diff --git a/esphome/components/improv_serial/improv_serial_component.cpp b/esphome/components/improv_serial/improv_serial_component.cpp
index 281e95d12b..6111973f3f 100644
--- a/esphome/components/improv_serial/improv_serial_component.cpp
+++ b/esphome/components/improv_serial/improv_serial_component.cpp
@@ -263,8 +263,10 @@ bool ImprovSerialComponent::parse_improv_payload_(improv::ImprovCommand &command
         if (std::find(networks.begin(), networks.end(), ssid) != networks.end())
           continue;
         // Send each ssid separately to avoid overflowing the buffer
-        std::vector<uint8_t> data = improv::build_rpc_response(
-            improv::GET_WIFI_NETWORKS, {ssid, str_sprintf("%d", scan.get_rssi()), YESNO(scan.get_with_auth())}, false);
+        char rssi_buf[8];  // RSSI range: -127 to 0
+        snprintf(rssi_buf, sizeof(rssi_buf), "%d", scan.get_rssi());
+        std::vector<uint8_t> data =
+            improv::build_rpc_response(improv::GET_WIFI_NETWORKS, {ssid, rssi_buf, YESNO(scan.get_with_auth())}, false);
         this->send_response_(data);
         networks.push_back(ssid);
       }