diff --git a/src/llamafactory/data/formatter.py b/src/llamafactory/data/formatter.py
index ed83a2d2..c68f0a52 100644
--- a/src/llamafactory/data/formatter.py
+++ b/src/llamafactory/data/formatter.py
@@ -86,19 +86,20 @@ class StringFormatter(Formatter):
             elif isinstance(slot, (dict, set)):
                 elements.append(slot)
             else:
-                raise RuntimeError(f"Input must be string, set[str] or dict[str, str], got {type(slot)}")
+                raise RuntimeError(f"Input must be string, set[str] or dict[str, str], got {type(slot)}.")
 
         return elements
 
 
 @dataclass
-class FunctionFormatter(Formatter):
+class FunctionFormatter(StringFormatter):
     def __post_init__(self):
+        super().__post_init__()
         self.tool_utils = get_tool_utils(self.tool_format)
 
     @override
     def apply(self, **kwargs) -> SLOTS:
-        content = kwargs.pop("content")
+        content: str = kwargs.pop("content")
         regex = re.compile(r"<think>(.*)</think>", re.DOTALL)
         thought = re.search(regex, content)
         if thought:
@@ -116,19 +117,13 @@ class FunctionFormatter(Formatter):
                 )
 
         except json.JSONDecodeError:
-            raise RuntimeError(f"Invalid JSON format in function message: {str([content])}")  # flat string
+            raise RuntimeError(f"Invalid JSON format in function message: {str([content])}.")  # flat string
 
-        elements = []
-        for slot in self.slots:
-            if slot == "{{content}}":
-                if thought:
-                    elements.append(thought.group(1))
+        function_str = self.tool_utils.function_formatter(functions)
+        if thought:
+            function_str = thought.group(1) + function_str
 
-                elements += self.tool_utils.function_formatter(functions)
-            else:
-                elements.append(slot)
-
-        return elements
+        return super().apply(content=function_str)
 
 
 @dataclass
@@ -143,7 +138,7 @@ class ToolFormatter(Formatter):
             tools = json.loads(content)
             return [self.tool_utils.tool_formatter(tools) if len(tools) != 0 else ""]
         except json.JSONDecodeError:
-            raise RuntimeError(f"Invalid JSON format in tool description: {str([content])}")  # flat string
+            raise RuntimeError(f"Invalid JSON format in tool description: {str([content])}.")  # flat string
 
     @override
     def extract(self, content: str) -> Union[str, List["FunctionCall"]]:
diff --git a/src/llamafactory/data/tool_utils.py b/src/llamafactory/data/tool_utils.py
index 6132e982..f3269413 100644
--- a/src/llamafactory/data/tool_utils.py
+++ b/src/llamafactory/data/tool_utils.py
@@ -21,8 +21,6 @@ from typing import Any, Dict, List, NamedTuple, Tuple, Union
 
 from typing_extensions import override
 
-from .data_utils import SLOTS
-
 
 class FunctionCall(NamedTuple):
     name: str
@@ -76,7 +74,7 @@ class ToolUtils(ABC):
 
     @staticmethod
     @abstractmethod
-    def function_formatter(functions: List["FunctionCall"]) -> SLOTS:
+    def function_formatter(functions: List["FunctionCall"]) -> str:
         r"""
         Generates the assistant message including all the tool calls.
         """
@@ -134,12 +132,12 @@ class DefaultToolUtils(ToolUtils):
 
     @override
     @staticmethod
-    def function_formatter(functions: List["FunctionCall"]) -> SLOTS:
+    def function_formatter(functions: List["FunctionCall"]) -> str:
         function_text = ""
         for name, arguments in functions:
             function_text += f"Action: {name}\nAction Input: {arguments}\n"
 
-        return [function_text]
+        return function_text
 
     @override
     @staticmethod
@@ -180,11 +178,11 @@ class GLM4ToolUtils(ToolUtils):
 
     @override
     @staticmethod
-    def function_formatter(functions: List["FunctionCall"]) -> SLOTS:
+    def function_formatter(functions: List["FunctionCall"]) -> str:
         if len(functions) > 1:
             raise ValueError("GLM-4 does not support parallel functions.")
 
-        return [f"{functions[0].name}\n{functions[0].arguments}"]
+        return f"{functions[0].name}\n{functions[0].arguments}"
 
     @override
     @staticmethod
@@ -221,11 +219,11 @@ class Llama3ToolUtils(ToolUtils):
 
     @override
     @staticmethod
-    def function_formatter(functions: List["FunctionCall"]) -> SLOTS:
+    def function_formatter(functions: List["FunctionCall"]) -> str:
         if len(functions) > 1:
             raise ValueError("Llama-3 does not support parallel functions.")
 
-        return [f'{{"name": "{functions[0].name}", "parameters": {functions[0].arguments}}}']
+        return f'{{"name": "{functions[0].name}", "parameters": {functions[0].arguments}}}'
 
     @override
     @staticmethod
@@ -257,12 +255,12 @@ class MistralToolUtils(ToolUtils):
 
     @override
     @staticmethod
-    def function_formatter(functions: List["FunctionCall"]) -> SLOTS:
+    def function_formatter(functions: List["FunctionCall"]) -> str:
         function_texts = []
         for name, arguments in functions:
             function_texts.append(f'{{"name": "{name}", "arguments": {arguments}}}')
 
-        return ["[" + ", ".join(function_texts) + "]"]
+        return "[" + ", ".join(function_texts) + "]"
 
     @override
     @staticmethod
@@ -302,14 +300,14 @@ class QwenToolUtils(ToolUtils):
 
     @override
     @staticmethod
-    def function_formatter(functions: List["FunctionCall"]) -> SLOTS:
+    def function_formatter(functions: List["FunctionCall"]) -> str:
         function_texts = []
         for name, arguments in functions:
             function_texts.append(
                 "<tool_call>\n" + f'{{"name": "{name}", "arguments": {arguments}}}' + "\n</tool_call>"
             )
 
-        return ["\n".join(function_texts)]
+        return "\n".join(function_texts)
 
     @override
     @staticmethod
diff --git a/tests/data/test_formatter.py b/tests/data/test_formatter.py
index 2aaf48a2..542bafb9 100644
--- a/tests/data/test_formatter.py
+++ b/tests/data/test_formatter.py
@@ -123,11 +123,10 @@ def test_glm4_tool_extractor():
 
 
 def test_llama3_function_formatter():
-    formatter = FunctionFormatter(slots=["{{content}}", "<|eot_id|>"], tool_format="llama3")
+    formatter = FunctionFormatter(slots=["{{content}}<|eot_id|>"], tool_format="llama3")
     tool_calls = json.dumps({"name": "tool_name", "arguments": {"foo": "bar", "size": 10}})
     assert formatter.apply(content=tool_calls) == [
-        """{"name": "tool_name", "parameters": {"foo": "bar", "size": 10}}""",
-        "<|eot_id|>",
+        """{"name": "tool_name", "parameters": {"foo": "bar", "size": 10}}<|eot_id|>"""
     ]
 
 
@@ -150,20 +149,19 @@ def test_llama3_tool_extractor():
 
 
 def test_mistral_function_formatter():
-    formatter = FunctionFormatter(slots=["[TOOL_CALLS] ", "{{content}}", "</s>"], tool_format="mistral")
+    formatter = FunctionFormatter(slots=["[TOOL_CALLS] {{content}}", "</s>"], tool_format="mistral")
     tool_calls = json.dumps(FUNCTION)
     assert formatter.apply(content=tool_calls) == [
-        "[TOOL_CALLS] ",
-        """[{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}]""",
+        "[TOOL_CALLS] " """[{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}]""",
         "</s>",
     ]
 
 
 def test_mistral_multi_function_formatter():
-    formatter = FunctionFormatter(slots=["[TOOL_CALLS] ", "{{content}}", "</s>"], tool_format="mistral")
+    formatter = FunctionFormatter(slots=["[TOOL_CALLS] {{content}}", "</s>"], tool_format="mistral")
     tool_calls = json.dumps([FUNCTION] * 2)
     assert formatter.apply(content=tool_calls) == [
-        "[TOOL_CALLS] ",
+        "[TOOL_CALLS] "
         """[{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}, """
         """{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}]""",
         "</s>",
@@ -197,21 +195,20 @@ def test_mistral_multi_tool_extractor():
 
 
 def test_qwen_function_formatter():
-    formatter = FunctionFormatter(slots=["{{content}}", "<|im_end|>"], tool_format="qwen")
+    formatter = FunctionFormatter(slots=["{{content}}<|im_end|>\n"], tool_format="qwen")
     tool_calls = json.dumps(FUNCTION)
     assert formatter.apply(content=tool_calls) == [
-        """<tool_call>\n{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}\n</tool_call>""",
-        "<|im_end|>",
+        """<tool_call>\n{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}\n</tool_call><|im_end|>\n"""
     ]
 
 
 def test_qwen_multi_function_formatter():
-    formatter = FunctionFormatter(slots=["{{content}}", "<|im_end|>"], tool_format="qwen")
+    formatter = FunctionFormatter(slots=["{{content}}<|im_end|>\n"], tool_format="qwen")
     tool_calls = json.dumps([FUNCTION] * 2)
     assert formatter.apply(content=tool_calls) == [
         """<tool_call>\n{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}\n</tool_call>\n"""
-        """<tool_call>\n{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}\n</tool_call>""",
-        "<|im_end|>",
+        """<tool_call>\n{"name": "tool_name", "arguments": {"foo": "bar", "size": 10}}\n</tool_call>"""
+        "<|im_end|>\n"
     ]