diff --git a/langfuse/_utils/prompt_cache.py b/langfuse/_utils/prompt_cache.py
index e286dca49..b4632b11f 100644
--- a/langfuse/_utils/prompt_cache.py
+++ b/langfuse/_utils/prompt_cache.py
@@ -27,11 +27,13 @@ def __init__(self, prompt: PromptClient, ttl_seconds: int):
         self._expiry = ttl_seconds + self.get_epoch_seconds()
 
     def is_expired(self) -> bool:
-        return self.get_epoch_seconds() > self._expiry
+        """Return whether the full cache lifetime has elapsed."""
+        return self.get_epoch_seconds() >= self._expiry
 
     @staticmethod
-    def get_epoch_seconds() -> int:
-        return int(datetime.now().timestamp())
+    def get_epoch_seconds() -> float:
+        """Keep fractional seconds so expiry reflects the actual insertion time."""
+        return datetime.now().timestamp()
 
 
 class PromptCacheRefreshConsumer(Thread):
diff --git a/tests/unit/test_prompt.py b/tests/unit/test_prompt.py
index eadfb8221..33724784a 100644
--- a/tests/unit/test_prompt.py
+++ b/tests/unit/test_prompt.py
@@ -748,3 +748,14 @@ def test_get_fresh_prompt_when_version_changes(langfuse: Langfuse):
     result_call_2 = langfuse.get_prompt(prompt_name, version=2)
     assert mock_server_call.call_count == 2
     assert result_call_2 == version_changed_prompt_client
+
+
+def test_prompt_cache_item_expires_at_exact_ttl_without_rounding():
+    """A cached prompt stays fresh only until its full TTL has elapsed."""
+    with patch("langfuse._utils.prompt_cache.datetime") as clock:
+        clock.now.return_value.timestamp.return_value = 100.75
+        item = PromptCacheItem(prompt=Mock(), ttl_seconds=1)
+        clock.now.return_value.timestamp.return_value = 101.70
+        assert not item.is_expired()
+        clock.now.return_value.timestamp.return_value = 101.75
+        assert item.is_expired()