diff --git a/esphome/config_validation.py b/esphome/config_validation.py
index 69f8ea9ef8..368b4f9f4a 100644
--- a/esphome/config_validation.py
+++ b/esphome/config_validation.py
@@ -398,6 +398,11 @@ def string_strict(value):
     )
 
 
+# Max icon string length (63 chars + null = 64-byte PROGMEM buffer)
+# Keep in sync with MAX_ICON_LENGTH in esphome/core/entity_base.h
+ICON_MAX_LENGTH = 63
+
+
 def icon(value):
     """Validate that a given config value is a valid icon."""
     value = string_strict(value)
@@ -407,9 +412,9 @@ def icon(value):
         raise Invalid(
             'Icons must match the format "[icon pack]:[icon]", e.g. "mdi:home-assistant"'
         )
-    if len(value) > 63:
+    if len(value) > ICON_MAX_LENGTH:
         raise Invalid(
-            f"Icon string is too long ({len(value)} chars, max 63). "
+            f"Icon string is too long ({len(value)} chars, max {ICON_MAX_LENGTH}). "
             "Icons are stored in PROGMEM with a 64-byte buffer limit."
         )
     return value
diff --git a/esphome/core/entity_base.h b/esphome/core/entity_base.h
index 6b6fe83834..a03d615c02 100644
--- a/esphome/core/entity_base.h
+++ b/esphome/core/entity_base.h
@@ -136,13 +136,13 @@ class EntityBase {
 #ifdef USE_ESP8266
   // On ESP8266, rodata is RAM. Icons are in PROGMEM and cannot be accessed
   // directly as const char*. Use get_icon_to() with a stack buffer instead.
-  template<typename T = void> StringRef get_icon_ref() const {
-    static_assert(!sizeof(T),
+  template<typename T = int> StringRef get_icon_ref() const {
+    static_assert(sizeof(T) == 0,
                   "get_icon_ref() unavailable on ESP8266 (rodata is RAM). Use get_icon_to() with a stack buffer.");
     return StringRef("");
   }
-  template<typename T = void> std::string get_icon() const {
-    static_assert(!sizeof(T),
+  template<typename T = int> std::string get_icon() const {
+    static_assert(sizeof(T) == 0,
                   "get_icon() unavailable on ESP8266 (rodata is RAM). Use get_icon_to() with a stack buffer.");
     return "";
   }
diff --git a/esphome/core/entity_helpers.py b/esphome/core/entity_helpers.py
index c699ac3dda..a8ca2f7432 100644
--- a/esphome/core/entity_helpers.py
+++ b/esphome/core/entity_helpers.py
@@ -188,14 +188,11 @@ def register_unit_of_measurement(value: str) -> int:
     return _register_string(value, _get_pool().units, _MAX_UNITS, "unit_of_measurement")
 
 
-_MAX_ICON_LENGTH = 63  # Max icon string length (64-byte buffer with null terminator)
-
-
 def register_icon(value: str) -> int:
     """Register an icon string and return its 1-based index."""
-    if value and len(value) > _MAX_ICON_LENGTH:
+    if value and len(value) > cv.ICON_MAX_LENGTH:
         raise ValueError(
-            f"Icon string too long ({len(value)} chars, max {_MAX_ICON_LENGTH}): '{value}'"
+            f"Icon string too long ({len(value)} chars, max {cv.ICON_MAX_LENGTH}): '{value}'"
         )
     return _register_string(value, _get_pool().icons, _MAX_ICONS, "icon")