diff --git a/lib/filterx/tests/test_object_boolean.c b/lib/filterx/tests/test_object_boolean.c
index b2fd6eab6..d4c6e1634 100644
--- a/lib/filterx/tests/test_object_boolean.c
+++ b/lib/filterx/tests/test_object_boolean.c
@@ -124,8 +124,11 @@ Test(filterx_boolean, test_filterx_boolean_repr)
 {
   FilterXObject *obj = filterx_boolean_new(TRUE);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(obj, repr));
   cr_assert_str_eq("true", repr->str);
+  cr_assert(filterx_object_repr_append(obj, repr));
+  cr_assert_str_eq("truetrue", repr->str);
   filterx_object_unref(obj);
 }
 
diff --git a/lib/filterx/tests/test_object_bytes.c b/lib/filterx/tests/test_object_bytes.c
index b4c3d6d7f..040d0484e 100644
--- a/lib/filterx/tests/test_object_bytes.c
+++ b/lib/filterx/tests/test_object_bytes.c
@@ -122,6 +122,18 @@ Test(filterx_bytes, test_filterx_bytes_typecast_from_protobuf)
   filterx_object_unref(obj);
 }
 
+Test(filterx_bytes, filterx_bytes_repr)
+{
+  FilterXObject *obj = filterx_bytes_new("\0\1\2\3", 4);
+  GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
+  cr_assert(filterx_object_repr(obj, repr));
+  cr_assert_str_eq("00010203", repr->str);
+  cr_assert(filterx_object_repr_append(obj, repr));
+  cr_assert_str_eq("0001020300010203", repr->str);
+  filterx_object_unref(obj);
+}
+
 static void
 setup(void)
 {
diff --git a/lib/filterx/tests/test_object_datetime.c b/lib/filterx/tests/test_object_datetime.c
index a760c4142..6f3d2fc3f 100644
--- a/lib/filterx/tests/test_object_datetime.c
+++ b/lib/filterx/tests/test_object_datetime.c
@@ -172,9 +172,8 @@ Test(filterx_datetime, test_filterx_datetime_typecast_from_datetime)
 
 Test(filterx_datetime, test_filterx_datetime_repr)
 {
-  const gchar *test_time_str = "2024-03-18T12:34:13+234";
   GPtrArray *args = g_ptr_array_new_with_free_func((GDestroyNotify) filterx_object_unref);
-  FilterXObject *in = filterx_string_new(test_time_str, -1);
+  FilterXObject *in = filterx_string_new("2024-03-18T12:34:13+234", -1);
   g_ptr_array_add(args, in);
 
   FilterXObject *obj = filterx_typecast_datetime(args);
@@ -182,9 +181,11 @@ Test(filterx_datetime, test_filterx_datetime_repr)
   cr_assert(filterx_object_is_type(obj, &FILTERX_TYPE_NAME(datetime)));
 
   GString *repr = scratch_buffers_alloc();
-
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(obj, repr));
   cr_assert_str_eq("2024-03-18T12:34:13.000+09:00", repr->str);
+  cr_assert(filterx_object_repr_append(obj, repr));
+  cr_assert_str_eq("2024-03-18T12:34:13.000+09:002024-03-18T12:34:13.000+09:00", repr->str);
 
   g_ptr_array_free(args, TRUE);
   filterx_object_unref(obj);
diff --git a/lib/filterx/tests/test_object_double.c b/lib/filterx/tests/test_object_double.c
index c71145bd3..8abdcdae3 100644
--- a/lib/filterx/tests/test_object_double.c
+++ b/lib/filterx/tests/test_object_double.c
@@ -152,8 +152,11 @@ Test(filterx_double, test_filterx_double_repr)
 {
   FilterXObject *obj = filterx_double_new(123.456);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(obj, repr));
   cr_assert_str_eq("123.456", repr->str);
+  cr_assert(filterx_object_repr_append(obj, repr));
+  cr_assert_str_eq("123.456123.456", repr->str);
   filterx_object_unref(obj);
 }
 
diff --git a/lib/filterx/tests/test_object_integer.c b/lib/filterx/tests/test_object_integer.c
index e3d803d52..67798ef5b 100644
--- a/lib/filterx/tests/test_object_integer.c
+++ b/lib/filterx/tests/test_object_integer.c
@@ -206,8 +206,11 @@ Test(filterx_integer, test_filterx_integer_repr)
 {
   FilterXObject *obj = filterx_integer_new(65566);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(obj, repr));
   cr_assert_str_eq("65566", repr->str);
+  cr_assert(filterx_object_repr_append(obj, repr));
+  cr_assert_str_eq("6556665566", repr->str);
   filterx_object_unref(obj);
 }
 
diff --git a/lib/filterx/tests/test_object_json.c b/lib/filterx/tests/test_object_json.c
index 55810b362..cbe76d752 100644
--- a/lib/filterx/tests/test_object_json.c
+++ b/lib/filterx/tests/test_object_json.c
@@ -28,6 +28,7 @@
 #include "filterx/object-message-value.h"
 #include "filterx/expr-function.h"
 #include "apphook.h"
+#include "scratch-buffers.h"
 
 Test(filterx_json, test_filterx_object_json_from_repr)
 {
@@ -164,6 +165,30 @@ Test(filterx_json, test_json_array_function)
   filterx_object_unref(fobj);
 }
 
+Test(filterx_json, filterx_json_object_repr)
+{
+  FilterXObject *obj = filterx_json_object_new_from_repr("{\"foo\": \"foovalue\"}", -1);
+  GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
+  cr_assert(filterx_object_repr(obj, repr));
+  cr_assert_str_eq("{\"foo\":\"foovalue\"}", repr->str);
+  cr_assert(filterx_object_repr_append(obj, repr));
+  cr_assert_str_eq("{\"foo\":\"foovalue\"}{\"foo\":\"foovalue\"}", repr->str);
+  filterx_object_unref(obj);
+}
+
+Test(filterx_json, filterx_json_array_repr)
+{
+  FilterXObject *obj = filterx_json_array_new_from_repr("[\"foo\", \"bar\"]", -1);
+  GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
+  cr_assert(filterx_object_repr(obj, repr));
+  cr_assert_str_eq("[\"foo\",\"bar\"]", repr->str);
+  cr_assert(filterx_object_repr_append(obj, repr));
+  cr_assert_str_eq("[\"foo\",\"bar\"][\"foo\",\"bar\"]", repr->str);
+  filterx_object_unref(obj);
+}
+
 static void
 setup(void)
 {
diff --git a/lib/filterx/tests/test_object_message.c b/lib/filterx/tests/test_object_message.c
index 946d600c3..9ffb61c82 100644
--- a/lib/filterx/tests/test_object_message.c
+++ b/lib/filterx/tests/test_object_message.c
@@ -77,52 +77,59 @@ Test(filterx_message, test_filterx_message_type_null_repr)
 {
   FilterXObject *fobj = filterx_message_value_new(NULL, 0, LM_VT_NULL);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
   cr_assert_str_eq("null", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("nullnull", repr->str);
   filterx_object_unref(fobj);
 }
 
 Test(filterx_message, test_filterx_message_type_string_repr)
 {
-  gchar *test_str = "any string";
-
-  FilterXObject *fobj = filterx_message_value_new(test_str, -1, LM_VT_STRING);
+  FilterXObject *fobj = filterx_message_value_new("any string", -1, LM_VT_STRING);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
-  cr_assert_str_eq(test_str, repr->str);
+  cr_assert_str_eq("any string", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("any stringany string", repr->str);
   filterx_object_unref(fobj);
 }
 
 Test(filterx_message, test_filterx_message_type_bytes_repr)
 {
-  gchar *test_str = "any bytes";
-
-  FilterXObject *fobj = filterx_message_value_new(test_str, -1, LM_VT_BYTES);
+  FilterXObject *fobj = filterx_message_value_new("any bytes", -1, LM_VT_BYTES);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
-  cr_assert_str_eq(test_str, repr->str);
+  cr_assert_str_eq("any bytes", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("any bytesany bytes", repr->str);
   filterx_object_unref(fobj);
 }
 
 Test(filterx_message, test_filterx_message_type_protobuf_repr)
 {
-  gchar *test_str = "not a valid protobuf!";
-
-  FilterXObject *fobj = filterx_message_value_new(test_str, -1, LM_VT_PROTOBUF);
+  FilterXObject *fobj = filterx_message_value_new("not a valid protobuf!", -1, LM_VT_PROTOBUF);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
-  cr_assert_str_eq(test_str, repr->str);
+  cr_assert_str_eq("not a valid protobuf!", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("not a valid protobuf!not a valid protobuf!", repr->str);
   filterx_object_unref(fobj);
 }
 
 Test(filterx_message, test_filterx_message_type_json_repr)
 {
-  gchar *test_str = "{\"test\":\"json\"}";
-
-  FilterXObject *fobj = filterx_message_value_new(test_str, -1, LM_VT_JSON);
+  FilterXObject *fobj = filterx_message_value_new("{\"test\": \"json\"}", -1, LM_VT_JSON);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
-  cr_assert_str_eq(test_str, repr->str);
+  cr_assert_str_eq("{\"test\": \"json\"}", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("{\"test\": \"json\"}{\"test\": \"json\"}", repr->str);
   filterx_object_unref(fobj);
 }
 
@@ -131,8 +138,11 @@ Test(filterx_message, test_filterx_message_type_boolean_repr)
   gchar *val = "T";
   FilterXObject *fobj = filterx_message_value_new(val, -1, LM_VT_BOOLEAN);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
   cr_assert_str_eq("true", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("truetrue", repr->str);
   filterx_object_unref(fobj);
 }
 
@@ -141,8 +151,11 @@ Test(filterx_message, test_filterx_message_type_int_repr)
   gchar *val = "443";
   FilterXObject *fobj = filterx_message_value_new(val, -1, LM_VT_INTEGER);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
   cr_assert_str_eq("443", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("443443", repr->str);
   filterx_object_unref(fobj);
 }
 
@@ -151,8 +164,11 @@ Test(filterx_message, test_filterx_message_type_double_repr)
   gchar *val = "17.756";
   FilterXObject *fobj = filterx_message_value_new(val, -1, LM_VT_DOUBLE);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
   cr_assert_str_eq("17.756", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("17.75617.756", repr->str);
   filterx_object_unref(fobj);
 }
 
@@ -161,8 +177,11 @@ Test(filterx_message, test_filterx_message_type_datetime_repr)
   gchar *val = "1713520972.000000+02:00";
   FilterXObject *fobj = filterx_message_value_new(val, -1, LM_VT_DATETIME);
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
   cr_assert_str_eq("2024-04-19T12:02:52.000+02:00", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("2024-04-19T12:02:52.000+02:002024-04-19T12:02:52.000+02:00", repr->str);
   filterx_object_unref(fobj);
 }
 
diff --git a/lib/filterx/tests/test_object_null.c b/lib/filterx/tests/test_object_null.c
index 0f17d5b30..b17694d0c 100644
--- a/lib/filterx/tests/test_object_null.c
+++ b/lib/filterx/tests/test_object_null.c
@@ -45,8 +45,12 @@ Test(filterx_null, test_filterx_object_null_repr)
 {
   FilterXObject *fobj = filterx_null_new();
   GString *repr = scratch_buffers_alloc();
+  g_string_assign(repr, "foo");
   cr_assert(filterx_object_repr(fobj, repr));
   cr_assert_str_eq("null", repr->str);
+  cr_assert(filterx_object_repr_append(fobj, repr));
+  cr_assert_str_eq("nullnull", repr->str);
+  filterx_object_unref(fobj);
 }
 
 static void