diff --git a/gateway/platforms/telegram.py b/gateway/platforms/telegram.py
index 1bc4ec2b1..fbfb942ec 100644
--- a/gateway/platforms/telegram.py
+++ b/gateway/platforms/telegram.py
@@ -2264,8 +2264,9 @@ class TelegramAdapter(BasePlatformAdapter):
             yield getattr(message, "caption", None) or "", getattr(message, "caption_entities", None) or []
 
         for source_text, entities in _iter_sources():
-            if bot_username and f"@{bot_username}" in source_text.lower():
-                return True
+            if bot_username:
+                if re.search(rf'(?<!\w)@{re.escape(bot_username)}(?!\w)', source_text, re.IGNORECASE):
+                    return True
             for entity in entities:
                 entity_type = str(getattr(entity, "type", "")).split(".")[-1].lower()
                 if entity_type == "mention" and bot_username:
diff --git a/tests/gateway/test_telegram_mention_boundaries.py b/tests/gateway/test_telegram_mention_boundaries.py
new file mode 100644
index 000000000..161995016
--- /dev/null
+++ b/tests/gateway/test_telegram_mention_boundaries.py
@@ -0,0 +1,110 @@
+"""Tests for Telegram bot mention word-boundary detection (bug #12545).
+
+The old implementation used a naive substring check (`f"@{bot_username}" in text.lower()`),
+which incorrectly matched partial substrings like 'foo@hermes_bot.example'.
+
+These tests verify that the regex-based word-boundary check correctly delimits mentions.
+"""
+from types import SimpleNamespace
+
+from gateway.platforms.telegram import TelegramAdapter
+
+
+def _make_adapter():
+    """Build a minimal TelegramAdapter with a mocked bot."""
+    from gateway.config import Platform, PlatformConfig
+
+    adapter = object.__new__(TelegramAdapter)
+    adapter.platform = Platform.TELEGRAM
+    adapter.config = PlatformConfig(enabled=True, token="***", extra={})
+    adapter._bot = SimpleNamespace(id=999, username="hermes_bot")
+    return adapter
+
+
+def _group_message(text, entities=None, caption=None, caption_entities=None):
+    """Produce a minimal group-message-like SimpleNamespace."""
+    return SimpleNamespace(
+        text=text,
+        caption=caption,
+        entities=entities or [],
+        caption_entities=caption_entities or [],
+        message_thread_id=None,
+        chat=SimpleNamespace(id=-100, type="group"),
+        reply_to_message=None,
+    )
+
+
+class TestTelegramMentionBoundaries:
+    """Test that _message_mentions_bot correctly respects word boundaries."""
+
+    def test_exact_mention_is_recognized(self):
+        """'@hermes_bot' at any position should be detected."""
+        adapter = _make_adapter()
+        msg = _group_message("hello @hermes_bot")
+        assert adapter._message_mentions_bot(msg) is True
+
+    def test_mention_at_start_of_string(self):
+        """'@hermes_bot hello' should be detected."""
+        adapter = _make_adapter()
+        msg = _group_message("@hermes_bot hello world")
+        assert adapter._message_mentions_bot(msg) is True
+
+    def test_mention_followed_by_punctuation(self):
+        """'@hermes_bot,' should be detected."""
+        adapter = _make_adapter()
+        msg = _group_message("@hermes_bot, how are you?")
+        assert adapter._message_mentions_bot(msg) is True
+
+    def test_mention_in_subdomain_is_not_recognized(self):
+        """'foo@hermes_bot.example' should NOT match (bug #12545)."""
+        adapter = _make_adapter()
+        msg = _group_message("foo@hermes_bot.example")
+        assert adapter._message_mentions_bot(msg) is False
+
+    def test_mention_in_longer_hostname_is_not_recognized(self):
+        """'email me at user@hermes_bot.domain.com' should NOT match."""
+        adapter = _make_adapter()
+        msg = _group_message("email me at user@hermes_bot.domain.com")
+        assert adapter._message_mentions_bot(msg) is False
+
+    def test_superstring_username_is_not_recognized(self):
+        """'@hermes_botx' should NOT match (different username)."""
+        adapter = _make_adapter()
+        msg = _group_message("@hermes_botx hello")
+        assert adapter._message_mentions_bot(msg) is False
+
+    def test_prefixed_superstring_is_not_recognized(self):
+        """'foo@hermes_bot_bar' should NOT match."""
+        adapter = _make_adapter()
+        msg = _group_message("foo@hermes_bot_bar")
+        assert adapter._message_mentions_bot(msg) is False
+
+    def test_mention_case_insensitive(self):
+        """'@HERMES_BOT' should be detected (case-insensitive)."""
+        adapter = _make_adapter()
+        msg = _group_message("@HERMES_BOT hello")
+        assert adapter._message_mentions_bot(msg) is True
+
+    def test_mention_mixed_case(self):
+        """'@Hermes_Bot' should be detected."""
+        adapter = _make_adapter()
+        msg = _group_message("@Hermes_Bot hello")
+        assert adapter._message_mentions_bot(msg) is True
+
+    def test_no_mention_returns_false(self):
+        """Plain text with no mention should return False."""
+        adapter = _make_adapter()
+        msg = _group_message("just a regular message in the group")
+        assert adapter._message_mentions_bot(msg) is False
+
+    def test_mention_in_caption(self):
+        """Mention in caption should be detected."""
+        adapter = _make_adapter()
+        msg = _group_message(None, caption="check this out @hermes_bot")
+        assert adapter._message_mentions_bot(msg) is True
+
+    def test_subdomain_mention_in_caption_not_recognized(self):
+        """'foo@hermes_bot.example' in caption should NOT match."""
+        adapter = _make_adapter()
+        msg = _group_message(None, caption="foo@hermes_bot.example")
+        assert adapter._message_mentions_bot(msg) is False