.Net: Feature: ChatHistoryCompactionReducer — an alternative IChatHistoryReducer that preserves structured history

name: ChatHistoryCompactionReducer
about: An alternative IChatHistoryReducer that compacts history into a structured
message list rather than a single free-text summary.

Summary

This is a proposal to add ChatHistoryCompactionReducer, an experimental alternative
implementation of IChatHistoryReducer.

Unlike summarization-based reducers that replace older messages with a single free-text
summary, this reducer asks an LLM to compact the history via a required tool call
(submit_history) and returns a structured list of ChatMessageContent objects.
The LLM decides what to keep, shorten, or remove — but the result remains a typed
message list rather than a plain-text summary. This means function call / result pairs
can be retained in the compacted history when the LLM considers them relevant, avoiding
redundant agent invocations to recover information lost after context reduction.

The reducer is fail-fast: when the model returns structurally invalid output, it throws
rather than attempting silent correction. Known edge cases and their potential fixup
strategies are documented in the README.

The implementation is experimental and not extensively tested. I am sharing it with
the community to get feedback on whether this approach is worth pursuing further.

Implementation (part of a working project):
https://github.com/00wz/DesktopAssistant/tree/main/src/DesktopAssistant.Infrastructure/AI/Summarization

Intent

I am willing to submit a PR with unit tests following SK contribution guidelines.
Happy to discuss the design before proceeding.

Start by reviewing the proposed implementation in the linked DesktopAssistant.Infrastructure/AI/Summarization path and the existing IChatHistoryReducer contract in Semantic Kernel. Read the README for documented edge cases, then clarify the design before adding the reducer and unit tests; done means an agreed approach with repository changes and test coverage for structured compaction and fail-fast invalid output.