diff --git a/td/telegram/ClientJson.cpp b/td/telegram/ClientJson.cpp
index 2f218d03095b..e67f620333cb 100644
--- a/td/telegram/ClientJson.cpp
+++ b/td/telegram/ClientJson.cpp
@@ -58,21 +58,18 @@ static string from_response(const td_api::Object &object, const string &extra, i
   auto buf = StackAllocator::alloc(1 << 18);
   JsonBuilder jb(StringBuilder(buf.as_slice(), true), -1);
   jb.enter_value() << ToJson(object);
-  auto slice = jb.string_builder().as_cslice();
+  auto &sb = jb.string_builder();
+  auto slice = sb.as_cslice();
   CHECK(!slice.empty() && slice.back() == '}');
-  string str;
-  str.reserve(slice.size() + (extra.empty() ? 0 : 10 + extra.size()) + (client_id == 0 ? 0 : 14 + 10));
-  str.append(slice.begin(), slice.size() - 1);
+  sb.pop_back();
   if (!extra.empty()) {
-    str += ",\"@extra\":";
-    str += extra;
+    sb << ",\"@extra\":" << extra;
   }
   if (client_id != 0) {
-    str += ",\"@client_id\":";
-    str += to_string(client_id);
+    sb << ",\"@client_id\":" << client_id;
   }
-  str += '}';
-  return str;
+  sb << '}';
+  return sb.as_cslice().str();
 }
 
 static TD_THREAD_LOCAL string *current_output;
diff --git a/tdutils/td/utils/StringBuilder.h b/tdutils/td/utils/StringBuilder.h
index 1d7da54ffe67..1a1987caa108 100644
--- a/tdutils/td/utils/StringBuilder.h
+++ b/tdutils/td/utils/StringBuilder.h
@@ -27,6 +27,11 @@ class StringBuilder {
     error_flag_ = false;
   }
 
+  void pop_back() {
+    CHECK(current_ptr_ > begin_ptr_);
+    current_ptr_--;
+  }
+
   MutableCSlice as_cslice() {
     if (current_ptr_ >= end_ptr_ + RESERVED_SIZE) {
       std::abort();  // shouldn't happen