diff --git a/esphome/core/entity_base.cpp b/esphome/core/entity_base.cpp
index 05f545c861..d869026a85 100644
--- a/esphome/core/entity_base.cpp
+++ b/esphome/core/entity_base.cpp
@@ -50,22 +50,22 @@ __attribute__((weak)) const char *entity_device_class_lookup(uint16_t) { return
 __attribute__((weak)) const char *entity_uom_lookup(uint16_t) { return ""; }
 __attribute__((weak)) const char *entity_icon_lookup(uint16_t) { return ""; }
 
+static constexpr auto EMPTY_REF = StringRef::from_lit("");
+
 // Entity device class (from packed index)
 StringRef EntityBase::get_device_class_ref() const {
-  static constexpr auto EMPTY = StringRef::from_lit("");
   uint16_t idx = (this->entity_string_packed_ >> ENTITY_STR_DC_SHIFT) & ENTITY_STR_DC_MASK;
   if (idx == 0)
-    return EMPTY;
+    return EMPTY_REF;
   return StringRef(entity_device_class_lookup(idx));
 }
 std::string EntityBase::get_device_class() const { return std::string(this->get_device_class_ref().c_str()); }
 
 // Entity unit of measurement (from packed index)
 StringRef EntityBase::get_unit_of_measurement_ref() const {
-  static constexpr auto EMPTY = StringRef::from_lit("");
   uint16_t idx = (this->entity_string_packed_ >> ENTITY_STR_UOM_SHIFT) & ENTITY_STR_UOM_MASK;
   if (idx == 0)
-    return EMPTY;
+    return EMPTY_REF;
   return StringRef(entity_uom_lookup(idx));
 }
 std::string EntityBase::get_unit_of_measurement() const {
@@ -74,14 +74,13 @@ std::string EntityBase::get_unit_of_measurement() const {
 
 // Entity icon (from packed index)
 StringRef EntityBase::get_icon_ref() const {
-  static constexpr auto EMPTY = StringRef::from_lit("");
 #ifdef USE_ENTITY_ICON
   uint16_t idx = (this->entity_string_packed_ >> ENTITY_STR_ICON_SHIFT) & ENTITY_STR_ICON_MASK;
   if (idx == 0)
-    return EMPTY;
+    return EMPTY_REF;
   return StringRef(entity_icon_lookup(idx));
 #else
-  return EMPTY;
+  return EMPTY_REF;
 #endif
 }
 std::string EntityBase::get_icon() const { return std::string(this->get_icon_ref().c_str()); }
diff --git a/esphome/core/entity_helpers.py b/esphome/core/entity_helpers.py
index 367da3d9ec..55d14d5d19 100644
--- a/esphome/core/entity_helpers.py
+++ b/esphome/core/entity_helpers.py
@@ -60,7 +60,7 @@ class EntityStringPool:
     device_classes: dict[str, int] = field(default_factory=dict)
     units: dict[str, int] = field(default_factory=dict)
     icons: dict[str, int] = field(default_factory=dict)
-    _tables_registered: bool = False
+    tables_registered: bool = False
 
 
 def _get_pool() -> EntityStringPool:
@@ -73,9 +73,9 @@ def _get_pool() -> EntityStringPool:
 def _ensure_tables_registered() -> None:
     """Schedule the table generation job (once)."""
     pool = _get_pool()
-    if pool._tables_registered:
+    if pool.tables_registered:
         return
-    pool._tables_registered = True
+    pool.tables_registered = True
     CORE.add_job(_generate_tables_job)
 
 
diff --git a/tests/unit_tests/core/test_entity_helpers.py b/tests/unit_tests/core/test_entity_helpers.py
index b937b2fcee..5dc017a5ed 100644
--- a/tests/unit_tests/core/test_entity_helpers.py
+++ b/tests/unit_tests/core/test_entity_helpers.py
@@ -11,6 +11,7 @@ from esphome.config_validation import Invalid
 from esphome.const import (
     CONF_DEVICE_ID,
     CONF_DISABLED_BY_DEFAULT,
+    CONF_ENTITY_CATEGORY,
     CONF_ICON,
     CONF_ID,
     CONF_INTERNAL,
@@ -18,6 +19,7 @@ from esphome.const import (
 )
 from esphome.core import CORE, ID, entity_helpers
 from esphome.core.entity_helpers import (
+    _register_string,
     _setup_entity_impl,
     entity_duplicate_validator,
     get_base_entity_object_id,
@@ -895,3 +897,30 @@ async def test_setup_entity_empty_name_no_mac_suffix_no_friendly_name(
     assert any('set_name("", 0)' in expr for expr in added_expressions), (
         f"Expected set_name with hash 0, got {added_expressions}"
     )
+
+
+def test_register_string_overflow() -> None:
+    """Test _register_string raises ValueError when max count is exceeded."""
+    category: dict[str, int] = {}
+    for i in range(3):
+        _register_string(f"val_{i}", category, 3, "test")
+    with pytest.raises(ValueError, match="Too many unique test values"):
+        _register_string("overflow", category, 3, "test")
+
+
+@pytest.mark.asyncio
+async def test_setup_entity_with_entity_category(
+    setup_test_environment: list[str],
+) -> None:
+    """Test setup_entity sets entity_category correctly."""
+    added_expressions = setup_test_environment
+    var = MockObj("sensor1")
+    config = {
+        CONF_NAME: "Temperature",
+        CONF_DISABLED_BY_DEFAULT: False,
+        CONF_ENTITY_CATEGORY: "diagnostic",
+    }
+    await _setup_entity_impl(var, config, "sensor")
+    assert any(
+        'set_entity_category("diagnostic")' in expr for expr in added_expressions
+    )