Python: Bug: Python 1.34.0 ChatHistoryTruncationReducer deletes the System Prompt

[imperonas]

Describe the bug
When using the ChatHistoryTruncationReducer the reducer reduces the system prompt (both if added to the constructor via system_message or when manually added as first message to the history )

To Reproduce
Steps to reproduce the behavior:

1. Create a History using the ChatHistoryTruncationReducer, and add a target_count
2. populate the ChatHistory with more elements than the target_count
3. call the reduce function

Expected behavior
I expect the System Prompt to be kept, as it is an integral part of the conversation

Screenshots
see first comment.

Platform

• Language: Python
• Source: pip semantic-kernel 1.34.0
• AI model: OpenAI:GPT-4o-mini(2024-12-01-preview)
• IDE: VS Code
• OS: Windows

Additional context
Nothing, but additional context can be provided if needed.

[imperonas]

here is an example

import asyncio

from semantic_kernel.contents import AuthorRole, ChatMessageContent
from semantic_kernel.contents.history_reducer.chat_history_truncation_reducer import (
    ChatHistoryTruncationReducer,
)


async def test():

    history = ChatHistoryTruncationReducer(
        target_count=2, system_message="a system message"
    )

    history.add_message(
        ChatMessageContent(role=AuthorRole.USER, content="user prompt 1")
    )
    history.add_message(
        ChatMessageContent(role=AuthorRole.ASSISTANT, content="response 1")
    )
    history.add_message(
        ChatMessageContent(role=AuthorRole.USER, content="user prompt 2")
    )
    history.add_message(
        ChatMessageContent(role=AuthorRole.ASSISTANT, content="response 2")
    )
    history.add_message(
        ChatMessageContent(role=AuthorRole.USER, content="user prompt 3")
    )
    history.add_message(
        ChatMessageContent(role=AuthorRole.ASSISTANT, content="response 3")
    )
    history.add_message(
        ChatMessageContent(role=AuthorRole.USER, content="user prompt 4")
    )
    history.add_message(
        ChatMessageContent(role=AuthorRole.ASSISTANT, content="response 4")
    )

    print(str(history))

    print("\n reduce hist \n")
    await history.reduce()
    print(str(history))


asyncio.run(test())

and the output:

<chat_history><message role="system"><text>a system message</text></message><message role="user"><text>user prompt 1</text></message><message role="assistant"><text>response 1</text></message><message role="user"><text>user prompt 2</text></message><message role="assistant"><text>response 2</text></message><message role="user"><text>user prompt 3</text></message><message role="assistant"><text>response 3</text></message><message role="user"><text>user prompt 4</text></message><message role="assistant"><text>response 4</text></message></chat_history>

 reduce hist

<chat_history><message role="user"><text>user prompt 4</text></message><message role="assistant"><text>response 4</text></message></chat_history>