From a1395af7636e787dda0e1e916d8afc4fbcd2b046 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Sat, 10 Jan 2026 17:07:21 -1000
Subject: [PATCH] [helpers] Add format_hex_prefixed_to for "0x" prefixed hex
 formatting (#13115)

---
 esphome/components/one_wire/one_wire.cpp      |  6 +++--
 .../sml/text_sensor/sml_text_sensor.cpp       |  6 ++---
 esphome/core/helpers.h                        | 23 +++++++++++++++++++
 3 files changed, 29 insertions(+), 6 deletions(-)

diff --git a/esphome/components/one_wire/one_wire.cpp b/esphome/components/one_wire/one_wire.cpp
index fd139d0ddc..187f559ca6 100644
--- a/esphome/components/one_wire/one_wire.cpp
+++ b/esphome/components/one_wire/one_wire.cpp
@@ -6,8 +6,10 @@ namespace one_wire {
 static const char *const TAG = "one_wire";
 
 const std::string &OneWireDevice::get_address_name() {
-  if (this->address_name_.empty())
-    this->address_name_ = std::string("0x") + format_hex(this->address_);
+  if (this->address_name_.empty()) {
+    char hex_buf[19];  // "0x" + 16 hex chars + null
+    this->address_name_ = format_hex_prefixed_to(hex_buf, this->address_);
+  }
   return this->address_name_;
 }
 
diff --git a/esphome/components/sml/text_sensor/sml_text_sensor.cpp b/esphome/components/sml/text_sensor/sml_text_sensor.cpp
index 6ceff26fe5..17b93ecccf 100644
--- a/esphome/components/sml/text_sensor/sml_text_sensor.cpp
+++ b/esphome/components/sml/text_sensor/sml_text_sensor.cpp
@@ -24,11 +24,9 @@ void SmlTextSensor::publish_val(const ObisInfo &obis_info) {
     case SML_HEX: {
       // Buffer for "0x" + up to 32 bytes as hex + null
       char buf[67];
-      buf[0] = '0';
-      buf[1] = 'x';
-      // Max 32 bytes of data fit in remaining buffer ((65-1)/2)
+      // Max 32 bytes of data fit in buffer ((67-3)/2)
       size_t hex_bytes = std::min(obis_info.value.size(), size_t(32));
-      format_hex_to(buf + 2, sizeof(buf) - 2, obis_info.value.begin(), hex_bytes);
+      format_hex_prefixed_to(buf, obis_info.value.begin(), hex_bytes);
       publish_state(buf, 2 + hex_bytes * 2);
       break;
     }
diff --git a/esphome/core/helpers.h b/esphome/core/helpers.h
index 05d2d475c1..cd43709f7d 100644
--- a/esphome/core/helpers.h
+++ b/esphome/core/helpers.h
@@ -759,6 +759,29 @@ inline char *format_hex_to(char (&buffer)[N], T val) {
 /// Calculate buffer size needed for format_hex_to: "XXXXXXXX...\0" = bytes * 2 + 1
 constexpr size_t format_hex_size(size_t byte_count) { return byte_count * 2 + 1; }
 
+/// Calculate buffer size needed for format_hex_prefixed_to: "0xXXXXXXXX...\0" = bytes * 2 + 3
+constexpr size_t format_hex_prefixed_size(size_t byte_count) { return byte_count * 2 + 3; }
+
+/// Format an unsigned integer as "0x" prefixed lowercase hex to buffer.
+template<size_t N, typename T, enable_if_t<std::is_unsigned<T>::value, int> = 0>
+inline char *format_hex_prefixed_to(char (&buffer)[N], T val) {
+  static_assert(N >= sizeof(T) * 2 + 3, "Buffer too small for prefixed hex");
+  buffer[0] = '0';
+  buffer[1] = 'x';
+  val = convert_big_endian(val);
+  format_hex_to(buffer + 2, N - 2, reinterpret_cast<const uint8_t *>(&val), sizeof(T));
+  return buffer;
+}
+
+/// Format byte array as "0x" prefixed lowercase hex to buffer.
+template<size_t N> inline char *format_hex_prefixed_to(char (&buffer)[N], const uint8_t *data, size_t length) {
+  static_assert(N >= 5, "Buffer must hold at least '0x' + one hex byte + null");
+  buffer[0] = '0';
+  buffer[1] = 'x';
+  format_hex_to(buffer + 2, N - 2, data, length);
+  return buffer;
+}
+
 /// Calculate buffer size needed for format_hex_pretty_to with separator: "XX:XX:...:XX\0"
 constexpr size_t format_hex_pretty_size(size_t byte_count) { return byte_count * 3; }