Function plan_compaction 

pub fn plan_compaction(
    ctx: &ContextState,
    keep_last: usize,
    target_tokens: Option<u64>,
) -> Option<CompactionRequest>

Decide what to fold. keep_last messages stay verbatim; a fold always ends before the kept window, never splits an assistant tool-call from its tool results, and always leaves a User message first in the kept window. Returns None when there is nothing worth folding (fewer than keep_last + 2 messages, or no user message to fold up to).