive.go
@@ -34,7 +34,7 @@ package txtar
 import (
 	"bytes"
 	"fmt"
-	"os"
+	"io/ioutil"
 	"strings"
 )
 
@@ -66,7 +66,7 @@ func Format(a *Archive) []byte {
 
 // ParseFile parses the named file as an archive.
 func ParseFile(file string) (*Archive, error) {
-	data, err := os.ReadFile(file)
+	data, err := ioutil.ReadFile(file)
 	if err != nil {
 		return nil, err
 	}
@@ -121,7 +121,7 @@ func isMarker(data []byte) (name string, after []byte) {
 	if i := bytes.IndexByte(data, '\n'); i >= 0 {
 		data, after = data[:i], data[i+1:]
 	}
-	if !bytes.HasSuffix(data, markerEnd) {
+	if !(bytes.HasSuffix(data, markerEnd) && len(data) >= len(marker)+len(markerEnd)) {
 		return "", nil
 	}
 	return strings.TrimSpace(string(data[len(marker) : len(data)-len(markerEnd)])), after
diff --git a/src/cmd/vendor/modules.txt b/src/cmd/vendor/modules.txt
index c98bdcd3440..1765e3e1fc1 100644
--- a/src/cmd/vendor/modules.txt
+++ b/src/cmd/vendor/modules.txt
@@ -93,6 +93,7 @@ golang.org/x/tools/go/types/typeutil
 golang.org/x/tools/internal/analysisinternal
 golang.org/x/tools/internal/lsp/fuzzy
 golang.org/x/tools/internal/typeparams
+golang.org/x/tools/txtar
 # golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1
 ## explicit; go 1.11
 golang.org/x/xerrors