omp uses StablePrefix to freeze system prompt + tool specs, AppendOnlyLog for append-only messages, and digest-based longestStablePrefix algorithm. When prune/shake/steering rewrite history, only the tail after the divergence point is resent. This maximizes Anthropic/DeepSeek prompt cache hit rate, fixing the old issue where every turn forced ~40k token re-prefill on llama.cpp (issue #3406).
omp doesn't have just one compaction: context-full uses LLM summarization with iterative windows and budget halving retries; snapcompact skips the LLM entirely, rendering history as dense PNG bitmaps for vision models to read — solving no-API-key, low-latency, vision-model-cheaper scenarios; branch summary summarizes the abandoned branch during `/tree` navigation so file ops aren't lost; shake mechanically replaces tool-result text and large fenced/XML blocks with placeholders — an emergency hatch when summary is too heavy and prune isn't enough. Four strategies, distinct failure modes, orchestrated by session maintenance or manual triggers.