diff --git a/esphome/components/api/api_pb2.cpp b/esphome/components/api/api_pb2.cpp
index 92efb6f0d8..21056e4899 100644
--- a/esphome/components/api/api_pb2.cpp
+++ b/esphome/components/api/api_pb2.cpp
@@ -356,7 +356,7 @@ void ListEntitiesFanResponse::calculate_size(uint32_t &total_size) const {
   ProtoSize::add_enum_field(total_size, 1, static_cast<uint32_t>(this->entity_category));
   if (!this->supported_preset_modes.empty()) {
     for (const auto &it : this->supported_preset_modes) {
-      ProtoSize::add_string_field(total_size, 1, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 1, it);
     }
   }
 #ifdef USE_DEVICES
@@ -480,7 +480,7 @@ void ListEntitiesLightResponse::calculate_size(uint32_t &total_size) const {
   ProtoSize::add_float_field(total_size, 1, this->max_mireds);
   if (!this->effects.empty()) {
     for (const auto &it : this->effects) {
-      ProtoSize::add_string_field(total_size, 1, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 1, it);
     }
   }
   ProtoSize::add_bool_field(total_size, 1, this->disabled_by_default);
@@ -1130,7 +1130,7 @@ void ListEntitiesClimateResponse::calculate_size(uint32_t &total_size) const {
   }
   if (!this->supported_custom_fan_modes.empty()) {
     for (const auto &it : this->supported_custom_fan_modes) {
-      ProtoSize::add_string_field(total_size, 1, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 1, it);
     }
   }
   if (!this->supported_presets.empty()) {
@@ -1140,7 +1140,7 @@ void ListEntitiesClimateResponse::calculate_size(uint32_t &total_size) const {
   }
   if (!this->supported_custom_presets.empty()) {
     for (const auto &it : this->supported_custom_presets) {
-      ProtoSize::add_string_field(total_size, 2, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 2, it);
     }
   }
   ProtoSize::add_bool_field(total_size, 2, this->disabled_by_default);
@@ -1392,7 +1392,7 @@ void ListEntitiesSelectResponse::calculate_size(uint32_t &total_size) const {
 #endif
   if (!this->options.empty()) {
     for (const auto &it : this->options) {
-      ProtoSize::add_string_field(total_size, 1, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 1, it);
     }
   }
   ProtoSize::add_bool_field(total_size, 1, this->disabled_by_default);
@@ -1479,7 +1479,7 @@ void ListEntitiesSirenResponse::calculate_size(uint32_t &total_size) const {
   ProtoSize::add_bool_field(total_size, 1, this->disabled_by_default);
   if (!this->tones.empty()) {
     for (const auto &it : this->tones) {
-      ProtoSize::add_string_field(total_size, 1, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 1, it);
     }
   }
   ProtoSize::add_bool_field(total_size, 1, this->supports_duration);
@@ -2347,7 +2347,7 @@ void VoiceAssistantWakeWord::calculate_size(uint32_t &total_size) const {
   ProtoSize::add_string_field(total_size, 1, this->wake_word_ref_.size());
   if (!this->trained_languages.empty()) {
     for (const auto &it : this->trained_languages) {
-      ProtoSize::add_string_field(total_size, 1, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 1, it);
     }
   }
 }
@@ -2364,7 +2364,7 @@ void VoiceAssistantConfigurationResponse::calculate_size(uint32_t &total_size) c
   ProtoSize::add_repeated_message(total_size, 1, this->available_wake_words);
   if (!this->active_wake_words.empty()) {
     for (const auto &it : this->active_wake_words) {
-      ProtoSize::add_string_field(total_size, 1, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 1, it);
     }
   }
   ProtoSize::add_uint32_field(total_size, 1, this->max_active_wake_words);
@@ -2735,7 +2735,7 @@ void ListEntitiesEventResponse::calculate_size(uint32_t &total_size) const {
   ProtoSize::add_string_field(total_size, 1, this->device_class_ref_.size());
   if (!this->event_types.empty()) {
     for (const auto &it : this->event_types) {
-      ProtoSize::add_string_field(total_size, 1, it.length());
+      ProtoSize::add_string_field_repeated(total_size, 1, it);
     }
   }
 #ifdef USE_DEVICES
diff --git a/esphome/components/api/proto.h b/esphome/components/api/proto.h
index 9b95454328..58242495d9 100644
--- a/esphome/components/api/proto.h
+++ b/esphome/components/api/proto.h
@@ -699,6 +699,15 @@ class ProtoSize {
     total_size += field_id_size + varint(static_cast<uint32_t>(len)) + static_cast<uint32_t>(len);
   }
 
+  /**
+   * @brief Calculates and adds the size of a string/bytes field to the total message size (repeated field version)
+   */
+  static inline void add_string_field_repeated(uint32_t &total_size, uint32_t field_id_size, const std::string &str) {
+    // Always calculate size for repeated fields (no empty check)
+    const uint32_t str_size = static_cast<uint32_t>(str.size());
+    total_size += field_id_size + varint(str_size) + str_size;
+  }
+
   /**
    * @brief Calculates and adds the size of a bytes field to the total message size
    */
diff --git a/script/api_protobuf/api_protobuf.py b/script/api_protobuf/api_protobuf.py
index 148e04b7e2..d6f6159050 100755
--- a/script/api_protobuf/api_protobuf.py
+++ b/script/api_protobuf/api_protobuf.py
@@ -598,10 +598,11 @@ class StringType(TypeInfo):
             return self._get_simple_size_calculation(name, force, "add_string_field")
 
         # Check if this is being called from a repeated field context
-        # In that case, 'name' will be 'it' and we need to use .length()
+        # In that case, 'name' will be 'it' and we need to use the repeated version
         if name == "it":
+            # For repeated fields, we need to use add_string_field_repeated which includes field ID
             field_id_size = self.calculate_field_id_size()
-            return f"ProtoSize::add_string_field(total_size, {field_id_size}, it.length());"
+            return f"ProtoSize::add_string_field_repeated(total_size, {field_id_size}, it);"
 
         # For messages that need encoding, use the StringRef size
         field_id_size = self.calculate_field_id_size()