diff --git a/diagnose.go b/diagnose.go
index aaea4848..6403f985 100644
--- a/diagnose.go
+++ b/diagnose.go
@@ -66,6 +66,7 @@ func Diag(data []byte, opts *DiagOptions) ([]byte, error) {
 	return di.diag()
 }
 
+// loosest decode options for diagnostic purpose.
 var diagnoseDecMode, _ = DecOptions{
 	MaxNestedLevels: 256,
 	UTF8:            UTF8DecodeInvalid,
@@ -446,43 +447,72 @@ func (di *diagnose) encodeByteString(val []byte) error {
 var utf16SurrSelf = rune(0x10000)
 
 // quote should be either `'` or `"`
-func (di *diagnose) encodeTextString(val string, quote rune) error {
-	if err := di.writeByte(byte(quote)); err != nil {
+func (di *diagnose) encodeTextString(val string, quote byte) error {
+	if err := di.writeByte(quote); err != nil {
 		return err
 	}
 
-	for _, r := range val {
-		switch {
-		case r == '\t', r == '\n', r == '\r', r == '\\', r == quote:
-			if err := di.writeByte('\\'); err != nil {
-				return err
-			}
-			if err := di.writeByte(byte(r)); err != nil {
-				return err
+	for i := 0; i < len(val); {
+		if b := val[i]; b < utf8.RuneSelf {
+			switch {
+			case b == '\t', b == '\n', b == '\r', b == '\\', b == quote:
+				if err := di.writeByte('\\'); err != nil {
+					return err
+				}
+
+				switch b {
+				case '\t':
+					b = 't'
+				case '\n':
+					b = 'n'
+				case '\r':
+					b = 'r'
+				}
+				if err := di.writeByte(b); err != nil {
+					return err
+				}
+
+			case b >= ' ' && b <= '~':
+				if err := di.writeByte(b); err != nil {
+					return err
+				}
+
+			default:
+				if err := di.writeU16(rune(b)); err != nil {
+					return err
+				}
 			}
 
-		case r >= ' ' && r <= '~':
-			if err := di.writeByte(byte(r)); err != nil {
+			i++
+			continue
+		}
+
+		c, size := utf8.DecodeRuneInString(val[i:])
+		switch {
+		case c == utf8.RuneError:
+			if err := di.writeU16(rune(val[i])); err != nil {
 				return err
 			}
 
-		case r < utf16SurrSelf:
-			if err := di.writeU16(r); err != nil {
+		case c < utf16SurrSelf:
+			if err := di.writeU16(c); err != nil {
 				return err
 			}
 
 		default:
-			r1, r2 := utf16.EncodeRune(r)
-			if err := di.writeU16(r1); err != nil {
+			c1, c2 := utf16.EncodeRune(c)
+			if err := di.writeU16(c1); err != nil {
 				return err
 			}
-			if err := di.writeU16(r2); err != nil {
+			if err := di.writeU16(c2); err != nil {
 				return err
 			}
 		}
+
+		i += size
 	}
 
-	return di.writeByte(byte(quote))
+	return di.writeByte(quote)
 }
 
 func (di *diagnose) encodeFloat(ai byte, val uint64) error {
diff --git a/diagnose_test.go b/diagnose_test.go
index d27ec731..770eb5eb 100644
--- a/diagnose_test.go
+++ b/diagnose_test.go
@@ -549,6 +549,92 @@ func TestDiagnoseByteString(t *testing.T) {
 	})
 }
 
+func TestDiagnoseTextString(t *testing.T) {
+	testCases := []struct {
+		title string
+		cbor  []byte
+		diag  string
+		opts  *DiagOptions
+	}{
+		{
+			"valid UTF-8 text in byte string",
+			hexDecode("4d68656c6c6f2c20e4bda0e5a5bd"),
+			`'hello, \u4f60\u597d'`,
+			&DiagOptions{
+				ByteStringText: true,
+			},
+		},
+		{
+			"valid UTF-8 text in text string",
+			hexDecode("6d68656c6c6f2c20e4bda0e5a5bd"),
+			`"hello, \u4f60\u597d"`, // "hello, 你好"
+			&DiagOptions{
+				ByteStringText: true,
+			},
+		},
+		{
+			"invalid UTF-8 text in byte string",
+			hexDecode("4d68656c6c6fffeee4bda0e5a5bd"),
+			`h'68656c6c6fffeee4bda0e5a5bd'`,
+			&DiagOptions{
+				ByteStringText: true,
+			},
+		},
+		{
+			"invalid UTF-8 text in text string",
+			hexDecode("6d68656c6c6fffeee4bda0e5a5bd"),
+			`"hello\u00ff\u00ee\u4f60\u597d"`,
+			&DiagOptions{
+				ByteStringText: true,
+			},
+		},
+		{
+			"valid grapheme cluster text in byte string",
+			hexDecode("583448656c6c6f2c2027e29da4efb88fe2808df09f94a5270ae4bda0e5a5bdefbc8c22f09fa791e2808df09fa49de2808df09fa79122"),
+			`'Hello, \'\u2764\ufe0f\u200d\ud83d\udd25\'\n\u4f60\u597d\uff0c"\ud83e\uddd1\u200d\ud83e\udd1d\u200d\ud83e\uddd1"'`,
+			&DiagOptions{
+				ByteStringText: true,
+			},
+		},
+		{
+			"valid grapheme cluster text in text string",
+			hexDecode("783448656c6c6f2c2027e29da4efb88fe2808df09f94a5270ae4bda0e5a5bdefbc8c22f09fa791e2808df09fa49de2808df09fa79122"),
+			`"Hello, '\u2764\ufe0f\u200d\ud83d\udd25'\n\u4f60\u597d\uff0c\"\ud83e\uddd1\u200d\ud83e\udd1d\u200d\ud83e\uddd1\""`, // "Hello, '❤️‍🔥'\n你好，\"🧑‍🤝‍🧑\""
+			&DiagOptions{
+				ByteStringText: true,
+			},
+		},
+		{
+			"invalid grapheme cluster text in byte string",
+			hexDecode("583448656c6c6feeff27e29da4efb88fe2808df09f94a5270de4bda0e5a5bdefbc8c22f09fa791e2808df09fa49de2808df09fa79122"),
+			`h'48656c6c6feeff27e29da4efb88fe2808df09f94a5270de4bda0e5a5bdefbc8c22f09fa791e2808df09fa49de2808df09fa79122'`,
+			&DiagOptions{
+				ByteStringText: true,
+			},
+		},
+		{
+			"invalid grapheme cluster text in text string",
+			hexDecode("783448656c6c6feeff27e29da4efb88fe2808df09f94a5270de4bda0e5a5bdefbc8c22f09fa791e2808df09fa49de2808df09fa79122"),
+			`"Hello\u00ee\u00ff'\u2764\ufe0f\u200d\ud83d\udd25'\r\u4f60\u597d\uff0c\"\ud83e\uddd1\u200d\ud83e\udd1d\u200d\ud83e\uddd1\""`,
+			&DiagOptions{
+				ByteStringText: true,
+			},
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.title, func(t *testing.T) {
+
+			data, err := Diag(tc.cbor, tc.opts)
+			if err != nil {
+				t.Errorf("Diag(0x%x) returned error %q", tc.cbor, err)
+			} else if string(data) != tc.diag {
+				t.Errorf("Diag(0x%x) returned `%s`, want %s", tc.cbor, string(data), tc.diag)
+			}
+		})
+	}
+}
+
 func TestDiagnoseFloatingPointNumber(t *testing.T) {
 	testCases := []struct {
 		title string