fix(api): Preserving the content transform logic in fetch_prompt_messages (#33666)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

api/dify_graph/nodes/llm/llm_utils.py

@@ -256,9 +256,13 @@ def fetch_prompt_messages(
                 ):
                     continue
                 prompt_message_content.append(content_item)
-            if prompt_message_content:
+            if not prompt_message_content:
+                continue
+            if len(prompt_message_content) == 1 and prompt_message_content[0].type == PromptMessageContentType.TEXT:
+                prompt_message.content = prompt_message_content[0].data
+            else:
                 prompt_message.content = prompt_message_content
-                filtered_prompt_messages.append(prompt_message)
+            filtered_prompt_messages.append(prompt_message)
         elif not prompt_message.is_empty():
             filtered_prompt_messages.append(prompt_message)
 

api/tests/unit_tests/core/workflow/nodes/llm/test_llm_utils.py

@@ -0,0 +1,106 @@
+from unittest import mock
+
+import pytest
+
+from core.model_manager import ModelInstance
+from dify_graph.model_runtime.entities import ImagePromptMessageContent, PromptMessageRole, TextPromptMessageContent
+from dify_graph.model_runtime.entities.message_entities import SystemPromptMessage
+from dify_graph.nodes.llm import llm_utils
+from dify_graph.nodes.llm.entities import LLMNodeChatModelMessage
+from dify_graph.nodes.llm.exc import NoPromptFoundError
+from dify_graph.runtime import VariablePool
+
+
+def _fetch_prompt_messages_with_mocked_content(content):
+    variable_pool = VariablePool.empty()
+    model_instance = mock.MagicMock(spec=ModelInstance)
+    prompt_template = [
+        LLMNodeChatModelMessage(
+            text="You are a classifier.",
+            role=PromptMessageRole.SYSTEM,
+            edition_type="basic",
+        )
+    ]
+
+    with (
+        mock.patch(
+            "dify_graph.nodes.llm.llm_utils.fetch_model_schema",
+            return_value=mock.MagicMock(features=[]),
+        ),
+        mock.patch(
+            "dify_graph.nodes.llm.llm_utils.handle_list_messages",
+            return_value=[SystemPromptMessage(content=content)],
+        ),
+        mock.patch(
+            "dify_graph.nodes.llm.llm_utils.handle_memory_chat_mode",
+            return_value=[],
+        ),
+    ):
+        return llm_utils.fetch_prompt_messages(
+            sys_query=None,
+            sys_files=[],
+            context=None,
+            memory=None,
+            model_instance=model_instance,
+            prompt_template=prompt_template,
+            stop=["END"],
+            memory_config=None,
+            vision_enabled=False,
+            vision_detail=ImagePromptMessageContent.DETAIL.HIGH,
+            variable_pool=variable_pool,
+            jinja2_variables=[],
+            template_renderer=None,
+        )
+
+
+def test_fetch_prompt_messages_skips_messages_when_all_contents_are_filtered_out():
+    with pytest.raises(NoPromptFoundError):
+        _fetch_prompt_messages_with_mocked_content(
+            [
+                ImagePromptMessageContent(
+                    format="url",
+                    url="https://example.com/image.png",
+                    mime_type="image/png",
+                ),
+            ]
+        )
+
+
+def test_fetch_prompt_messages_flattens_single_text_content_after_filtering_unsupported_multimodal_items():
+    prompt_messages, stop = _fetch_prompt_messages_with_mocked_content(
+        [
+            TextPromptMessageContent(data="You are a classifier."),
+            ImagePromptMessageContent(
+                format="url",
+                url="https://example.com/image.png",
+                mime_type="image/png",
+            ),
+        ]
+    )
+
+    assert stop == ["END"]
+    assert prompt_messages == [SystemPromptMessage(content="You are a classifier.")]
+
+
+def test_fetch_prompt_messages_keeps_list_content_when_multiple_supported_items_remain():
+    prompt_messages, stop = _fetch_prompt_messages_with_mocked_content(
+        [
+            TextPromptMessageContent(data="You are"),
+            TextPromptMessageContent(data=" a classifier."),
+            ImagePromptMessageContent(
+                format="url",
+                url="https://example.com/image.png",
+                mime_type="image/png",
+            ),
+        ]
+    )
+
+    assert stop == ["END"]
+    assert prompt_messages == [
+        SystemPromptMessage(
+            content=[
+                TextPromptMessageContent(data="You are"),
+                TextPromptMessageContent(data=" a classifier."),
+            ]
+        )
+    ]