Abstract compaction service. Implementations own token estimation, retention, and summarization, but a successful run must replace the selected surface span with one summary node and prevent concurrent compaction of the same session. Load one implementation per context as `ctx.compact`.
Check token pressure and compact if the conversation is too large. Estimate the next request, including its session prefix, derived history, and system prompt. Above threshold, compact a head-anchored range ending at a balanced tool boundary and reconsolidate any prior automatic checkpoint. Return `null` when no compaction is needed or an open tail leaves no safe cutoff. A single oversized retained unit or prefix cannot be repaired here.
Forcibly compact a range of surface nodes into a single summary node. `start` and `end` name an inclusive span by surface position, not numeric seq order; replacements can make visible seqs non-monotonic. Both edges must be balanced so assistant tool calls remain paired with their results. A model- backed implementation forwards cancellation and rejects active, missing, reversed, or unbalanced ranges.
