From 8b46610281a6faeb3ff7b2cc4e16946575bfa53b Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Wed, 7 Jan 2026 19:39:11 -1000
Subject: [PATCH] tweaks

---
 esphome/components/mqtt/mqtt_component.cpp | 5 ++---
 esphome/core/config.py                     | 2 ++
 esphome/core/entity_base.h                 | 5 ++++-
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/esphome/components/mqtt/mqtt_component.cpp b/esphome/components/mqtt/mqtt_component.cpp
index 8d04d376f6..d838d1789f 100644
--- a/esphome/components/mqtt/mqtt_component.cpp
+++ b/esphome/components/mqtt/mqtt_component.cpp
@@ -28,10 +28,9 @@ inline char *append_char(char *p, char c) {
 // These limits are enforced at Python config validation time in mqtt/__init__.py
 // using cv.Length() validators for topic_prefix and discovery_prefix.
 // MQTT_COMPONENT_TYPE_MAX_LEN and MQTT_SUFFIX_MAX_LEN are defined in mqtt_component.h.
-// OBJECT_ID_MAX_LEN is defined in entity_base.h and derived from friendly_name limits.
+// ESPHOME_DEVICE_NAME_MAX_LEN and OBJECT_ID_MAX_LEN are defined in entity_base.h.
 // This ensures the stack buffers below are always large enough.
 static constexpr size_t TOPIC_PREFIX_MAX_LEN = 64;      // Validated in Python: cv.Length(max=64)
-static constexpr size_t SANITIZED_NAME_MAX_LEN = 64;    // Safe: hostname validation limits to 31 chars
 static constexpr size_t DISCOVERY_PREFIX_MAX_LEN = 64;  // Validated in Python: cv.Length(max=64)
 
 // Stack buffer sizes - safe because all inputs are length-validated at config time
@@ -40,7 +39,7 @@ static constexpr size_t DEFAULT_TOPIC_MAX_LEN =
     TOPIC_PREFIX_MAX_LEN + 1 + MQTT_COMPONENT_TYPE_MAX_LEN + 1 + OBJECT_ID_MAX_LEN + 1 + MQTT_SUFFIX_MAX_LEN + 1;
 // Format: prefix + "/" + type + "/" + name + "/" + object_id + "/config" + null
 static constexpr size_t DISCOVERY_TOPIC_MAX_LEN = DISCOVERY_PREFIX_MAX_LEN + 1 + MQTT_COMPONENT_TYPE_MAX_LEN + 1 +
-                                                  SANITIZED_NAME_MAX_LEN + 1 + OBJECT_ID_MAX_LEN + 7 + 1;
+                                                  ESPHOME_DEVICE_NAME_MAX_LEN + 1 + OBJECT_ID_MAX_LEN + 7 + 1;
 
 void MQTTComponent::set_qos(uint8_t qos) { this->qos_ = qos; }
 
diff --git a/esphome/core/config.py b/esphome/core/config.py
index f9c3011507..b7e6ab9bee 100644
--- a/esphome/core/config.py
+++ b/esphome/core/config.py
@@ -76,6 +76,7 @@ VALID_INCLUDE_EXTS = {".h", ".hpp", ".tcc", ".ino", ".cpp", ".c"}
 
 
 def validate_hostname(config):
+    # Keep in sync with ESPHOME_DEVICE_NAME_MAX_LEN in esphome/core/entity_base.h
     max_length = 31
     if config[CONF_NAME_ADD_MAC_SUFFIX]:
         max_length -= 7  # "-AABBCC" is appended when add mac suffix option is used
@@ -207,6 +208,7 @@ CONFIG_SCHEMA = cv.All(
     cv.Schema(
         {
             cv.Required(CONF_NAME): cv.valid_name,
+            # Keep max=120 in sync with OBJECT_ID_MAX_LEN in esphome/core/entity_base.h
             cv.Optional(CONF_FRIENDLY_NAME, ""): cv.All(
                 cv.string_no_slash, cv.Length(max=120)
             ),
diff --git a/esphome/core/entity_base.h b/esphome/core/entity_base.h
index a45c7795bf..1649077dd0 100644
--- a/esphome/core/entity_base.h
+++ b/esphome/core/entity_base.h
@@ -13,7 +13,10 @@
 
 namespace esphome {
 
-// Maximum size for object_id buffer (friendly_name max ~120 + margin)
+// Maximum device name length - keep in sync with validate_hostname() in esphome/core/config.py
+static constexpr size_t ESPHOME_DEVICE_NAME_MAX_LEN = 31;
+
+// Maximum size for object_id buffer - keep in sync with friendly_name cv.Length(max=120) in esphome/core/config.py
 static constexpr size_t OBJECT_ID_MAX_LEN = 128;
 
 enum EntityCategory : uint8_t {