diff --git a/tests/test_normalize_messages.py b/tests/test_normalize_messages.py
new file mode 100644
index 000000000..6d88437ad
--- /dev/null
+++ b/tests/test_normalize_messages.py
@@ -0,0 +1,174 @@
+# SPDX-License-Identifier: Apache-2.0
+"""
+Tests for _normalize_messages() in vllm_mlx.server.
+
+_normalize_messages() maps non-standard roles (developer -> system) and merges
+consecutive same-role messages before chat template application. This prevents
+crashes from Qwen 3.5 and Llama templates that require alternating roles.
+"""
+
+
+class TestNormalizeMessages:
+    """Test _normalize_messages() for handling real-world client formats."""
+
+    def test_merge_consecutive_system_messages(self):
+        """Consecutive system messages are merged into one."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "system", "content": "You are a helpful assistant."},
+            {"role": "system", "content": "Always respond in JSON."},
+            {"role": "user", "content": "Hello"},
+        ]
+        result = _normalize_messages(messages)
+        assert len(result) == 2
+        assert result[0]["role"] == "system"
+        assert "helpful assistant" in result[0]["content"]
+        assert "JSON" in result[0]["content"]
+        assert result[1]["role"] == "user"
+        assert result[1]["content"] == "Hello"
+
+    def test_merge_consecutive_user_messages(self):
+        """Consecutive user messages are merged into one."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "system", "content": "You are a helper."},
+            {"role": "user", "content": "First part"},
+            {"role": "user", "content": "Second part"},
+        ]
+        result = _normalize_messages(messages)
+        assert len(result) == 2
+        assert result[1]["role"] == "user"
+        assert "First part" in result[1]["content"]
+        assert "Second part" in result[1]["content"]
+
+    def test_opencode_format(self):
+        """OpenCode's system+system+user+user format is normalized."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "system", "content": "System prompt part 1"},
+            {"role": "system", "content": "System prompt part 2"},
+            {"role": "user", "content": "User instruction"},
+            {"role": "user", "content": "User question"},
+        ]
+        result = _normalize_messages(messages)
+        assert len(result) == 2
+        assert result[0]["role"] == "system"
+        assert result[1]["role"] == "user"
+
+    def test_developer_role_mapped_to_system(self):
+        """OpenAI Responses API 'developer' role is mapped to 'system'."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "developer", "content": "You are a helpful assistant."},
+            {"role": "user", "content": "Hello"},
+        ]
+        result = _normalize_messages(messages)
+        assert result[0]["role"] == "system"
+        assert result[1]["role"] == "user"
+
+    def test_developer_and_system_merged(self):
+        """developer + system consecutive messages are merged after role mapping."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "developer", "content": "Part 1"},
+            {"role": "system", "content": "Part 2"},
+            {"role": "user", "content": "Hello"},
+        ]
+        result = _normalize_messages(messages)
+        assert len(result) == 2
+        assert result[0]["role"] == "system"
+        assert "Part 1" in result[0]["content"]
+        assert "Part 2" in result[0]["content"]
+
+    def test_already_alternating_unchanged(self):
+        """Well-formed alternating messages pass through unchanged."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "system", "content": "You are a helper."},
+            {"role": "user", "content": "Hello"},
+            {"role": "assistant", "content": "Hi!"},
+            {"role": "user", "content": "Bye"},
+        ]
+        result = _normalize_messages(messages)
+        assert result == messages
+
+    def test_single_message_unchanged(self):
+        """Single message passes through unchanged."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [{"role": "user", "content": "Hello"}]
+        result = _normalize_messages(messages)
+        assert result == messages
+
+    def test_empty_messages(self):
+        """Empty message list passes through."""
+        from vllm_mlx.server import _normalize_messages
+
+        assert _normalize_messages([]) == []
+
+    def test_multimodal_content_preserved(self):
+        """Messages with list content (multimodal) are not merged."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "user", "content": "Describe this:"},
+            {
+                "role": "user",
+                "content": [
+                    {"type": "text", "text": "What is in this image?"},
+                    {
+                        "type": "image_url",
+                        "image_url": {"url": "http://example.com/img.png"},
+                    },
+                ],
+            },
+        ]
+        result = _normalize_messages(messages)
+        # List content can't be trivially merged with string - kept separate
+        assert len(result) >= 1
+
+    def test_preserves_non_content_fields(self):
+        """Fields other than role/content are preserved on the first merged message."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "system", "content": "Part 1", "name": "sys1"},
+            {"role": "system", "content": "Part 2"},
+            {"role": "user", "content": "Hello"},
+        ]
+        result = _normalize_messages(messages)
+        assert len(result) == 2
+        assert result[0]["role"] == "system"
+
+    def test_null_content_not_merged(self):
+        """Messages with None content (tool_calls pattern) are not merged."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "assistant", "content": None, "tool_calls": [{"id": "tc1"}]},
+            {"role": "assistant", "content": "Follow-up"},
+        ]
+        result = _normalize_messages(messages)
+        # None content can't be merged with string - kept separate
+        assert len(result) == 2
+
+    def test_three_consecutive_system_messages(self):
+        """Three consecutive system messages merge into one."""
+        from vllm_mlx.server import _normalize_messages
+
+        messages = [
+            {"role": "system", "content": "Part 1"},
+            {"role": "system", "content": "Part 2"},
+            {"role": "system", "content": "Part 3"},
+            {"role": "user", "content": "Hello"},
+        ]
+        result = _normalize_messages(messages)
+        assert len(result) == 2
+        assert "Part 1" in result[0]["content"]
+        assert "Part 3" in result[0]["content"]
diff --git a/vllm_mlx/server.py b/vllm_mlx/server.py
index f0328d4e6..8de98efef 100644
--- a/vllm_mlx/server.py
+++ b/vllm_mlx/server.py
@@ -1326,12 +1326,14 @@ async def create_chat_completion(request: ChatCompletionRequest, raw_request: Re
             messages.append(msg_dict)
         images, videos = [], []  # MLLM extracts these from messages
         logger.debug(f"MLLM: Processing {len(messages)} messages")
+        messages = _normalize_messages(messages)
     else:
         # For LLM, extract text, images, and videos separately
         messages, images, videos = extract_multimodal_content(
             request.messages,
             preserve_native_format=engine.preserve_native_tool_format,
         )
+        messages = _normalize_messages(messages)
 
     has_media = bool(images or videos)
 
@@ -1434,6 +1436,64 @@ async def create_chat_completion(request: ChatCompletionRequest, raw_request: Re
     )
 
 
+def _normalize_messages(messages: list[dict]) -> list[dict]:
+    """Normalize message roles and merge consecutive same-role messages.
+
+    1. Maps non-standard roles to standard ones (e.g. ``developer`` -> ``system``).
+    2. Merges consecutive same-role messages to satisfy chat template constraints
+       (Qwen 3.5, Llama, etc. require alternating roles).
+
+    Only merges when both messages have string content. Messages with list
+    content (multimodal) are left as-is to preserve image/video attachments.
+
+    Args:
+        messages: List of message dicts with 'role' and 'content' keys.
+
+    Returns:
+        New list with normalized roles and consecutive same-role messages merged.
+    """
+    # OpenAI Responses API uses "developer" instead of "system".
+    # Map it so chat templates don't fail and fall back to raw prefill.
+    _ROLE_MAP = {"developer": "system"}
+
+    if not messages:
+        return messages
+
+    merged = [messages[0].copy()]
+    if merged[0]["role"] in _ROLE_MAP:
+        merged[0]["role"] = _ROLE_MAP[merged[0]["role"]]
+    for msg in messages[1:]:
+        prev = merged[-1]
+        role = _ROLE_MAP.get(msg["role"], msg["role"])
+        if (
+            role == prev["role"]
+            and isinstance(prev.get("content"), str)
+            and isinstance(msg.get("content"), str)
+        ):
+            # Merge string content with double newline separator
+            prev["content"] = prev["content"] + "\n\n" + msg["content"]
+            logger.debug(
+                f"Merged consecutive {role} messages "
+                f"({len(prev['content'])} chars total)"
+            )
+        else:
+            copy = msg.copy()
+            copy["role"] = role
+            merged.append(copy)
+
+    mapped_roles = sum(1 for m in messages if m["role"] in _ROLE_MAP)
+    merged_count = len(messages) - len(merged)
+    if mapped_roles or merged_count:
+        parts = []
+        if mapped_roles:
+            parts.append(f"mapped {mapped_roles} role(s)")
+        if merged_count:
+            parts.append(f"merged {len(messages)} -> {len(merged)}")
+        logger.info(f"Normalized messages: {', '.join(parts)}")
+
+    return merged
+
+
 def _inject_json_instruction(messages: list, instruction: str) -> list:
     """
     Inject JSON instruction into messages.
@@ -1529,6 +1589,7 @@ async def create_anthropic_message(
         openai_request.messages,
         preserve_native_format=engine.preserve_native_tool_format,
     )
+    messages = _normalize_messages(messages)
 
     chat_kwargs = {
         "max_tokens": openai_request.max_tokens or _default_max_tokens,
@@ -1686,6 +1747,7 @@ async def _stream_anthropic_messages(
         openai_request.messages,
         preserve_native_format=engine.preserve_native_tool_format,
     )
+    messages = _normalize_messages(messages)
 
     chat_kwargs = {
         "max_tokens": openai_request.max_tokens or _default_max_tokens,