chore(mlx-lm): support text type content in messages (#1225)

* chore(mlx-lm): support text type content

* chore: optimize the messagef content processing

* nits + format

---------

Co-authored-by: Awni Hannun <awni@apple.com>

llms/mlx_lm/server.py

@@ -114,6 +114,33 @@ def convert_chat(messages: List[dict], role_mapping: Optional[dict] = None):
     return prompt.rstrip()
 
 
+def process_message_content(messages):
+    """
+    Convert message content to a format suitable for `apply_chat_template`.
+
+    The function operates on messages in place. It converts the 'content' field
+    to a string instead of a list of text fragments.
+
+    Args:
+        message_list (list): A list of dictionaries, where each dictionary may
+          have a 'content' key containing a list of dictionaries with 'type' and
+          'text' keys.
+
+    Raises:
+        ValueError: If the 'content' type is not supported or if 'text' is missing.
+
+    """
+    for message in messages:
+        content = message["content"]
+        if isinstance(content, list):
+            text_fragments = [
+                fragment["text"] for fragment in content if fragment["type"] == "text"
+            ]
+            if len(text_fragments) != len(content):
+                raise ValueError("Only 'text' content type is supported.")
+            message["content"] = "".join(text_fragments)
+
+
 @dataclass
 class PromptCache:
     cache: List[Any] = field(default_factory=list)
@@ -591,8 +618,10 @@ class APIHandler(BaseHTTPRequestHandler):
         self.request_id = f"chatcmpl-{uuid.uuid4()}"
         self.object_type = "chat.completion.chunk" if self.stream else "chat.completion"
         if self.tokenizer.chat_template:
+            messages = body["messages"]
+            process_message_content(messages)
             prompt = self.tokenizer.apply_chat_template(
-                body["messages"],
+                messages,
                 body.get("tools", None),
                 add_generation_prompt=True,
             )

llms/tests/test_server.py

@@ -80,6 +80,29 @@ class TestServer(unittest.TestCase):
         self.assertIn("id", response_body)
         self.assertIn("choices", response_body)
 
+    def test_handle_chat_completions_with_content_fragments(self):
+        url = f"http://localhost:{self.port}/v1/chat/completions"
+        chat_post_data = {
+            "model": "chat_model",
+            "max_tokens": 10,
+            "temperature": 0.7,
+            "top_p": 0.85,
+            "repetition_penalty": 1.2,
+            "messages": [
+                {
+                    "role": "system",
+                    "content": [
+                        {"type": "text", "text": "You are a helpful assistant."}
+                    ],
+                },
+                {"role": "user", "content": [{"type": "text", "text": "Hello!"}]},
+            ],
+        }
+        response = requests.post(url, json=chat_post_data)
+        response_body = response.text
+        self.assertIn("id", response_body)
+        self.assertIn("choices", response_body)
+
     def test_handle_models(self):
         url = f"http://localhost:{self.port}/v1/models"
         response = requests.get(url)