diff --git a/api/apps/restful_apis/openai_api.py b/api/apps/restful_apis/openai_api.py
index 1c58e1c964..4df0eed3df 100644
--- a/api/apps/restful_apis/openai_api.py
+++ b/api/apps/restful_apis/openai_api.py
@@ -91,6 +91,43 @@ def _build_sse_response(body):
     return resp
 
 
+def _normalize_message_content(content):
+    """Convert OpenAI message content to a string for the dialog layer.
+
+    Supports string content and array parts with ``type: text``. Other part types
+    (e.g. image_url) are ignored until vision is wired through this route.
+    """
+    if content is None:
+        return ""
+    if isinstance(content, str):
+        return content
+    if isinstance(content, list):
+        parts = []
+        for part in content:
+            if isinstance(part, dict) and part.get("type") == "text":
+                text = part.get("text", "")
+                if text is not None:
+                    parts.append(str(text))
+        return "\n".join(parts)
+    return None
+
+
+def _normalize_openai_messages(messages):
+    """Return (normalized_messages, error_message). error_message is set on failure."""
+    if not isinstance(messages, list):
+        return None, "messages must be an array."
+
+    normalized = []
+    for message in messages:
+        if not isinstance(message, dict):
+            return None, "Each message must be an object."
+        content = _normalize_message_content(message.get("content"))
+        if content is None:
+            return None, "messages[].content must be a string or an array of content parts."
+        normalized.append({**message, "content": content})
+    return normalized, None
+
+
 @manager.route("/openai/<chat_id>/chat/completions", methods=["POST"])  # noqa: F821
 @login_required
 @validate_request("model", "messages")
@@ -113,6 +150,9 @@ async def openai_chat_completions(chat_id):
     messages = req.get("messages", [])
     if len(messages) < 1:
         return get_error_data_result("You have to provide messages.")
+    messages, normalize_error = _normalize_openai_messages(messages)
+    if normalize_error:
+        return get_error_data_result(normalize_error)
     if messages[-1]["role"] != "user":
         return get_error_data_result("The last content of this conversation is not from user.")
 
diff --git a/test/testcases/test_http_api/test_session_management/test_session_sdk_routes_unit.py b/test/testcases/test_http_api/test_session_management/test_session_sdk_routes_unit.py
index cdb2736e12..2fa7f824c4 100644
--- a/test/testcases/test_http_api/test_session_management/test_session_sdk_routes_unit.py
+++ b/test/testcases/test_http_api/test_session_management/test_session_sdk_routes_unit.py
@@ -1045,7 +1045,112 @@ def test_openai_nonstream_branch_unit(monkeypatch):
 
     res = _run(inspect.unwrap(module.openai_chat_completions)("chat-1"))
     assert res["choices"][0]["message"]["content"] == "world"
-    
+
+
+@pytest.mark.p2
+def test_openai_defaults_to_nonstream_when_stream_omitted_unit(monkeypatch):
+    """Omitted stream must default to false (OpenAI API compat), not SSE."""
+    module = _load_openai_api_module(monkeypatch)
+
+    monkeypatch.setattr(module, "num_tokens_from_string", lambda text: len(text or ""))
+    monkeypatch.setattr(
+        module.DialogService,
+        "query",
+        lambda **_kwargs: [SimpleNamespace(kb_ids=[], llm_id="chat-model", tenant_id="tenant-1")],
+    )
+
+    stream_flags = []
+
+    async def fake_async_chat(_dia, _msg, stream, **_kwargs):
+        stream_flags.append(stream)
+        yield {"answer": "hello", "reference": {}}
+
+    monkeypatch.setattr(module, "async_chat", fake_async_chat)
+    monkeypatch.setattr(
+        module,
+        "get_request_json",
+        lambda: _AwaitableValue(
+            {
+                "model": "model",
+                "messages": [{"role": "user", "content": "hi"}],
+            }
+        ),
+    )
+
+    res = _run(inspect.unwrap(module.openai_chat_completions)("chat-1"))
+    assert stream_flags == [False]
+    assert isinstance(res, dict)
+    assert res["object"] == "chat.completion"
+    assert res["choices"][0]["message"]["content"] == "hello"
+
+
+@pytest.mark.p2
+def test_openai_array_text_content_normalized_unit(monkeypatch):
+    """OpenAI-style array content with text parts must not crash async_chat."""
+    module = _load_openai_api_module(monkeypatch)
+
+    monkeypatch.setattr(module, "num_tokens_from_string", lambda text: len(text or ""))
+    monkeypatch.setattr(
+        module.DialogService,
+        "query",
+        lambda **_kwargs: [SimpleNamespace(kb_ids=[], llm_id="chat-model", tenant_id="tenant-1")],
+    )
+
+    captured_msg = []
+
+    async def fake_async_chat(_dia, msg, _stream, **_kwargs):
+        captured_msg.append(msg)
+        yield {"answer": "ok", "reference": {}}
+
+    monkeypatch.setattr(module, "async_chat", fake_async_chat)
+    monkeypatch.setattr(
+        module,
+        "get_request_json",
+        lambda: _AwaitableValue(
+            {
+                "model": "model",
+                "stream": False,
+                "messages": [
+                    {
+                        "role": "user",
+                        "content": [
+                            {"type": "text", "text": "Hello"},
+                            {"type": "text", "text": "World"},
+                        ],
+                    }
+                ],
+            }
+        ),
+    )
+
+    res = _run(inspect.unwrap(module.openai_chat_completions)("chat-1"))
+    assert captured_msg[0][0]["content"] == "Hello\nWorld"
+    assert res["choices"][0]["message"]["content"] == "ok"
+
+
+@pytest.mark.p2
+def test_openai_invalid_message_content_type_unit(monkeypatch):
+    module = _load_openai_api_module(monkeypatch)
+
+    monkeypatch.setattr(
+        module.DialogService,
+        "query",
+        lambda **_kwargs: [SimpleNamespace(kb_ids=[], llm_id="chat-model", tenant_id="tenant-1")],
+    )
+    monkeypatch.setattr(
+        module,
+        "get_request_json",
+        lambda: _AwaitableValue(
+            {
+                "model": "model",
+                "messages": [{"role": "user", "content": 12345}],
+            }
+        ),
+    )
+
+    res = _run(inspect.unwrap(module.openai_chat_completions)("chat-1"))
+    assert "messages[].content must be a string or an array of content parts." in res["message"]
+
 
 @pytest.mark.p2
 def test_agents_openai_compatibility_unit(monkeypatch):