diff --git a/esphome/components/api/api.proto b/esphome/components/api/api.proto
index 278e977a73..56423ca231 100644
--- a/esphome/components/api/api.proto
+++ b/esphome/components/api/api.proto
@@ -308,9 +308,10 @@ enum EntityCategory {
   ENTITY_CATEGORY_DIAGNOSTIC = 2;
 }
 
-// Entity name/object_id max_data_length = 120 matches NAME_MAX_LENGTH,
-// icon max_data_length = 63 matches ICON_MAX_LENGTH.
-// Both defined in esphome/core/config.py and validated at config time.
+// Entity field max_data_length values match constants in esphome/core/config.py:
+//   name/object_id = 120 (NAME_MAX_LENGTH)
+//   icon = 63 (ICON_MAX_LENGTH)
+//   device_class = 47 (DEVICE_CLASS_MAX_LENGTH)
 
 // ==================== BINARY SENSOR ====================
 message ListEntitiesBinarySensorResponse {
@@ -324,7 +325,7 @@ message ListEntitiesBinarySensorResponse {
   string name = 3 [(max_data_length) = 120, (force) = true];
   reserved 4; // Deprecated: was string unique_id
 
-  string device_class = 5;
+  string device_class = 5 [(max_data_length) = 47];
   bool is_status_binary_sensor = 6;
   bool disabled_by_default = 7;
   string icon = 8 [(field_ifdef) = "USE_ENTITY_ICON", (max_data_length) = 63];
@@ -361,7 +362,7 @@ message ListEntitiesCoverResponse {
   bool assumed_state = 5;
   bool supports_position = 6;
   bool supports_tilt = 7;
-  string device_class = 8;
+  string device_class = 8 [(max_data_length) = 47];
   bool disabled_by_default = 9;
   string icon = 10 [(field_ifdef) = "USE_ENTITY_ICON", (max_data_length) = 63];
   EntityCategory entity_category = 11;
@@ -639,7 +640,7 @@ message ListEntitiesSensorResponse {
   string unit_of_measurement = 6;
   int32 accuracy_decimals = 7;
   bool force_update = 8;
-  string device_class = 9;
+  string device_class = 9 [(max_data_length) = 47];
   SensorStateClass state_class = 10;
   // Last reset type removed in 2021.9.0
   // Deprecated in API version 1.5
@@ -679,7 +680,7 @@ message ListEntitiesSwitchResponse {
   bool assumed_state = 6;
   bool disabled_by_default = 7;
   EntityCategory entity_category = 8;
-  string device_class = 9;
+  string device_class = 9 [(max_data_length) = 47];
   uint32 device_id = 10 [(field_ifdef) = "USE_DEVICES"];
 }
 message SwitchStateResponse {
@@ -720,7 +721,7 @@ message ListEntitiesTextSensorResponse {
   string icon = 5 [(field_ifdef) = "USE_ENTITY_ICON", (max_data_length) = 63];
   bool disabled_by_default = 6;
   EntityCategory entity_category = 7;
-  string device_class = 8;
+  string device_class = 8 [(max_data_length) = 47];
   uint32 device_id = 9 [(field_ifdef) = "USE_DEVICES"];
 }
 message TextSensorStateResponse {
@@ -1260,7 +1261,7 @@ message ListEntitiesNumberResponse {
   EntityCategory entity_category = 10;
   string unit_of_measurement = 11;
   NumberMode mode = 12;
-  string device_class = 13;
+  string device_class = 13 [(max_data_length) = 47];
   uint32 device_id = 14 [(field_ifdef) = "USE_DEVICES"];
 }
 message NumberStateResponse {
@@ -1460,7 +1461,7 @@ message ListEntitiesButtonResponse {
   string icon = 5 [(field_ifdef) = "USE_ENTITY_ICON", (max_data_length) = 63];
   bool disabled_by_default = 6;
   EntityCategory entity_category = 7;
-  string device_class = 8;
+  string device_class = 8 [(max_data_length) = 47];
   uint32 device_id = 9 [(field_ifdef) = "USE_DEVICES"];
 }
 message ButtonCommandRequest {
@@ -2304,7 +2305,7 @@ message ListEntitiesEventResponse {
   string icon = 5 [(field_ifdef) = "USE_ENTITY_ICON", (max_data_length) = 63];
   bool disabled_by_default = 6;
   EntityCategory entity_category = 7;
-  string device_class = 8;
+  string device_class = 8 [(max_data_length) = 47];
 
   repeated string event_types = 9 [(container_pointer_no_template) = "FixedVector<const char *>"];
   uint32 device_id = 10 [(field_ifdef) = "USE_DEVICES"];
@@ -2335,7 +2336,7 @@ message ListEntitiesValveResponse {
   string icon = 5 [(field_ifdef) = "USE_ENTITY_ICON", (max_data_length) = 63];
   bool disabled_by_default = 6;
   EntityCategory entity_category = 7;
-  string device_class = 8;
+  string device_class = 8 [(max_data_length) = 47];
 
   bool assumed_state = 9;
   bool supports_position = 10;
@@ -2433,7 +2434,7 @@ message ListEntitiesUpdateResponse {
   string icon = 5 [(field_ifdef) = "USE_ENTITY_ICON", (max_data_length) = 63];
   bool disabled_by_default = 6;
   EntityCategory entity_category = 7;
-  string device_class = 8;
+  string device_class = 8 [(max_data_length) = 47];
   uint32 device_id = 9 [(field_ifdef) = "USE_DEVICES"];
 }
 message UpdateStateResponse {
diff --git a/esphome/components/api/api_pb2.cpp b/esphome/components/api/api_pb2.cpp
index 8dea47bb6b..d00a61b692 100644
--- a/esphome/components/api/api_pb2.cpp
+++ b/esphome/components/api/api_pb2.cpp
@@ -226,7 +226,7 @@ uint32_t ListEntitiesBinarySensorResponse::calculate_size() const {
   size += 2 + this->object_id.size();
   size += 5;
   size += 2 + this->name.size();
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
   size += ProtoSize::calc_bool(1, this->is_status_binary_sensor);
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
 #ifdef USE_ENTITY_ICON
@@ -284,7 +284,7 @@ uint32_t ListEntitiesCoverResponse::calculate_size() const {
   size += ProtoSize::calc_bool(1, this->assumed_state);
   size += ProtoSize::calc_bool(1, this->supports_position);
   size += ProtoSize::calc_bool(1, this->supports_tilt);
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
 #ifdef USE_ENTITY_ICON
   size += !this->icon.empty() ? 2 + this->icon.size() : 0;
@@ -710,7 +710,7 @@ uint32_t ListEntitiesSensorResponse::calculate_size() const {
   size += ProtoSize::calc_length(1, this->unit_of_measurement.size());
   size += ProtoSize::calc_int32(1, this->accuracy_decimals);
   size += ProtoSize::calc_bool(1, this->force_update);
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->state_class));
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->entity_category));
@@ -765,7 +765,7 @@ uint32_t ListEntitiesSwitchResponse::calculate_size() const {
   size += ProtoSize::calc_bool(1, this->assumed_state);
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->entity_category));
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
 #ifdef USE_DEVICES
   size += ProtoSize::calc_uint32(1, this->device_id);
 #endif
@@ -838,7 +838,7 @@ uint32_t ListEntitiesTextSensorResponse::calculate_size() const {
 #endif
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->entity_category));
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
 #ifdef USE_DEVICES
   size += ProtoSize::calc_uint32(1, this->device_id);
 #endif
@@ -1707,7 +1707,7 @@ uint32_t ListEntitiesNumberResponse::calculate_size() const {
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->entity_category));
   size += ProtoSize::calc_length(1, this->unit_of_measurement.size());
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->mode));
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
 #ifdef USE_DEVICES
   size += ProtoSize::calc_uint32(1, this->device_id);
 #endif
@@ -2076,7 +2076,7 @@ uint32_t ListEntitiesButtonResponse::calculate_size() const {
 #endif
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->entity_category));
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
 #ifdef USE_DEVICES
   size += ProtoSize::calc_uint32(1, this->device_id);
 #endif
@@ -3314,7 +3314,7 @@ uint32_t ListEntitiesEventResponse::calculate_size() const {
 #endif
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->entity_category));
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
   if (!this->event_types->empty()) {
     for (const char *it : *this->event_types) {
       size += ProtoSize::calc_length_force(1, strlen(it));
@@ -3370,7 +3370,7 @@ uint32_t ListEntitiesValveResponse::calculate_size() const {
 #endif
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->entity_category));
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
   size += ProtoSize::calc_bool(1, this->assumed_state);
   size += ProtoSize::calc_bool(1, this->supports_position);
   size += ProtoSize::calc_bool(1, this->supports_stop);
@@ -3527,7 +3527,7 @@ uint32_t ListEntitiesUpdateResponse::calculate_size() const {
 #endif
   size += ProtoSize::calc_bool(1, this->disabled_by_default);
   size += ProtoSize::calc_uint32(1, static_cast<uint32_t>(this->entity_category));
-  size += ProtoSize::calc_length(1, this->device_class.size());
+  size += !this->device_class.empty() ? 2 + this->device_class.size() : 0;
 #ifdef USE_DEVICES
   size += ProtoSize::calc_uint32(1, this->device_id);
 #endif