diff --git a/libs/core/langchain_core/language_models/fake_chat_models.py b/libs/core/langchain_core/language_models/fake_chat_models.py
index a1cdd49a7b3..b8dd7325242 100644
--- a/libs/core/langchain_core/language_models/fake_chat_models.py
+++ b/libs/core/langchain_core/language_models/fake_chat_models.py
@@ -36,6 +36,8 @@ class FakeMessagesListChatModel(BaseChatModel):
         run_manager: Optional[CallbackManagerForLLMRun] = None,
         **kwargs: Any,
     ) -> ChatResult:
+        if self.sleep is not None:
+            time.sleep(self.sleep)
         response = self.responses[self.i]
         if self.i < len(self.responses) - 1:
             self.i += 1
diff --git a/libs/core/langchain_core/utils/mustache.py b/libs/core/langchain_core/utils/mustache.py
index be0739568c2..12cab8da0a3 100644
--- a/libs/core/langchain_core/utils/mustache.py
+++ b/libs/core/langchain_core/utils/mustache.py
@@ -150,6 +150,11 @@ def parse_tag(template: str, l_del: str, r_del: str) -> tuple[tuple[str, str], s
         msg = f"unclosed tag at line {_CURRENT_LINE}"
         raise ChevronError(msg) from e
 
+    # Check for empty tags
+    if not tag.strip():
+        msg = f"empty tag at line {_CURRENT_LINE}"
+        raise ChevronError(msg)
+
     # Find the type meaning of the first character
     tag_type = tag_types.get(tag[0], "variable")
 
diff --git a/libs/core/tests/unit_tests/fake/test_fake_chat_model.py b/libs/core/tests/unit_tests/fake/test_fake_chat_model.py
index 7500e1640ac..ce262797535 100644
--- a/libs/core/tests/unit_tests/fake/test_fake_chat_model.py
+++ b/libs/core/tests/unit_tests/fake/test_fake_chat_model.py
@@ -1,5 +1,6 @@
 """Tests for verifying that testing utility code works as expected."""
 
+import time
 from itertools import cycle
 from typing import Any, Optional, Union
 from uuid import UUID
@@ -9,10 +10,11 @@ from typing_extensions import override
 from langchain_core.callbacks.base import AsyncCallbackHandler
 from langchain_core.language_models import (
     FakeListChatModel,
+    FakeMessagesListChatModel,
     GenericFakeChatModel,
     ParrotFakeChatModel,
 )
-from langchain_core.messages import AIMessage, AIMessageChunk, BaseMessage
+from langchain_core.messages import AIMessage, AIMessageChunk, BaseMessage, HumanMessage
 from langchain_core.outputs import ChatGenerationChunk, GenerationChunk
 from tests.unit_tests.stubs import (
     _any_id_ai_message,
@@ -230,3 +232,18 @@ def test_fake_list_chat_model_batch() -> None:
         fake = FakeListChatModel(responses=["a", "b", "c"])
         resp = fake.batch(["1", "2", "3"])
         assert resp == expected
+
+
+def test_fake_messages_list_chat_model_sleep_delay() -> None:
+    sleep_time = 0.1
+    model = FakeMessagesListChatModel(
+        responses=[AIMessage(content="A"), AIMessage(content="B")],
+        sleep=sleep_time,
+    )
+    messages = [HumanMessage(content="C")]
+
+    start = time.time()
+    model.invoke(messages)
+    elapsed = time.time() - start
+
+    assert elapsed >= sleep_time
diff --git a/libs/core/tests/unit_tests/prompts/test_structured.py b/libs/core/tests/unit_tests/prompts/test_structured.py
index 0b74b37cc6f..fda2d00b367 100644
--- a/libs/core/tests/unit_tests/prompts/test_structured.py
+++ b/libs/core/tests/unit_tests/prompts/test_structured.py
@@ -2,6 +2,7 @@ from functools import partial
 from inspect import isclass
 from typing import Any, Union, cast
 
+import pytest
 from pydantic import BaseModel
 
 from langchain_core.language_models import FakeListChatModel
@@ -10,6 +11,7 @@ from langchain_core.load.load import loads
 from langchain_core.messages import HumanMessage
 from langchain_core.prompts.structured import StructuredPrompt
 from langchain_core.runnables.base import Runnable, RunnableLambda
+from langchain_core.utils.mustache import ChevronError
 from langchain_core.utils.pydantic import is_basemodel_subclass
 
 
@@ -128,3 +130,8 @@ def test_structured_prompt_template_format() -> None:
     assert prompt.invoke({"person": {"name": "foo"}}).to_messages() == [
         HumanMessage("hi foo")
     ]
+
+
+def test_structured_prompt_template_empty_vars() -> None:
+    with pytest.raises(ChevronError, match="empty tag"):
+        StructuredPrompt([("human", "hi {{}}")], schema={}, template_format="mustache")