From 95974358ab718afb2fd97014ff8c138ce3d4280b Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Mon, 6 Apr 2026 18:03:26 -1000
Subject: [PATCH 1/5] [api] Add max_data_length and force to
 DeviceInfoResponse/HelloResponse proto fields

Add max_data_length + force proto options to string fields in
HelloResponse, AreaInfo, DeviceInfo, and DeviceInfoResponse where
we have strong compile-time or codegen guarantees on max length.

This enables encode_short_string_force() (direct byte writes,
no varint branching) for fields with single-byte tags, and
fixed-formula size calculations for all annotated fields.

Add static_asserts to cross-validate C++ constexpr constants
(MAC_ADDRESS_PRETTY_BUFFER_SIZE, BUILD_TIME_STR_SIZE,
ESPHOME_VERSION, ESPHOME_MANUFACTURER) against the proto
max_data_length annotations.
---
 esphome/components/api/api.proto          | 33 ++++++++++-------
 esphome/components/api/api_connection.cpp |  7 ++++
 esphome/components/api/api_pb2.cpp        | 44 +++++++++++------------
 3 files changed, 50 insertions(+), 34 deletions(-)

diff --git a/esphome/components/api/api.proto b/esphome/components/api/api.proto
index 07705baff6..8e3b5a1ffb 100644
--- a/esphome/components/api/api.proto
+++ b/esphome/components/api/api.proto
@@ -129,11 +129,12 @@ message HelloResponse {
 
   // A string identifying the server (ESP); like client info this may be empty
   // and only exists for debugging/logging purposes.
-  // For example "ESPHome v1.10.0 on ESP8266"
-  string server_info = 3;
+  // Currently set to ESPHOME_VERSION string literal.
+  string server_info = 3 [(max_data_length) = 32, (force) = true];
 
   // The name of the server (App.get_name())
-  string name = 4;
+  // max_data_length matches config_validation.NAME_MAX_LENGTH
+  string name = 4 [(max_data_length) = 120, (force) = true];
 }
 
 // DEPRECATED in ESPHome 2026.1.0 - Password authentication is no longer supported.
@@ -196,12 +197,14 @@ message DeviceInfoRequest {
 
 message AreaInfo {
   uint32 area_id = 1;
-  string name = 2;
+  // max_data_length matches core/config.FRIENDLY_NAME_MAX_LEN via AREA_SCHEMA
+  string name = 2 [(max_data_length) = 120, (force) = true];
 }
 
 message DeviceInfo {
   uint32 device_id = 1;
-  string name = 2;
+  // max_data_length matches core/config.FRIENDLY_NAME_MAX_LEN via DEVICE_SCHEMA
+  string name = 2 [(max_data_length) = 120, (force) = true];
   uint32 area_id = 3;
 }
 
@@ -216,6 +219,12 @@ message SerialProxyInfo {
   SerialProxyPortType port_type = 2;  // Port type (RS232, RS485)
 }
 
+// DeviceInfoResponse max_data_length values:
+//   name/friendly_name = 120 (config_validation.NAME_MAX_LENGTH / core/config.FRIENDLY_NAME_MAX_LEN)
+//   mac_address/bluetooth_mac_address = 17 (MAC_ADDRESS_PRETTY_BUFFER_SIZE - 1, constexpr)
+//   esphome_version = 32 (ESPHOME_VERSION string literal)
+//   compilation_time = 25 (Application::BUILD_TIME_STR_SIZE - 1, constexpr)
+//   manufacturer = 20 (longest hardcoded literal: "Nordic Semiconductor")
 message DeviceInfoResponse {
   option (id) = 10;
   option (source) = SOURCE_SERVER;
@@ -225,18 +234,18 @@ message DeviceInfoResponse {
   bool uses_password = 1 [deprecated = true];
 
   // The name of the node, given by "App.set_name()"
-  string name = 2;
+  string name = 2 [(max_data_length) = 120, (force) = true];
 
   // The mac address of the device. For example "AC:BC:32:89:0E:A9"
-  string mac_address = 3;
+  string mac_address = 3 [(max_data_length) = 17, (force) = true];
 
   // A string describing the ESPHome version. For example "1.10.0"
-  string esphome_version = 4;
+  string esphome_version = 4 [(max_data_length) = 32, (force) = true];
 
   // A string describing the date of compilation, this is generated by the compiler
   // and therefore may not be in the same format all the time.
   // If the user isn't using ESPHome, this will also not be set.
-  string compilation_time = 5;
+  string compilation_time = 5 [(max_data_length) = 25, (force) = true];
 
   // The model of the board. For example NodeMCU
   string model = 6;
@@ -253,9 +262,9 @@ message DeviceInfoResponse {
   uint32 legacy_bluetooth_proxy_version = 11 [deprecated=true, (field_ifdef) = "USE_BLUETOOTH_PROXY"];
   uint32 bluetooth_proxy_feature_flags = 15 [(field_ifdef) = "USE_BLUETOOTH_PROXY"];
 
-  string manufacturer = 12;
+  string manufacturer = 12 [(max_data_length) = 20, (force) = true];
 
-  string friendly_name = 13;
+  string friendly_name = 13 [(max_data_length) = 120, (force) = true];
 
   // Deprecated in API version 1.10
   uint32 legacy_voice_assistant_version = 14 [deprecated=true, (field_ifdef) = "USE_VOICE_ASSISTANT"];
@@ -264,7 +273,7 @@ message DeviceInfoResponse {
   string suggested_area = 16 [(field_ifdef) = "USE_AREAS"];
 
   // The Bluetooth mac address of the device. For example "AC:BC:32:89:0E:AA"
-  string bluetooth_mac_address = 18 [(field_ifdef) = "USE_BLUETOOTH_PROXY"];
+  string bluetooth_mac_address = 18 [(max_data_length) = 17, (force) = true, (field_ifdef) = "USE_BLUETOOTH_PROXY"];
 
   // Supports receiving and saving api encryption key
   bool api_encryption_supported = 19 [(field_ifdef) = "USE_API_NOISE"];
diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index feb16e4f4c..53c4aabcb7 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -72,6 +72,12 @@ static constexpr uint32_t HANDSHAKE_TIMEOUT_MS = 60000;
 
 static constexpr auto ESPHOME_VERSION_REF = StringRef::from_lit(ESPHOME_VERSION);
 
+// Cross-validate C++ constants against proto max_data_length annotations in api.proto
+static_assert(MAC_ADDRESS_PRETTY_BUFFER_SIZE - 1 == 17,
+              "Update max_data_length for mac_address/bluetooth_mac_address in api.proto");
+static_assert(Application::BUILD_TIME_STR_SIZE - 1 == 25, "Update max_data_length for compilation_time in api.proto");
+static_assert(sizeof(ESPHOME_VERSION) - 1 <= 32, "Update max_data_length for esphome_version in api.proto");
+
 static const char *const TAG = "api.connection";
 #ifdef USE_CAMERA
 static const int CAMERA_STOP_STREAM = 5000;
@@ -1716,6 +1722,7 @@ bool APIConnection::send_device_info_response_() {
   static constexpr auto MANUFACTURER = StringRef::from_lit(ESPHOME_MANUFACTURER);
   resp.manufacturer = MANUFACTURER;
 #endif
+  static_assert(sizeof(ESPHOME_MANUFACTURER) - 1 <= 20, "Update max_data_length for manufacturer in api.proto");
 #undef ESPHOME_MANUFACTURER
 
 #ifdef USE_ESP8266
diff --git a/esphome/components/api/api_pb2.cpp b/esphome/components/api/api_pb2.cpp
index f7c68b95a7..66996452fa 100644
--- a/esphome/components/api/api_pb2.cpp
+++ b/esphome/components/api/api_pb2.cpp
@@ -35,29 +35,29 @@ uint8_t *HelloResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM
   uint8_t *__restrict__ pos = buffer.get_pos();
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 1, this->api_version_major);
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 2, this->api_version_minor);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 3, this->server_info);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 4, this->name);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 26, this->server_info);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 34, this->name);
   return pos;
 }
 uint32_t HelloResponse::calculate_size() const {
   uint32_t size = 0;
   size += ProtoSize::calc_uint32(1, this->api_version_major);
   size += ProtoSize::calc_uint32(1, this->api_version_minor);
-  size += ProtoSize::calc_length(1, this->server_info.size());
-  size += ProtoSize::calc_length(1, this->name.size());
+  size += 2 + this->server_info.size();
+  size += 2 + this->name.size();
   return size;
 }
 #ifdef USE_AREAS
 uint8_t *AreaInfo::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
   uint8_t *__restrict__ pos = buffer.get_pos();
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 1, this->area_id);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 2, this->name);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 18, this->name);
   return pos;
 }
 uint32_t AreaInfo::calculate_size() const {
   uint32_t size = 0;
   size += ProtoSize::calc_uint32(1, this->area_id);
-  size += ProtoSize::calc_length(1, this->name.size());
+  size += 2 + this->name.size();
   return size;
 }
 #endif
@@ -65,14 +65,14 @@ uint32_t AreaInfo::calculate_size() const {
 uint8_t *DeviceInfo::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
   uint8_t *__restrict__ pos = buffer.get_pos();
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 1, this->device_id);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 2, this->name);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 18, this->name);
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 3, this->area_id);
   return pos;
 }
 uint32_t DeviceInfo::calculate_size() const {
   uint32_t size = 0;
   size += ProtoSize::calc_uint32(1, this->device_id);
-  size += ProtoSize::calc_length(1, this->name.size());
+  size += 2 + this->name.size();
   size += ProtoSize::calc_uint32(1, this->area_id);
   return size;
 }
@@ -93,10 +93,10 @@ uint32_t SerialProxyInfo::calculate_size() const {
 #endif
 uint8_t *DeviceInfoResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_PARAM) const {
   uint8_t *__restrict__ pos = buffer.get_pos();
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 2, this->name);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 3, this->mac_address);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 4, this->esphome_version);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 5, this->compilation_time);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 18, this->name);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 26, this->mac_address);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 34, this->esphome_version);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 42, this->compilation_time);
   ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 6, this->model);
 #ifdef USE_DEEP_SLEEP
   ProtoEncode::encode_bool(pos PROTO_ENCODE_DEBUG_ARG, 7, this->has_deep_sleep);
@@ -113,8 +113,8 @@ uint8_t *DeviceInfoResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_
 #ifdef USE_BLUETOOTH_PROXY
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 15, this->bluetooth_proxy_feature_flags);
 #endif
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 12, this->manufacturer);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 13, this->friendly_name);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 98, this->manufacturer);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 106, this->friendly_name);
 #ifdef USE_VOICE_ASSISTANT
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 17, this->voice_assistant_feature_flags);
 #endif
@@ -122,7 +122,7 @@ uint8_t *DeviceInfoResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_
   ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 16, this->suggested_area);
 #endif
 #ifdef USE_BLUETOOTH_PROXY
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 18, this->bluetooth_mac_address);
+  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 18, this->bluetooth_mac_address, true);
 #endif
 #ifdef USE_API_NOISE
   ProtoEncode::encode_bool(pos PROTO_ENCODE_DEBUG_ARG, 19, this->api_encryption_supported);
@@ -155,10 +155,10 @@ uint8_t *DeviceInfoResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_
 }
 uint32_t DeviceInfoResponse::calculate_size() const {
   uint32_t size = 0;
-  size += ProtoSize::calc_length(1, this->name.size());
-  size += ProtoSize::calc_length(1, this->mac_address.size());
-  size += ProtoSize::calc_length(1, this->esphome_version.size());
-  size += ProtoSize::calc_length(1, this->compilation_time.size());
+  size += 2 + this->name.size();
+  size += 2 + this->mac_address.size();
+  size += 2 + this->esphome_version.size();
+  size += 2 + this->compilation_time.size();
   size += ProtoSize::calc_length(1, this->model.size());
 #ifdef USE_DEEP_SLEEP
   size += ProtoSize::calc_bool(1, this->has_deep_sleep);
@@ -175,8 +175,8 @@ uint32_t DeviceInfoResponse::calculate_size() const {
 #ifdef USE_BLUETOOTH_PROXY
   size += ProtoSize::calc_uint32(1, this->bluetooth_proxy_feature_flags);
 #endif
-  size += ProtoSize::calc_length(1, this->manufacturer.size());
-  size += ProtoSize::calc_length(1, this->friendly_name.size());
+  size += 2 + this->manufacturer.size();
+  size += 2 + this->friendly_name.size();
 #ifdef USE_VOICE_ASSISTANT
   size += ProtoSize::calc_uint32(2, this->voice_assistant_feature_flags);
 #endif
@@ -184,7 +184,7 @@ uint32_t DeviceInfoResponse::calculate_size() const {
   size += ProtoSize::calc_length(2, this->suggested_area.size());
 #endif
 #ifdef USE_BLUETOOTH_PROXY
-  size += ProtoSize::calc_length(2, this->bluetooth_mac_address.size());
+  size += 3 + this->bluetooth_mac_address.size();
 #endif
 #ifdef USE_API_NOISE
   size += ProtoSize::calc_bool(2, this->api_encryption_supported);

From 5a92a3b16e62967b06a24db903078b56b72d6826 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Mon, 6 Apr 2026 18:14:18 -1000
Subject: [PATCH 2/5] [api] Add max_data_length/force to model and project
 fields, validate board/project lengths

Add BOARD_MAX_LENGTH and PROJECT_MAX_LENGTH constants (127) and
enforce them in YAML validation for all platform board schemas
and project name/version.

Add max_data_length + force proto options to model, project_name,
and project_version fields in DeviceInfoResponse.
---
 esphome/components/api/api.proto         | 10 +++++++---
 esphome/components/api/api_pb2.cpp       | 12 ++++++------
 esphome/components/esp32/__init__.py     |  5 ++++-
 esphome/components/esp8266/__init__.py   |  5 ++++-
 esphome/components/libretiny/__init__.py |  5 ++++-
 esphome/components/nrf52/__init__.py     |  5 ++++-
 esphome/components/rp2040/__init__.py    |  5 ++++-
 esphome/core/config.py                   | 14 ++++++++++++--
 8 files changed, 45 insertions(+), 16 deletions(-)

diff --git a/esphome/components/api/api.proto b/esphome/components/api/api.proto
index 8e3b5a1ffb..0a49a76c11 100644
--- a/esphome/components/api/api.proto
+++ b/esphome/components/api/api.proto
@@ -225,6 +225,8 @@ message SerialProxyInfo {
 //   esphome_version = 32 (ESPHOME_VERSION string literal)
 //   compilation_time = 25 (Application::BUILD_TIME_STR_SIZE - 1, constexpr)
 //   manufacturer = 20 (longest hardcoded literal: "Nordic Semiconductor")
+//   model = 127 (core/config.BOARD_MAX_LENGTH, validated in platform schemas)
+//   project_name/project_version = 127 (core/config.PROJECT_MAX_LENGTH)
 message DeviceInfoResponse {
   option (id) = 10;
   option (source) = SOURCE_SERVER;
@@ -248,13 +250,15 @@ message DeviceInfoResponse {
   string compilation_time = 5 [(max_data_length) = 25, (force) = true];
 
   // The model of the board. For example NodeMCU
-  string model = 6;
+  // max_data_length matches core/config.BOARD_MAX_LENGTH (validated in platform schemas)
+  string model = 6 [(max_data_length) = 127, (force) = true];
 
   bool has_deep_sleep = 7 [(field_ifdef) = "USE_DEEP_SLEEP"];
 
   // The esphome project details if set
-  string project_name = 8 [(field_ifdef) = "ESPHOME_PROJECT_NAME"];
-  string project_version = 9 [(field_ifdef) = "ESPHOME_PROJECT_NAME"];
+  // max_data_length matches core/config.PROJECT_MAX_LENGTH
+  string project_name = 8 [(max_data_length) = 127, (force) = true, (field_ifdef) = "ESPHOME_PROJECT_NAME"];
+  string project_version = 9 [(max_data_length) = 127, (force) = true, (field_ifdef) = "ESPHOME_PROJECT_NAME"];
 
   uint32 webserver_port = 10 [(field_ifdef) = "USE_WEBSERVER"];
 
diff --git a/esphome/components/api/api_pb2.cpp b/esphome/components/api/api_pb2.cpp
index 66996452fa..64adc8bc9e 100644
--- a/esphome/components/api/api_pb2.cpp
+++ b/esphome/components/api/api_pb2.cpp
@@ -97,15 +97,15 @@ uint8_t *DeviceInfoResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_
   ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 26, this->mac_address);
   ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 34, this->esphome_version);
   ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 42, this->compilation_time);
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 6, this->model);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 50, this->model);
 #ifdef USE_DEEP_SLEEP
   ProtoEncode::encode_bool(pos PROTO_ENCODE_DEBUG_ARG, 7, this->has_deep_sleep);
 #endif
 #ifdef ESPHOME_PROJECT_NAME
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 8, this->project_name);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 66, this->project_name);
 #endif
 #ifdef ESPHOME_PROJECT_NAME
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 9, this->project_version);
+  ProtoEncode::encode_short_string_force(pos PROTO_ENCODE_DEBUG_ARG, 74, this->project_version);
 #endif
 #ifdef USE_WEBSERVER
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 10, this->webserver_port);
@@ -159,15 +159,15 @@ uint32_t DeviceInfoResponse::calculate_size() const {
   size += 2 + this->mac_address.size();
   size += 2 + this->esphome_version.size();
   size += 2 + this->compilation_time.size();
-  size += ProtoSize::calc_length(1, this->model.size());
+  size += 2 + this->model.size();
 #ifdef USE_DEEP_SLEEP
   size += ProtoSize::calc_bool(1, this->has_deep_sleep);
 #endif
 #ifdef ESPHOME_PROJECT_NAME
-  size += ProtoSize::calc_length(1, this->project_name.size());
+  size += 2 + this->project_name.size();
 #endif
 #ifdef ESPHOME_PROJECT_NAME
-  size += ProtoSize::calc_length(1, this->project_version.size());
+  size += 2 + this->project_version.size();
 #endif
 #ifdef USE_WEBSERVER
   size += ProtoSize::calc_uint32(1, this->webserver_port);
diff --git a/esphome/components/esp32/__init__.py b/esphome/components/esp32/__init__.py
index 0d8a221524..54eecb5ad1 100644
--- a/esphome/components/esp32/__init__.py
+++ b/esphome/components/esp32/__init__.py
@@ -44,6 +44,7 @@ from esphome.const import (
     __version__,
 )
 from esphome.core import CORE, HexInt
+from esphome.core.config import BOARD_MAX_LENGTH
 from esphome.coroutine import CoroPriority, coroutine_with_priority
 import esphome.final_validate as fv
 from esphome.helpers import copy_file_if_changed, rmtree, write_file_if_changed
@@ -1403,7 +1404,9 @@ CONF_PARTITIONS = "partitions"
 CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
-            cv.Optional(CONF_BOARD): cv.string_strict,
+            cv.Optional(CONF_BOARD): cv.All(
+                cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+            ),
             cv.Optional(CONF_CPU_FREQUENCY): cv.one_of(
                 *FULL_CPU_FREQUENCIES, upper=True
             ),
diff --git a/esphome/components/esp8266/__init__.py b/esphome/components/esp8266/__init__.py
index fcd3499b15..76c8b1e870 100644
--- a/esphome/components/esp8266/__init__.py
+++ b/esphome/components/esp8266/__init__.py
@@ -20,6 +20,7 @@ from esphome.const import (
     ThreadModel,
 )
 from esphome.core import CORE, CoroPriority, Lambda, coroutine_with_priority
+from esphome.core.config import BOARD_MAX_LENGTH
 from esphome.helpers import copy_file_if_changed
 from esphome.types import ConfigType
 
@@ -203,7 +204,9 @@ BUILD_FLASH_MODES = ["qio", "qout", "dio", "dout"]
 CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
-            cv.Required(CONF_BOARD): cv.string_strict,
+            cv.Required(CONF_BOARD): cv.All(
+                cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+            ),
             cv.Optional(CONF_FRAMEWORK, default={}): ARDUINO_FRAMEWORK_SCHEMA,
             cv.Optional(CONF_RESTORE_FROM_FLASH, default=False): cv.boolean,
             cv.Optional(CONF_EARLY_PIN_INIT, default=True): cv.boolean,
diff --git a/esphome/components/libretiny/__init__.py b/esphome/components/libretiny/__init__.py
index 8f99124604..00be620831 100644
--- a/esphome/components/libretiny/__init__.py
+++ b/esphome/components/libretiny/__init__.py
@@ -23,6 +23,7 @@ from esphome.const import (
     __version__,
 )
 from esphome.core import CORE
+from esphome.core.config import BOARD_MAX_LENGTH
 from esphome.storage_json import StorageJSON
 
 from . import gpio  # noqa
@@ -266,7 +267,9 @@ CONFIG_SCHEMA = cv.All(_notify_old_style)
 BASE_SCHEMA = cv.Schema(
     {
         cv.GenerateID(): cv.declare_id(LTComponent),
-        cv.Required(CONF_BOARD): cv.string_strict,
+        cv.Required(CONF_BOARD): cv.All(
+            cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+        ),
         cv.Optional(CONF_FAMILY): cv.one_of(*FAMILIES, upper=True),
         cv.Optional(CONF_FRAMEWORK, default={}): FRAMEWORK_SCHEMA,
     },
diff --git a/esphome/components/nrf52/__init__.py b/esphome/components/nrf52/__init__.py
index 5054e5e0df..566aefb209 100644
--- a/esphome/components/nrf52/__init__.py
+++ b/esphome/components/nrf52/__init__.py
@@ -46,6 +46,7 @@ from esphome.const import (
     ThreadModel,
 )
 from esphome.core import CORE, CoroPriority, EsphomeError, coroutine_with_priority
+from esphome.core.config import BOARD_MAX_LENGTH
 import esphome.final_validate as fv
 from esphome.storage_json import StorageJSON
 from esphome.types import ConfigType
@@ -145,7 +146,9 @@ CONFIG_SCHEMA = cv.All(
     set_core_data,
     cv.Schema(
         {
-            cv.Required(CONF_BOARD): cv.string_strict,
+            cv.Required(CONF_BOARD): cv.All(
+                cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+            ),
             cv.Optional(KEY_BOOTLOADER): cv.one_of(*BOOTLOADERS, lower=True),
             cv.Optional(CONF_DFU): cv.Schema(
                 {
diff --git a/esphome/components/rp2040/__init__.py b/esphome/components/rp2040/__init__.py
index 0bb1811069..628cdf370a 100644
--- a/esphome/components/rp2040/__init__.py
+++ b/esphome/components/rp2040/__init__.py
@@ -22,6 +22,7 @@ from esphome.const import (
     ThreadModel,
 )
 from esphome.core import CORE, CoroPriority, EsphomeError, coroutine_with_priority
+from esphome.core.config import BOARD_MAX_LENGTH
 from esphome.helpers import copy_file_if_changed, read_file, write_file_if_changed
 
 from . import boards
@@ -168,7 +169,9 @@ ARDUINO_FRAMEWORK_SCHEMA = cv.All(
 CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
-            cv.Required(CONF_BOARD): cv.string_strict,
+            cv.Required(CONF_BOARD): cv.All(
+                cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+            ),
             cv.Optional(CONF_FRAMEWORK, default={}): ARDUINO_FRAMEWORK_SCHEMA,
             cv.Optional(CONF_WATCHDOG_TIMEOUT, default="8388ms"): cv.All(
                 cv.positive_time_period_milliseconds,
diff --git a/esphome/core/config.py b/esphome/core/config.py
index 31cfd00ef7..efc6d62f48 100644
--- a/esphome/core/config.py
+++ b/esphome/core/config.py
@@ -236,6 +236,12 @@ ICON_MAX_LENGTH = 63
 # Max unit of measurement string length
 UNIT_OF_MEASUREMENT_MAX_LENGTH = 63
 
+# Max project name/version string length (must fit in single-byte varint for proto encoding)
+PROJECT_MAX_LENGTH = 127
+
+# Max board/model string length (must fit in single-byte varint for proto encoding)
+BOARD_MAX_LENGTH = 127
+
 AREA_SCHEMA = cv.Schema(
     {
         cv.GenerateID(CONF_ID): cv.declare_id(Area),
@@ -306,9 +312,13 @@ CONFIG_SCHEMA = cv.All(
             cv.Optional(CONF_PROJECT): cv.Schema(
                 {
                     cv.Required(CONF_NAME): cv.All(
-                        cv.string_strict, valid_project_name
+                        cv.string_strict,
+                        valid_project_name,
+                        cv.Length(max=PROJECT_MAX_LENGTH),
+                    ),
+                    cv.Required(CONF_VERSION): cv.All(
+                        cv.string_strict, cv.Length(max=PROJECT_MAX_LENGTH)
                     ),
-                    cv.Required(CONF_VERSION): cv.string_strict,
                     cv.Optional(CONF_ON_UPDATE): automation.validate_automation(
                         {
                             cv.GenerateID(CONF_TRIGGER_ID): cv.declare_id(

From 52d76f8db09bd3f60140c801bb4e2e2c8eb8b813 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Mon, 6 Apr 2026 18:17:04 -1000
Subject: [PATCH 3/5] [api] Fix name max_data_length to match hostname limit,
 add suggested_area

- Fix HelloResponse.name and DeviceInfoResponse.name max_data_length
  from 120 to 31 to match ESPHOME_DEVICE_NAME_MAX_LEN (hostname limit)
- Add max_data_length=120 + force to suggested_area field
- Add static_asserts for ESPHOME_DEVICE_NAME_MAX_LEN and
  ESPHOME_FRIENDLY_NAME_MAX_LEN
- Fix comments to accurately describe guarantees
---
 esphome/components/api/api.proto          | 16 +++++++++-------
 esphome/components/api/api_connection.cpp |  2 ++
 esphome/components/api/api_pb2.cpp        |  4 ++--
 3 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/esphome/components/api/api.proto b/esphome/components/api/api.proto
index 0a49a76c11..33d16f0339 100644
--- a/esphome/components/api/api.proto
+++ b/esphome/components/api/api.proto
@@ -132,9 +132,9 @@ message HelloResponse {
   // Currently set to ESPHOME_VERSION string literal.
   string server_info = 3 [(max_data_length) = 32, (force) = true];
 
-  // The name of the server (App.get_name())
-  // max_data_length matches config_validation.NAME_MAX_LENGTH
-  string name = 4 [(max_data_length) = 120, (force) = true];
+  // The name of the server (App.get_name() - device hostname)
+  // max_data_length matches ESPHOME_DEVICE_NAME_MAX_LEN (validated by validate_hostname)
+  string name = 4 [(max_data_length) = 31, (force) = true];
 }
 
 // DEPRECATED in ESPHome 2026.1.0 - Password authentication is no longer supported.
@@ -220,13 +220,15 @@ message SerialProxyInfo {
 }
 
 // DeviceInfoResponse max_data_length values:
-//   name/friendly_name = 120 (config_validation.NAME_MAX_LENGTH / core/config.FRIENDLY_NAME_MAX_LEN)
+//   name = 31 (ESPHOME_DEVICE_NAME_MAX_LEN, validated by validate_hostname)
+//   friendly_name = 120 (core/config.FRIENDLY_NAME_MAX_LEN)
 //   mac_address/bluetooth_mac_address = 17 (MAC_ADDRESS_PRETTY_BUFFER_SIZE - 1, constexpr)
 //   esphome_version = 32 (ESPHOME_VERSION string literal)
 //   compilation_time = 25 (Application::BUILD_TIME_STR_SIZE - 1, constexpr)
 //   manufacturer = 20 (longest hardcoded literal: "Nordic Semiconductor")
 //   model = 127 (core/config.BOARD_MAX_LENGTH, validated in platform schemas)
 //   project_name/project_version = 127 (core/config.PROJECT_MAX_LENGTH)
+//   suggested_area = 120 (core/config.FRIENDLY_NAME_MAX_LEN via AREA_SCHEMA)
 message DeviceInfoResponse {
   option (id) = 10;
   option (source) = SOURCE_SERVER;
@@ -235,8 +237,8 @@ message DeviceInfoResponse {
   // with older ESPHome versions that still send this field.
   bool uses_password = 1 [deprecated = true];
 
-  // The name of the node, given by "App.set_name()"
-  string name = 2 [(max_data_length) = 120, (force) = true];
+  // The name of the node, given by "App.set_name()" - device hostname
+  string name = 2 [(max_data_length) = 31, (force) = true];
 
   // The mac address of the device. For example "AC:BC:32:89:0E:A9"
   string mac_address = 3 [(max_data_length) = 17, (force) = true];
@@ -274,7 +276,7 @@ message DeviceInfoResponse {
   uint32 legacy_voice_assistant_version = 14 [deprecated=true, (field_ifdef) = "USE_VOICE_ASSISTANT"];
   uint32 voice_assistant_feature_flags = 17 [(field_ifdef) = "USE_VOICE_ASSISTANT"];
 
-  string suggested_area = 16 [(field_ifdef) = "USE_AREAS"];
+  string suggested_area = 16 [(max_data_length) = 120, (force) = true, (field_ifdef) = "USE_AREAS"];
 
   // The Bluetooth mac address of the device. For example "AC:BC:32:89:0E:AA"
   string bluetooth_mac_address = 18 [(max_data_length) = 17, (force) = true, (field_ifdef) = "USE_BLUETOOTH_PROXY"];
diff --git a/esphome/components/api/api_connection.cpp b/esphome/components/api/api_connection.cpp
index 53c4aabcb7..bfb3ec291c 100644
--- a/esphome/components/api/api_connection.cpp
+++ b/esphome/components/api/api_connection.cpp
@@ -77,6 +77,8 @@ static_assert(MAC_ADDRESS_PRETTY_BUFFER_SIZE - 1 == 17,
               "Update max_data_length for mac_address/bluetooth_mac_address in api.proto");
 static_assert(Application::BUILD_TIME_STR_SIZE - 1 == 25, "Update max_data_length for compilation_time in api.proto");
 static_assert(sizeof(ESPHOME_VERSION) - 1 <= 32, "Update max_data_length for esphome_version in api.proto");
+static_assert(ESPHOME_DEVICE_NAME_MAX_LEN <= 31, "Update max_data_length for name in api.proto");
+static_assert(ESPHOME_FRIENDLY_NAME_MAX_LEN <= 120, "Update max_data_length for friendly_name in api.proto");
 
 static const char *const TAG = "api.connection";
 #ifdef USE_CAMERA
diff --git a/esphome/components/api/api_pb2.cpp b/esphome/components/api/api_pb2.cpp
index 64adc8bc9e..d27cfa57cf 100644
--- a/esphome/components/api/api_pb2.cpp
+++ b/esphome/components/api/api_pb2.cpp
@@ -119,7 +119,7 @@ uint8_t *DeviceInfoResponse::encode(ProtoWriteBuffer &buffer PROTO_ENCODE_DEBUG_
   ProtoEncode::encode_uint32(pos PROTO_ENCODE_DEBUG_ARG, 17, this->voice_assistant_feature_flags);
 #endif
 #ifdef USE_AREAS
-  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 16, this->suggested_area);
+  ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 16, this->suggested_area, true);
 #endif
 #ifdef USE_BLUETOOTH_PROXY
   ProtoEncode::encode_string(pos PROTO_ENCODE_DEBUG_ARG, 18, this->bluetooth_mac_address, true);
@@ -181,7 +181,7 @@ uint32_t DeviceInfoResponse::calculate_size() const {
   size += ProtoSize::calc_uint32(2, this->voice_assistant_feature_flags);
 #endif
 #ifdef USE_AREAS
-  size += ProtoSize::calc_length(2, this->suggested_area.size());
+  size += 3 + this->suggested_area.size();
 #endif
 #ifdef USE_BLUETOOTH_PROXY
   size += 3 + this->bluetooth_mac_address.size();

From cfcf9216f3e53bd2aeae6a9291b7670bc75687b6 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Mon, 6 Apr 2026 18:33:04 -1000
Subject: [PATCH 4/5] [core] Add cv.ByteLength validator, switch all
 proto-backed length checks to byte length

Add cv.ByteLength() validator that checks UTF-8 byte length instead
of character count. This ensures multibyte characters don't cause
the encoded string to exceed the proto max_data_length limit.

Switch all length validators that feed into proto fields with
max_data_length annotations to use byte length:
- Entity names, friendly names, area names, device names
- Icons, device classes, units of measurement
- Board names, project name/version
---
 esphome/components/esp32/__init__.py     |  2 +-
 esphome/components/esp8266/__init__.py   |  2 +-
 esphome/components/libretiny/__init__.py |  2 +-
 esphome/components/nrf52/__init__.py     |  2 +-
 esphome/components/number/__init__.py    |  2 +-
 esphome/components/rp2040/__init__.py    |  2 +-
 esphome/components/sensor/__init__.py    |  2 +-
 esphome/config_validation.py             | 34 +++++++++++++++++++-----
 esphome/core/config.py                   | 10 +++----
 esphome/core/entity_helpers.py           | 15 ++++++-----
 10 files changed, 49 insertions(+), 24 deletions(-)

diff --git a/esphome/components/esp32/__init__.py b/esphome/components/esp32/__init__.py
index 54eecb5ad1..f27690c97b 100644
--- a/esphome/components/esp32/__init__.py
+++ b/esphome/components/esp32/__init__.py
@@ -1405,7 +1405,7 @@ CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
             cv.Optional(CONF_BOARD): cv.All(
-                cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+                cv.string_strict, cv.ByteLength(max=BOARD_MAX_LENGTH)
             ),
             cv.Optional(CONF_CPU_FREQUENCY): cv.one_of(
                 *FULL_CPU_FREQUENCIES, upper=True
diff --git a/esphome/components/esp8266/__init__.py b/esphome/components/esp8266/__init__.py
index 76c8b1e870..bef7e36470 100644
--- a/esphome/components/esp8266/__init__.py
+++ b/esphome/components/esp8266/__init__.py
@@ -205,7 +205,7 @@ CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
             cv.Required(CONF_BOARD): cv.All(
-                cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+                cv.string_strict, cv.ByteLength(max=BOARD_MAX_LENGTH)
             ),
             cv.Optional(CONF_FRAMEWORK, default={}): ARDUINO_FRAMEWORK_SCHEMA,
             cv.Optional(CONF_RESTORE_FROM_FLASH, default=False): cv.boolean,
diff --git a/esphome/components/libretiny/__init__.py b/esphome/components/libretiny/__init__.py
index 00be620831..656eee6d7b 100644
--- a/esphome/components/libretiny/__init__.py
+++ b/esphome/components/libretiny/__init__.py
@@ -268,7 +268,7 @@ BASE_SCHEMA = cv.Schema(
     {
         cv.GenerateID(): cv.declare_id(LTComponent),
         cv.Required(CONF_BOARD): cv.All(
-            cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+            cv.string_strict, cv.ByteLength(max=BOARD_MAX_LENGTH)
         ),
         cv.Optional(CONF_FAMILY): cv.one_of(*FAMILIES, upper=True),
         cv.Optional(CONF_FRAMEWORK, default={}): FRAMEWORK_SCHEMA,
diff --git a/esphome/components/nrf52/__init__.py b/esphome/components/nrf52/__init__.py
index 566aefb209..5d92a4fa80 100644
--- a/esphome/components/nrf52/__init__.py
+++ b/esphome/components/nrf52/__init__.py
@@ -147,7 +147,7 @@ CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
             cv.Required(CONF_BOARD): cv.All(
-                cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+                cv.string_strict, cv.ByteLength(max=BOARD_MAX_LENGTH)
             ),
             cv.Optional(KEY_BOOTLOADER): cv.one_of(*BOOTLOADERS, lower=True),
             cv.Optional(CONF_DFU): cv.Schema(
diff --git a/esphome/components/number/__init__.py b/esphome/components/number/__init__.py
index 26d2602ba4..1a0dc42407 100644
--- a/esphome/components/number/__init__.py
+++ b/esphome/components/number/__init__.py
@@ -190,7 +190,7 @@ validate_device_class = cv.one_of(*DEVICE_CLASSES, lower=True, space="_")
 validate_unit_of_measurement = cv.All(
     cv.string_strict,
     # Keep in sync with max_data_length in api.proto
-    cv.Length(max=UNIT_OF_MEASUREMENT_MAX_LENGTH),
+    cv.ByteLength(max=UNIT_OF_MEASUREMENT_MAX_LENGTH),
 )
 
 _NUMBER_SCHEMA = (
diff --git a/esphome/components/rp2040/__init__.py b/esphome/components/rp2040/__init__.py
index 628cdf370a..e452780d41 100644
--- a/esphome/components/rp2040/__init__.py
+++ b/esphome/components/rp2040/__init__.py
@@ -170,7 +170,7 @@ CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
             cv.Required(CONF_BOARD): cv.All(
-                cv.string_strict, cv.Length(max=BOARD_MAX_LENGTH)
+                cv.string_strict, cv.ByteLength(max=BOARD_MAX_LENGTH)
             ),
             cv.Optional(CONF_FRAMEWORK, default={}): ARDUINO_FRAMEWORK_SCHEMA,
             cv.Optional(CONF_WATCHDOG_TIMEOUT, default="8388ms"): cv.All(
diff --git a/esphome/components/sensor/__init__.py b/esphome/components/sensor/__init__.py
index 275c4542fb..550e32c506 100644
--- a/esphome/components/sensor/__init__.py
+++ b/esphome/components/sensor/__init__.py
@@ -294,7 +294,7 @@ RoundMultipleFilter = sensor_ns.class_("RoundMultipleFilter", Filter)
 validate_unit_of_measurement = cv.All(
     cv.string_strict,
     # Keep in sync with max_data_length in api.proto
-    cv.Length(max=UNIT_OF_MEASUREMENT_MAX_LENGTH),
+    cv.ByteLength(max=UNIT_OF_MEASUREMENT_MAX_LENGTH),
 )
 validate_accuracy_decimals = cv.int_
 validate_icon = cv.icon
diff --git a/esphome/config_validation.py b/esphome/config_validation.py
index c6b67e9f35..cbf81de147 100644
--- a/esphome/config_validation.py
+++ b/esphome/config_validation.py
@@ -130,6 +130,26 @@ RequiredFieldInvalid = vol.RequiredFieldInvalid
 # the rest of the error path is relative to the root config path
 ROOT_CONFIG_PATH = object()
 
+
+def ByteLength(*, max: int):
+    """Validate that the UTF-8 byte length of a string does not exceed max.
+
+    Use instead of Length() when the limit must apply to encoded bytes,
+    not characters (e.g. for protobuf length-varint constraints).
+    """
+
+    def validator(value):
+        byte_len = len(str(value).encode("utf-8"))
+        if byte_len > max:
+            raise Invalid(
+                f"String is too long ({byte_len} bytes, max {max}). "
+                f"Multibyte characters count as multiple bytes."
+            )
+        return value
+
+    return validator
+
+
 RESERVED_IDS = [
     # C++ keywords https://en.cppreference.com/w/cpp/keyword
     "alarm",
@@ -411,9 +431,10 @@ def icon(value):
         raise Invalid(
             'Icons must match the format "[icon pack]:[icon]", e.g. "mdi:home-assistant"'
         )
-    if len(value) > ICON_MAX_LENGTH:
+    byte_len = len(value.encode("utf-8"))
+    if byte_len > ICON_MAX_LENGTH:
         raise Invalid(
-            f"Icon string is too long ({len(value)} chars, max {ICON_MAX_LENGTH}). "
+            f"Icon string is too long ({byte_len} bytes, max {ICON_MAX_LENGTH}). "
             "Icons are stored in PROGMEM with a 64-byte buffer limit."
         )
     return value
@@ -2067,11 +2088,12 @@ def _validate_entity_name(value):
             "Name cannot be None when esphome->friendly_name is not set!"
         )(value)
     if value is not None:
-        # Validate length for web server URL compatibility
-        if len(value) > NAME_MAX_LENGTH:
+        # Validate byte length for web server URL and proto encoding compatibility
+        byte_len = len(value.encode("utf-8"))
+        if byte_len > NAME_MAX_LENGTH:
             raise Invalid(
-                f"Name is too long ({len(value)} chars). "
-                f"Maximum length is {NAME_MAX_LENGTH} characters."
+                f"Name is too long ({byte_len} bytes). "
+                f"Maximum length is {NAME_MAX_LENGTH} bytes."
             )
         # Validate no '/' in name for web server URL compatibility
         value = _validate_no_slash(value)
diff --git a/esphome/core/config.py b/esphome/core/config.py
index efc6d62f48..bf210876df 100644
--- a/esphome/core/config.py
+++ b/esphome/core/config.py
@@ -246,7 +246,7 @@ AREA_SCHEMA = cv.Schema(
     {
         cv.GenerateID(CONF_ID): cv.declare_id(Area),
         cv.Required(CONF_NAME): cv.All(
-            cv.string_no_slash, cv.Length(max=FRIENDLY_NAME_MAX_LEN)
+            cv.string_no_slash, cv.ByteLength(max=FRIENDLY_NAME_MAX_LEN)
         ),
     }
 )
@@ -255,7 +255,7 @@ DEVICE_SCHEMA = cv.Schema(
     {
         cv.GenerateID(CONF_ID): cv.declare_id(Device),
         cv.Required(CONF_NAME): cv.All(
-            cv.string_no_slash, cv.Length(max=FRIENDLY_NAME_MAX_LEN)
+            cv.string_no_slash, cv.ByteLength(max=FRIENDLY_NAME_MAX_LEN)
         ),
         cv.Optional(CONF_AREA_ID): cv.use_id(Area),
     }
@@ -272,7 +272,7 @@ CONFIG_SCHEMA = cv.All(
             cv.Required(CONF_NAME): cv.valid_name,
             # Keep max=120 in sync with OBJECT_ID_MAX_LEN in esphome/core/entity_base.h
             cv.Optional(CONF_FRIENDLY_NAME, ""): cv.All(
-                cv.string_no_slash, cv.Length(max=FRIENDLY_NAME_MAX_LEN)
+                cv.string_no_slash, cv.ByteLength(max=FRIENDLY_NAME_MAX_LEN)
             ),
             cv.Optional(CONF_AREA): validate_area_config,
             cv.Optional(CONF_COMMENT): cv.All(cv.string, cv.Length(max=255)),
@@ -314,10 +314,10 @@ CONFIG_SCHEMA = cv.All(
                     cv.Required(CONF_NAME): cv.All(
                         cv.string_strict,
                         valid_project_name,
-                        cv.Length(max=PROJECT_MAX_LENGTH),
+                        cv.ByteLength(max=PROJECT_MAX_LENGTH),
                     ),
                     cv.Required(CONF_VERSION): cv.All(
-                        cv.string_strict, cv.Length(max=PROJECT_MAX_LENGTH)
+                        cv.string_strict, cv.ByteLength(max=PROJECT_MAX_LENGTH)
                     ),
                     cv.Optional(CONF_ON_UPDATE): automation.validate_automation(
                         {
diff --git a/esphome/core/entity_helpers.py b/esphome/core/entity_helpers.py
index fc931c2baa..f09dd013fe 100644
--- a/esphome/core/entity_helpers.py
+++ b/esphome/core/entity_helpers.py
@@ -193,9 +193,10 @@ def _register_string(
 
 def register_device_class(value: str) -> int:
     """Register a device_class string and return its 1-based index."""
-    if value and len(value) > DEVICE_CLASS_MAX_LENGTH:
+    byte_len = len(value.encode("utf-8")) if value else 0
+    if byte_len > DEVICE_CLASS_MAX_LENGTH:
         raise ValueError(
-            f"Device class string too long ({len(value)} chars, max {DEVICE_CLASS_MAX_LENGTH}): '{value}'"
+            f"Device class string too long ({byte_len} bytes, max {DEVICE_CLASS_MAX_LENGTH}): '{value}'"
         )
     return _register_string(
         value, _get_pool().device_classes, _MAX_DEVICE_CLASSES, "device_class"
@@ -204,9 +205,10 @@ def register_device_class(value: str) -> int:
 
 def register_unit_of_measurement(value: str) -> int:
     """Register a unit_of_measurement string and return its 1-based index."""
-    if value and len(value) > UNIT_OF_MEASUREMENT_MAX_LENGTH:
+    byte_len = len(value.encode("utf-8")) if value else 0
+    if byte_len > UNIT_OF_MEASUREMENT_MAX_LENGTH:
         raise ValueError(
-            f"Unit of measurement string too long ({len(value)} chars, "
+            f"Unit of measurement string too long ({byte_len} bytes, "
             f"max {UNIT_OF_MEASUREMENT_MAX_LENGTH}): '{value}'"
         )
     return _register_string(value, _get_pool().units, _MAX_UNITS, "unit_of_measurement")
@@ -214,9 +216,10 @@ def register_unit_of_measurement(value: str) -> int:
 
 def register_icon(value: str) -> int:
     """Register an icon string and return its 1-based index."""
-    if value and len(value) > ICON_MAX_LENGTH:
+    byte_len = len(value.encode("utf-8")) if value else 0
+    if byte_len > ICON_MAX_LENGTH:
         raise ValueError(
-            f"Icon string too long ({len(value)} chars, max {ICON_MAX_LENGTH}): '{value}'"
+            f"Icon string too long ({byte_len} bytes, max {ICON_MAX_LENGTH}): '{value}'"
         )
     return _register_string(value, _get_pool().icons, _MAX_ICONS, "icon")
 

From b86ab0763297cbde2ff0d986570bd923f5e195af Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Mon, 6 Apr 2026 18:38:42 -1000
Subject: [PATCH 5/5] [core] Fix test for byte-length error message, add
 ByteLength and multibyte tests

---
 tests/unit_tests/test_config_validation.py | 44 ++++++++++++++++++----
 1 file changed, 36 insertions(+), 8 deletions(-)

diff --git a/tests/unit_tests/test_config_validation.py b/tests/unit_tests/test_config_validation.py
index c1849daf4b..ce941b40dc 100644
--- a/tests/unit_tests/test_config_validation.py
+++ b/tests/unit_tests/test_config_validation.py
@@ -149,17 +149,36 @@ def test_icon__invalid():
 
 
 def test_icon__max_length():
-    """Test that icons exceeding 63 characters are rejected."""
-    # Exactly 63 chars should pass
-    max_icon = "mdi:" + "a" * 59  # 63 chars total
+    """Test that icons exceeding 63 bytes are rejected."""
+    # Exactly 63 bytes should pass
+    max_icon = "mdi:" + "a" * 59  # 63 bytes total
     assert config_validation.icon(max_icon) == max_icon
 
-    # 64 chars should fail
-    too_long = "mdi:" + "a" * 60  # 64 chars total
+    # 64 bytes should fail
+    too_long = "mdi:" + "a" * 60  # 64 bytes total
     with pytest.raises(Invalid, match="Icon string is too long"):
         config_validation.icon(too_long)
 
 
+def test_byte_length() -> None:
+    """Test ByteLength validator checks UTF-8 byte length, not char count."""
+    validator = config_validation.ByteLength(max=10)  # pylint: disable=no-member
+
+    # ASCII: 10 chars = 10 bytes, should pass
+    assert validator("a" * 10) == "a" * 10
+
+    # ASCII: 11 chars = 11 bytes, should fail
+    with pytest.raises(Invalid, match="too long.*11 bytes.*max 10"):
+        validator("a" * 11)
+
+    # Multibyte: 3 chars × 3 bytes = 9 bytes, should pass
+    assert validator("温度传") == "温度传"
+
+    # Multibyte: 4 chars × 3 bytes = 12 bytes, should fail
+    with pytest.raises(Invalid, match="too long.*12 bytes.*max 10"):
+        validator("温度传感")
+
+
 @pytest.mark.parametrize("value", ("True", "YES", "on", "enAblE", True))
 def test_boolean__valid_true(value):
     assert config_validation.boolean(value) is True
@@ -567,14 +586,23 @@ def test_validate_entity_name__slash_replaced_with_warning(
 
 
 def test_validate_entity_name__max_length() -> None:
-    # 120 chars should pass
+    # 120 bytes should pass
     assert config_validation._validate_entity_name("x" * 120) == "x" * 120
 
-    # 121 chars should fail
-    with pytest.raises(Invalid, match="too long.*121 chars.*Maximum.*120"):
+    # 121 bytes should fail
+    with pytest.raises(Invalid, match="too long.*121 bytes.*Maximum.*120"):
         config_validation._validate_entity_name("x" * 121)
 
 
+def test_validate_entity_name__multibyte_byte_length() -> None:
+    # 40 chars of 3-byte UTF-8 = 120 bytes, should pass
+    assert config_validation._validate_entity_name("温" * 40) == "温" * 40
+
+    # 41 chars of 3-byte UTF-8 = 123 bytes, should fail (over 120 byte limit)
+    with pytest.raises(Invalid, match="too long.*123 bytes.*Maximum.*120"):
+        config_validation._validate_entity_name("温" * 41)
+
+
 def test_validate_entity_name__none_without_friendly_name() -> None:
     # When name is "None" and friendly_name is not set, it should fail
     CORE.friendly_name = None