[api] Store Home Assistant state subscriptions in flash instead of heap (#12008)

esphome/components/api/api_connection.cpp

@@ -1580,7 +1580,12 @@ bool APIConnection::send_device_info_response(const DeviceInfoRequest &msg) {
 #ifdef USE_API_HOMEASSISTANT_STATES
 void APIConnection::on_home_assistant_state_response(const HomeAssistantStateResponse &msg) {
   for (auto &it : this->parent_->get_state_subs()) {
-    if (it.entity_id == msg.entity_id && it.attribute.value() == msg.attribute) {
+    // Compare entity_id and attribute with message fields
+    bool entity_match = (strcmp(it.entity_id, msg.entity_id.c_str()) == 0);
+    bool attribute_match = (it.attribute != nullptr && strcmp(it.attribute, msg.attribute.c_str()) == 0) ||
+                           (it.attribute == nullptr && msg.attribute.empty());
+
+    if (entity_match && attribute_match) {
       it.callback(msg.state);
     }
   }
@@ -1959,8 +1964,8 @@ void APIConnection::process_state_subscriptions_() {
   SubscribeHomeAssistantStateResponse resp;
   resp.set_entity_id(StringRef(it.entity_id));
 
-  // Avoid string copy by directly using the optional's value if it exists
-  resp.set_attribute(it.attribute.has_value() ? StringRef(it.attribute.value()) : StringRef(""));
+  // Avoid string copy by using the const char* pointer if it exists
+  resp.set_attribute(it.attribute != nullptr ? StringRef(it.attribute) : StringRef(""));
 
   resp.once = it.once;
   if (this->send_message(resp, SubscribeHomeAssistantStateResponse::MESSAGE_TYPE)) {

esphome/components/api/api_server.cpp

@@ -419,25 +419,56 @@ void APIServer::handle_action_response(uint32_t call_id, bool success, const std
 #endif  // USE_API_HOMEASSISTANT_SERVICES
 
 #ifdef USE_API_HOMEASSISTANT_STATES
+// Helper to add subscription (reduces duplication)
+void APIServer::add_state_subscription_(const char *entity_id, const char *attribute,
+                                        std::function<void(std::string)> f, bool once) {
+  this->state_subs_.push_back(HomeAssistantStateSubscription{
+      .entity_id = entity_id, .attribute = attribute, .callback = std::move(f), .once = once,
+      // entity_id_dynamic_storage and attribute_dynamic_storage remain nullptr (no heap allocation)
+  });
+}
+
+// Helper to add subscription with heap-allocated strings (reduces duplication)
+void APIServer::add_state_subscription_(std::string entity_id, optional<std::string> attribute,
+                                        std::function<void(std::string)> f, bool once) {
+  HomeAssistantStateSubscription sub;
+  // Allocate heap storage for the strings
+  sub.entity_id_dynamic_storage = std::make_unique<std::string>(std::move(entity_id));
+  sub.entity_id = sub.entity_id_dynamic_storage->c_str();
+
+  if (attribute.has_value()) {
+    sub.attribute_dynamic_storage = std::make_unique<std::string>(std::move(attribute.value()));
+    sub.attribute = sub.attribute_dynamic_storage->c_str();
+  } else {
+    sub.attribute = nullptr;
+  }
+
+  sub.callback = std::move(f);
+  sub.once = once;
+  this->state_subs_.push_back(std::move(sub));
+}
+
+// New const char* overload (for internal components - zero allocation)
+void APIServer::subscribe_home_assistant_state(const char *entity_id, const char *attribute,
+                                               std::function<void(std::string)> f) {
+  this->add_state_subscription_(entity_id, attribute, std::move(f), false);
+}
+
+void APIServer::get_home_assistant_state(const char *entity_id, const char *attribute,
+                                         std::function<void(std::string)> f) {
+  this->add_state_subscription_(entity_id, attribute, std::move(f), true);
+}
+
+// Existing std::string overload (for custom_api_device.h - heap allocation)
 void APIServer::subscribe_home_assistant_state(std::string entity_id, optional<std::string> attribute,
                                                std::function<void(std::string)> f) {
-  this->state_subs_.push_back(HomeAssistantStateSubscription{
-      .entity_id = std::move(entity_id),
-      .attribute = std::move(attribute),
-      .callback = std::move(f),
-      .once = false,
-  });
+  this->add_state_subscription_(std::move(entity_id), std::move(attribute), std::move(f), false);
 }
 
 void APIServer::get_home_assistant_state(std::string entity_id, optional<std::string> attribute,
                                          std::function<void(std::string)> f) {
-  this->state_subs_.push_back(HomeAssistantStateSubscription{
-      .entity_id = std::move(entity_id),
-      .attribute = std::move(attribute),
-      .callback = std::move(f),
-      .once = true,
-  });
-};
+  this->add_state_subscription_(std::move(entity_id), std::move(attribute), std::move(f), true);
+}
 
 const std::vector<APIServer::HomeAssistantStateSubscription> &APIServer::get_state_subs() const {
   return this->state_subs_;

esphome/components/api/api_server.h

@@ -190,16 +190,27 @@ class APIServer : public Component,
 
 #ifdef USE_API_HOMEASSISTANT_STATES
   struct HomeAssistantStateSubscription {
-    std::string entity_id;
-    optional<std::string> attribute;
+    const char *entity_id;  // Pointer to flash (internal) or heap (external)
+    const char *attribute;  // Pointer to flash or nullptr (nullptr means no attribute)
     std::function<void(std::string)> callback;
     bool once;
+
+    // Dynamic storage for external components using std::string API (custom_api_device.h)
+    // These are only allocated when using the std::string overload (nullptr for const char* overload)
+    std::unique_ptr<std::string> entity_id_dynamic_storage;
+    std::unique_ptr<std::string> attribute_dynamic_storage;
   };
 
+  // New const char* overload (for internal components - zero allocation)
+  void subscribe_home_assistant_state(const char *entity_id, const char *attribute, std::function<void(std::string)> f);
+  void get_home_assistant_state(const char *entity_id, const char *attribute, std::function<void(std::string)> f);
+
+  // Existing std::string overload (for custom_api_device.h - heap allocation)
   void subscribe_home_assistant_state(std::string entity_id, optional<std::string> attribute,
                                       std::function<void(std::string)> f);
   void get_home_assistant_state(std::string entity_id, optional<std::string> attribute,
                                 std::function<void(std::string)> f);
+
   const std::vector<HomeAssistantStateSubscription> &get_state_subs() const;
 #endif
 #ifdef USE_API_USER_DEFINED_ACTIONS
@@ -220,6 +231,13 @@ class APIServer : public Component,
   bool update_noise_psk_(const SavedNoisePsk &new_psk, const LogString *save_log_msg, const LogString *fail_log_msg,
                          const psk_t &active_psk, bool make_active);
 #endif  // USE_API_NOISE
+#ifdef USE_API_HOMEASSISTANT_STATES
+  // Helper methods to reduce code duplication
+  void add_state_subscription_(const char *entity_id, const char *attribute, std::function<void(std::string)> f,
+                               bool once);
+  void add_state_subscription_(std::string entity_id, optional<std::string> attribute,
+                               std::function<void(std::string)> f, bool once);
+#endif  // USE_API_HOMEASSISTANT_STATES
   // Pointers and pointer-like types first (4 bytes each)
   std::unique_ptr<socket::Socket> socket_ = nullptr;
 #ifdef USE_API_CLIENT_CONNECTED_TRIGGER