fix: render every content part of ChatPromptBuilder message templates (#12907)

Co-authored-by: anakin87 <[email protected]>

haystack/components/builders/chat_prompt_builder.py

@@ -176,12 +176,13 @@ class ChatPromptBuilder:
                         # infer variables from template
                         if message.text is None:
                             raise ValueError(NO_TEXT_ERROR_MESSAGE.format(role=message.role.value, message=message))
-                        if message.text and "templatize_part" in message.text:
-                            raise ValueError(FILTER_NOT_ALLOWED_ERROR_MESSAGE)
-                        assigned_variables, template_variables = _extract_template_variables_and_assignments(
-                            env=self._env, template=message.text
-                        )
-                        extracted_variables += list(template_variables - assigned_variables)
+                        for text in message.texts:
+                            if "templatize_part" in text:
+                                raise ValueError(FILTER_NOT_ALLOWED_ERROR_MESSAGE)
+                            assigned_variables, template_variables = _extract_template_variables_and_assignments(
+                                env=self._env, template=text
+                            )
+                            extracted_variables += list(template_variables - assigned_variables)
             elif isinstance(template, str):
                 assigned_variables, template_variables = _extract_template_variables_and_assignments(
                     env=self._env, template=template
@@ -263,12 +264,15 @@ class ChatPromptBuilder:
                     self._validate_variables(set(template_variables_combined.keys()))
                     if message.text is None:
                         raise ValueError(NO_TEXT_ERROR_MESSAGE.format(role=message.role.value, message=message))
-                    if message.text and "templatize_part" in message.text:
-                        raise ValueError(FILTER_NOT_ALLOWED_ERROR_MESSAGE)
-                    compiled_template = self._env.from_string(message.text)
-                    rendered_text = compiled_template.render(template_variables_combined)
+                    rendered_content = list(message._content)
+                    for index, part in enumerate(rendered_content):
+                        if isinstance(part, TextContent):
+                            if "templatize_part" in part.text:
+                                raise ValueError(FILTER_NOT_ALLOWED_ERROR_MESSAGE)
+                            rendered_text = self._env.from_string(part.text).render(template_variables_combined)
+                            rendered_content[index] = TextContent(text=rendered_text)
                     # use dataclasses.replace to avoid in-place mutation of the original message
-                    rendered_message: ChatMessage = replace(message, _content=[TextContent(text=rendered_text)])
+                    rendered_message: ChatMessage = replace(message, _content=rendered_content)
                     processed_messages.append(rendered_message)
                 else:
                     processed_messages.append(message)

releasenotes/notes/chat-prompt-builder-renders-all-content-parts-9c3f2a71d84b5e06.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed ``ChatPromptBuilder`` dropping content parts when the template is a list of ``ChatMessage`` objects.
+    Only the first ``TextContent`` part was rendered, and every other part - additional texts, images, files -
+    was removed from the rendered prompt without a warning. Template variables used in those dropped parts
+    were not detected either, so they were not exposed as inputs. Now all ``TextContent`` parts are
+    rendered and the remaining content parts are passed through unchanged.

test/components/builders/test_chat_prompt_builder.py

@@ -15,7 +15,7 @@ from haystack import component
 from haystack.components.builders.chat_prompt_builder import ChatPromptBuilder
 from haystack.components.retrievers.in_memory import InMemoryBM25Retriever
 from haystack.core.pipeline.pipeline import Pipeline
-from haystack.dataclasses.chat_message import ChatMessage, FileContent, ImageContent, ReasoningContent
+from haystack.dataclasses.chat_message import ChatMessage, FileContent, ImageContent, ReasoningContent, TextContent
 from haystack.dataclasses.document import Document
 
 
@@ -119,6 +119,36 @@ class TestChatPromptBuilder:
         res = builder.run(variable="test")
         assert res == {"prompt": [ChatMessage.from_user("This is a test")]}
 
+    def test_run_with_multiple_text_parts(self):
+        template = [
+            ChatMessage.from_user(
+                content_parts=[TextContent(text="Hello, {{ name }}!"), TextContent(text="Goodbye, {{ other }}!")]
+            )
+        ]
+        builder = ChatPromptBuilder(template=template)
+
+        assert set(builder.variables) == {"name", "other"}
+        assert builder.run(name="John", other="Jane")["prompt"][0].texts == ["Hello, John!", "Goodbye, Jane!"]
+
+    @pytest.mark.parametrize(
+        "content_part",
+        [
+            ImageContent(base64_image="cHJldGVuZC1wbmctYnl0ZXM=", mime_type="image/png"),
+            FileContent(base64_data="dGVzdA==", mime_type="application/pdf", filename="document.pdf"),
+        ],
+        ids=["image", "file"],
+    )
+    def test_run_preserves_non_text_content_parts(self, content_part):
+        template = [ChatMessage.from_user(content_parts=["Describe {{ thing }}", content_part, "Also {{ other }}"])]
+        builder = ChatPromptBuilder(template=template)
+
+        assert builder.run(thing="this", other="that") == {
+            "prompt": [ChatMessage.from_user(content_parts=["Describe this", content_part, "Also that"])]
+        }
+        assert template == [
+            ChatMessage.from_user(content_parts=["Describe {{ thing }}", content_part, "Also {{ other }}"])
+        ]
+
     def test_run_template_variable(self):
         builder = ChatPromptBuilder(template=[ChatMessage.from_user("This is a {{ variable }}")])
         res = builder.run(template_variables={"variable": "test"})
@@ -720,14 +750,24 @@ class TestChatPromptBuilderDynamic:
         assert comp._variables is None
         assert comp._required_variables == "*"
 
-    def test_chat_message_list_with_templatize_part_init_raises_error(self):
-        template = [ChatMessage.from_user("This is a {{ variable | templatize_part }}")]
+    @pytest.mark.parametrize(
+        "content_parts",
+        [["This is a {{ variable | templatize_part }}"], ["First text", "This is a {{ variable | templatize_part }}"]],
+        ids=["first_text", "second_text"],
+    )
+    def test_chat_message_list_with_templatize_part_init_raises_error(self, content_parts):
+        template = [ChatMessage.from_user(content_parts=content_parts)]
         with pytest.raises(ValueError, match="templatize_part filter cannot be used"):
             ChatPromptBuilder(template=template)
 
-    def test_chat_message_list_with_templatize_part_run_raises_error(self):
+    @pytest.mark.parametrize(
+        "content_parts",
+        [["This is a {{ variable | templatize_part }}"], ["First text", "This is a {{ variable | templatize_part }}"]],
+        ids=["first_text", "second_text"],
+    )
+    def test_chat_message_list_with_templatize_part_run_raises_error(self, content_parts):
         builder = ChatPromptBuilder()
-        template = [ChatMessage.from_user("This is a {{ variable | templatize_part }}")]
+        template = [ChatMessage.from_user(content_parts=content_parts)]
         with pytest.raises(ValueError, match="templatize_part filter cannot be used"):
             builder.run(template=template, variable="test")