diff --git a/esphome/components/api/__init__.py b/esphome/components/api/__init__.py
index 58828c131d..e8dacf51bc 100644
--- a/esphome/components/api/__init__.py
+++ b/esphome/components/api/__init__.py
@@ -380,12 +380,19 @@ async def homeassistant_service_to_code(
     var = cg.new_Pvariable(action_id, template_arg, serv, False)
     templ = await cg.templatable(config[CONF_ACTION], args, None)
     cg.add(var.set_service(templ))
+
+    # Initialize FixedVectors with exact sizes from config
+    cg.add(var.init_data(len(config[CONF_DATA])))
     for key, value in config[CONF_DATA].items():
         templ = await cg.templatable(value, args, None)
         cg.add(var.add_data(key, templ))
+
+    cg.add(var.init_data_template(len(config[CONF_DATA_TEMPLATE])))
     for key, value in config[CONF_DATA_TEMPLATE].items():
         templ = await cg.templatable(value, args, None)
         cg.add(var.add_data_template(key, templ))
+
+    cg.add(var.init_variables(len(config[CONF_VARIABLES])))
     for key, value in config[CONF_VARIABLES].items():
         templ = await cg.templatable(value, args, None)
         cg.add(var.add_variable(key, templ))
@@ -458,15 +465,23 @@ async def homeassistant_event_to_code(config, action_id, template_arg, args):
     var = cg.new_Pvariable(action_id, template_arg, serv, True)
     templ = await cg.templatable(config[CONF_EVENT], args, None)
     cg.add(var.set_service(templ))
+
+    # Initialize FixedVectors with exact sizes from config
+    cg.add(var.init_data(len(config[CONF_DATA])))
     for key, value in config[CONF_DATA].items():
         templ = await cg.templatable(value, args, None)
         cg.add(var.add_data(key, templ))
+
+    cg.add(var.init_data_template(len(config[CONF_DATA_TEMPLATE])))
     for key, value in config[CONF_DATA_TEMPLATE].items():
         templ = await cg.templatable(value, args, None)
         cg.add(var.add_data_template(key, templ))
+
+    cg.add(var.init_variables(len(config[CONF_VARIABLES])))
     for key, value in config[CONF_VARIABLES].items():
         templ = await cg.templatable(value, args, None)
         cg.add(var.add_variable(key, templ))
+
     return var
 
 
@@ -489,6 +504,8 @@ async def homeassistant_tag_scanned_to_code(config, action_id, template_arg, arg
     serv = await cg.get_variable(config[CONF_ID])
     var = cg.new_Pvariable(action_id, template_arg, serv, True)
     cg.add(var.set_service("esphome.tag_scanned"))
+    # Initialize FixedVector with exact size (1 data field)
+    cg.add(var.init_data(1))
     templ = await cg.templatable(config[CONF_TAG], args, cg.std_string)
     cg.add(var.add_data("tag_id", templ))
     return var
diff --git a/esphome/components/api/homeassistant_service.h b/esphome/components/api/homeassistant_service.h
index 46e89cb39f..4adda47b71 100644
--- a/esphome/components/api/homeassistant_service.h
+++ b/esphome/components/api/homeassistant_service.h
@@ -41,10 +41,14 @@ template<typename... X> class TemplatableStringValue : public TemplatableValue<s
 
 template<typename... Ts> class TemplatableKeyValuePair {
  public:
+  // Default constructor needed for FixedVector::emplace_back()
+  TemplatableKeyValuePair() = default;
+
   // Keys are always string literals from YAML dictionary keys (e.g., "code", "event")
   // and never templatable values or lambdas. Only the value parameter can be a lambda/template.
   // Using pass-by-value with std::move allows optimal performance for both lvalues and rvalues.
   template<typename T> TemplatableKeyValuePair(std::string key, T value) : key(std::move(key)), value(value) {}
+
   std::string key;
   TemplatableStringValue<Ts...> value;
 };
@@ -93,15 +97,28 @@ template<typename... Ts> class HomeAssistantServiceCallAction : public Action<Ts
 
   template<typename T> void set_service(T service) { this->service_ = service; }
 
+  // Initialize FixedVector members - called from Python codegen with compile-time known sizes
+  void init_data(size_t count) { this->data_.init(count); }
+  void init_data_template(size_t count) { this->data_template_.init(count); }
+  void init_variables(size_t count) { this->variables_.init(count); }
+
   // Keys are always string literals from the Python code generation (e.g., cg.add(var.add_data("tag_id", templ))).
   // The value parameter can be a lambda/template, but keys are never templatable.
   // Using pass-by-value allows the compiler to optimize for both lvalues and rvalues.
-  template<typename T> void add_data(std::string key, T value) { this->data_.emplace_back(std::move(key), value); }
+  template<typename T> void add_data(std::string key, T value) {
+    auto &kv = this->data_.emplace_back();
+    kv.key = std::move(key);
+    kv.value = value;
+  }
   template<typename T> void add_data_template(std::string key, T value) {
-    this->data_template_.emplace_back(std::move(key), value);
+    auto &kv = this->data_template_.emplace_back();
+    kv.key = std::move(key);
+    kv.value = value;
   }
   template<typename T> void add_variable(std::string key, T value) {
-    this->variables_.emplace_back(std::move(key), value);
+    auto &kv = this->variables_.emplace_back();
+    kv.key = std::move(key);
+    kv.value = value;
   }
 
 #ifdef USE_API_HOMEASSISTANT_ACTION_RESPONSES
@@ -186,9 +203,9 @@ template<typename... Ts> class HomeAssistantServiceCallAction : public Action<Ts
 
   APIServer *parent_;
   TemplatableStringValue<Ts...> service_{};
-  std::vector<TemplatableKeyValuePair<Ts...>> data_;
-  std::vector<TemplatableKeyValuePair<Ts...>> data_template_;
-  std::vector<TemplatableKeyValuePair<Ts...>> variables_;
+  FixedVector<TemplatableKeyValuePair<Ts...>> data_;
+  FixedVector<TemplatableKeyValuePair<Ts...>> data_template_;
+  FixedVector<TemplatableKeyValuePair<Ts...>> variables_;
 #ifdef USE_API_HOMEASSISTANT_ACTION_RESPONSES
 #ifdef USE_API_HOMEASSISTANT_ACTION_RESPONSES_JSON
   TemplatableStringValue<Ts...> response_template_{""};