fix(python): keep ChatMessageContent.to_prompt a str when encoding is set

python/semantic_kernel/contents/chat_message_content.py

@@ -294,11 +294,15 @@ def from_element(cls, element: Element) -> "ChatMessageContent":
     def to_prompt(self) -> str:
         """Convert the ChatMessageContent to a prompt.
 
+        The encoding attribute of the content is not used here,
+        this always returns a string, the encoding is only relevant
+        for the actual text content, not for the prompt representation.
+
         Returns:
             str - The prompt from the ChatMessageContent.
         """
         root = self.to_element()
-        return ElementTree.tostring(root, encoding=self.encoding or "unicode", short_empty_elements=False)
+        return ElementTree.tostring(root, encoding="unicode", short_empty_elements=False)
 
     def to_dict(self, role_key: str = "role", content_key: str = "content") -> dict[str, Any]:
         """Serialize the ChatMessageContent to a dictionary.

python/tests/unit/contents/test_chat_message_content.py

@@ -152,6 +152,24 @@ def test_cmc_to_prompt():
     assert prompt == '<message role="user"><text>Hello, world!</text></message>'
 
 
+@pytest.mark.parametrize(
+    "encoding",
+    [
+        pytest.param("utf-8", id="utf8"),
+        pytest.param("ascii", id="ascii"),
+        pytest.param("latin-1", id="latin1"),
+    ],
+)
+def test_cmc_to_prompt_with_encoding(encoding: str):
+    """The encoding of the content should not change the type or shape of the prompt."""
+    message = ChatMessageContent(role=AuthorRole.USER, content="Hello, wörld!", encoding=encoding)
+    prompt = message.to_prompt()
+    assert isinstance(prompt, str)
+    assert not prompt.startswith("<?xml")
+    assert "Hello, wörld!" in prompt
+    assert ChatMessageContent.from_element(XML(prompt)).content == "Hello, wörld!"
+
+
 def test_cmc_from_element():
     element = ChatMessageContent(role=AuthorRole.USER, content="Hello, world!").to_element()
     message = ChatMessageContent.from_element(element)