Python: Bug: AnnotationContent and StreamingAnnotationContent to_element writes CitationType.X, so annotations in a rendered chat history fail to parse #14590

Describe the bug

AnnotationContent.to_element() and StreamingAnnotationContent.to_element() store the CitationType enum object in the XML attribute:

if self.citation_type:
    element.set("type", self.citation_type)

CitationType is a str, Enum. When the element is serialized, the attribute is written with the enum's str() form, CitationType.FILE_CITATION, not the value file_citation. from_element() then cannot read it back. Any chat message that has an annotation with a citation type breaks as soon as the history is rendered to a prompt string and parsed back (ChatHistory.from_rendered_prompt, which is what prompt templates use for {{$chat_history}} style messages).

The existing round trip tests pass because they never serialize the element to a string, so the enum object stays in memory.

To Reproduce

Run against current main (5b4a326), Python 3.13:

from semantic_kernel.contents import ChatHistory, ChatMessageContent, AuthorRole, TextContent
from semantic_kernel.contents.annotation_content import AnnotationContent, CitationType

msg = ChatMessageContent(
    role=AuthorRole.ASSISTANT,
    items=[
        TextContent(text="see [1]"),
        AnnotationContent(file_id="file-1", quote="q", start_index=4, end_index=7, citation_type=CitationType.FILE_CITATION),
    ],
)
history = ChatHistory(messages=[msg])
print(msg.to_prompt())
ChatHistory.from_rendered_prompt(str(history))

Output:

<message role="assistant"><text>see [1]</text><annotation type="CitationType.FILE_CITATION" file_id="file-1" quote="q" start_index="4" end_index="7"></annotation></message>
pydantic_core._pydantic_core.ValidationError: 1 validation error for AnnotationContent
type
  Input should be 'url_citation', 'file_path', 'file_citation' or 'container_file_citation' [type=enum, input_value='CitationType.FILE_CITATION', input_type=str]

StreamingAnnotationContent has the same line and fails the same way:

from xml.etree.ElementTree import tostring, fromstring
from semantic_kernel.contents.streaming_annotation_content import StreamingAnnotationContent
c = StreamingAnnotationContent(file_id="f", citation_type=CitationType.FILE_CITATION)
StreamingAnnotationContent.from_element(fromstring(tostring(c.to_element(), encoding="unicode")))

Expected behavior

The attribute should be written as the enum value (type="file_citation") so that from_element can parse it. The fix is to use self.citation_type.value in both to_element methods.

I have a fix with tests ready and can open a PR.

Platform

• Language: Python
• Source: main branch (5b4a326)
• OS: Windows, Python 3.13