From a50654ef4d8f069e0b3b2612f7241681f20317b9 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Wed, 14 Jan 2026 15:13:54 -1000
Subject: [PATCH] [modbus_controller] Use stack buffers instead of
 str_sprintf/str_snprintf

---
 .../modbus_controller/modbus_controller.h        |  8 ++++++--
 .../text_sensor/modbus_textsensor.cpp            | 16 ++++++++++++----
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/esphome/components/modbus_controller/modbus_controller.h b/esphome/components/modbus_controller/modbus_controller.h
index 6ed05715cb..466598e9cd 100644
--- a/esphome/components/modbus_controller/modbus_controller.h
+++ b/esphome/components/modbus_controller/modbus_controller.h
@@ -285,8 +285,12 @@ class ServerRegister {
       case SensorValueType::S_QWORD_R:
         return std::to_string(value);
       case SensorValueType::FP32_R:
-      case SensorValueType::FP32:
-        return str_sprintf("%.1f", bit_cast<float>(static_cast<uint32_t>(value)));
+      case SensorValueType::FP32: {
+        // max 48: float with %.1f can be up to 41 chars (3.4e38 → 39 digits + sign + decimal + 1 digit) + null
+        char buf[48];
+        snprintf(buf, sizeof(buf), "%.1f", bit_cast<float>(static_cast<uint32_t>(value)));
+        return buf;
+      }
       default:
         return std::to_string(value);
     }
diff --git a/esphome/components/modbus_controller/text_sensor/modbus_textsensor.cpp b/esphome/components/modbus_controller/text_sensor/modbus_textsensor.cpp
index 89e86741b0..c50e8317fa 100644
--- a/esphome/components/modbus_controller/text_sensor/modbus_textsensor.cpp
+++ b/esphome/components/modbus_controller/text_sensor/modbus_textsensor.cpp
@@ -16,12 +16,20 @@ void ModbusTextSensor::parse_and_publish(const std::vector<uint8_t> &data) {
   while ((items_left > 0) && index < data.size()) {
     uint8_t b = data[index];
     switch (this->encode_) {
-      case RawEncoding::HEXBYTES:
-        output_str += str_snprintf("%02x", 2, b);
+      case RawEncoding::HEXBYTES: {
+        // max 3: 2 hex digits + null
+        char hex_buf[3];
+        snprintf(hex_buf, sizeof(hex_buf), "%02x", b);
+        output_str += hex_buf;
         break;
-      case RawEncoding::COMMA:
-        output_str += str_sprintf(index != this->offset ? ",%d" : "%d", b);
+      }
+      case RawEncoding::COMMA: {
+        // max 5: ","(1) + uint8(3) + null
+        char dec_buf[5];
+        snprintf(dec_buf, sizeof(dec_buf), index != this->offset ? ",%d" : "%d", b);
+        output_str += dec_buf;
         break;
+      }
       case RawEncoding::ANSI:
         if (b < 0x20)
           break;