diff --git a/esphome/components/mqtt/mqtt_client.cpp b/esphome/components/mqtt/mqtt_client.cpp
index 0ab5b238b5..be3ac16147 100644
--- a/esphome/components/mqtt/mqtt_client.cpp
+++ b/esphome/components/mqtt/mqtt_client.cpp
@@ -91,7 +91,15 @@ void MQTTClientComponent::send_device_info_() {
         uint8_t index = 0;
         for (auto &ip : network::get_ip_addresses()) {
           if (ip.is_set()) {
-            root["ip" + (index == 0 ? "" : esphome::to_string(index))] = ip.str();
+            char key[8];  // "ip" + up to 3 digits + null
+            char ip_buf[network::IP_ADDRESS_BUFFER_SIZE];
+            if (index == 0) {
+              strcpy(key, "ip");
+            } else {
+              snprintf(key, sizeof(key), "ip%u", index);
+            }
+            ip.str_to(ip_buf);
+            root[key] = ip_buf;
             index++;
           }
         }
diff --git a/esphome/components/mqtt/mqtt_component.cpp b/esphome/components/mqtt/mqtt_component.cpp
index 20c111de43..66f6a1e625 100644
--- a/esphome/components/mqtt/mqtt_component.cpp
+++ b/esphome/components/mqtt/mqtt_component.cpp
@@ -232,7 +232,10 @@ bool MQTTComponent::send_discovery_() {
 #else
         const char *fmt = ver_fmt;
 #endif
-        device_info[MQTT_DEVICE_SW_VERSION] = str_sprintf(fmt, App.get_config_hash());
+        // sizeof(ver_fmt) + 8: format specifier expands to 8 hex digits, plus safety margin
+        char version_buf[sizeof(ver_fmt) + 8];
+        snprintf(version_buf, sizeof(version_buf), fmt, App.get_config_hash());
+        device_info[MQTT_DEVICE_SW_VERSION] = version_buf;
         device_info[MQTT_DEVICE_MODEL] = ESPHOME_BOARD;
 #if defined(USE_ESP8266) || defined(USE_ESP32)
         device_info[MQTT_DEVICE_MANUFACTURER] = "Espressif";