diff --git a/README.md b/README.md
index fbf9d4c..adad6d0 100644
--- a/README.md
+++ b/README.md
@@ -246,6 +246,10 @@ new_chat = converter.chat_replace_variables(
 )
 ```
 
+> [!IMPORTANT]
+>
+> About the prompt format, each role key (e.g. `$system$` / `$user$` / `$assistant`) should be placed in a separate line.
+
 ### Substitute
 
 `PromptConverter` can also substitute placeholder variables like `%output_format%` stored in text files to make multiple prompts modular. A substitute map `substitute.txt` looks like this:
diff --git a/pyproject.toml b/pyproject.toml
index 7fd575b..6116259 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,13 +4,13 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "HandyLLM"
-version = "0.6.0"
+version = "0.6.1"
 authors = [
   { name="Atomie CHEN", email="atomic_cwh@163.com" },
 ]
 description = "A handy toolkit for using LLM."
 readme = "README.md"
-requires-python = ">=3.6"
+requires-python = ">=3.7"
 classifiers = [
     "Programming Language :: Python :: 3",
     # "License :: OSI Approved :: MIT License",
diff --git a/src/handyllm/openai_client.py b/src/handyllm/openai_client.py
index 4fc16a9..a5a6280 100644
--- a/src/handyllm/openai_client.py
+++ b/src/handyllm/openai_client.py
@@ -1,11 +1,10 @@
+from __future__ import annotations
+from typing import Union, TYPE_CHECKING
 import os
 import json
 import time
 from enum import Enum, auto
 import asyncio
-from typing import Union
-import requests
-import httpx
 
 from .endpoint_manager import Endpoint, EndpointManager
 from .requestor import Requestor
@@ -65,6 +64,9 @@ def __init__(
         api_version=None,
         model_engine_map=None,
         ) -> None:
+        self._sync_client = None
+        self._async_client = None
+        
         # convert string to enum
         if isinstance(mode, str):
             mode = mode.upper()
@@ -73,16 +75,16 @@ def __init__(
             mode = ClientMode[mode]
         elif not isinstance(mode, ClientMode):
             raise TypeError("Invalid client mode specified")
-
+        
         if mode == ClientMode.SYNC or mode == ClientMode.BOTH:
+            # lazy import
+            import requests
             self._sync_client = requests.Session()
-        else:
-            self._sync_client = None
         
         if mode == ClientMode.ASYNC or mode == ClientMode.BOTH:
+            # lazy import
+            import httpx
             self._async_client = httpx.AsyncClient()
-        else:
-            self._async_client = None
         
         self.api_base = api_base
         self.api_key = api_key
diff --git a/src/handyllm/prompt_converter.py b/src/handyllm/prompt_converter.py
index 8661eab..e847fd3 100644
--- a/src/handyllm/prompt_converter.py
+++ b/src/handyllm/prompt_converter.py
@@ -2,9 +2,16 @@
 
 class PromptConverter:
     
+    role_keys = ['system', 'user', 'assistant']
+
     def __init__(self):
         self.substitute_map = {}
 
+    @property
+    def split_pattern(self):
+        # build a regex pattern to split the prompt by role keys
+        return r'^\$(' + '|'.join(self.role_keys) + r')\$$'
+
     def read_substitute_content(self, path: str):
         # 从文本文件读取所有prompt中需要替换的内容
         with open(path, 'r', encoding='utf-8') as fin:
@@ -24,11 +31,11 @@ def raw2chat(self, raw_prompt: str):
 
         # convert plain text to chat format
         chat = []
-        blocks = re.split(r'(\$\w+\$)', raw_prompt)
+        blocks = re.split(self.split_pattern, raw_prompt, flags=re.MULTILINE)
         for idx in range(1, len(blocks), 2):
             key = blocks[idx]
             value = blocks[idx+1]
-            chat.append({"role": key[1:-1], "content": value.strip()})
+            chat.append({"role": key, "content": value.strip()})
         
         return chat
     
diff --git a/src/handyllm/requestor.py b/src/handyllm/requestor.py
index c0f6bb7..f8a82c6 100644
--- a/src/handyllm/requestor.py
+++ b/src/handyllm/requestor.py
@@ -1,10 +1,12 @@
+from __future__ import annotations
+from typing import Union, TYPE_CHECKING
 import asyncio
 import logging
 import json
-from typing import Union
-import requests
-import httpx
 import time
+if TYPE_CHECKING:
+    import requests
+    import httpx
 
 from ._constants import _API_TYPES_AZURE
 
diff --git a/tests/test_prompt.py b/tests/test_prompt.py
index 8cb50fe..1cb2041 100644
--- a/tests/test_prompt.py
+++ b/tests/test_prompt.py
@@ -2,10 +2,10 @@
 from handyllm import PromptConverter
 converter = PromptConverter()
 
-converter.read_substitute_content('../assets/substitute.txt')  # read substitute map
+converter.read_substitute_content('./assets/substitute.txt')  # read substitute map
 
 # chat can be used as the message parameter for OpenAI API
-chat = converter.rawfile2chat('../assets/prompt.txt')  # variables are substituted according to map
+chat = converter.rawfile2chat('./assets/prompt.txt')  # variables are substituted according to map
 # print(json.dumps(chat, indent=2))
 print(converter.chat2raw(chat))
 print('-----')