deepseek-harness/docs/core-data-structures/compaction.md

Compaction

The compaction seam — a capability seam split like bash: interface (dsh-compact, ctx.compact), implementation (a backend such as dsh-compact-basic), and consumer (a /compact tool, deferred). Compaction is one optional capability, not part of the agent-loop spine — so its vocabulary lives here, not in core.md. A tokenizer- or template-based backend is a sibling package implementing the same interface. Unlike bash, the interface necessarily depends on dsh-session and dsh-llm: its verbs act on an agent-owned Session, and its durable summary event uses the ContentBlock vocabulary (see the compaction capability-seam RFC).

Source: packages/compact/compact/src/types.ts

The compact/* session events

Compaction extends SessionEventMap with three event types via declaration merging. All three are log-only — they record the compaction lock and its provenance, and never join the surface. SurfaceEventType is deliberately NOT extended (only message-producing events reach the model), so the summary itself rides on a separate user/message with surfaceOp: { op: 'replace', start, end } — the only surface mutation. See the RFC for why reusing user/message is honest rather than a workaround.

Event	Payload	Role
compact/start	{ turn }	acquires the log-recorded lock
compact/summary	{ summary, shadowedRange, shadowedSeqs, shadowedTokenCount, model, maxTokens? }	provenance: the summary blocks, the shadowed surface-boundary pair (start/end seqs — a position span, not a numeric interval), the shadowed seqs in surface order, the estimated token count, and the summarize call's envelope (model, plus its generation cap when one applied) — logged so the one-shot request is reconstructable from log + code (the reconstructability RFC)
compact/end	{ turn, error? }	releases the lock (error set when summarization threw)

The lock brackets the whole operation: compact/start is appended first, then summarization, the compact/summary provenance record, and the user/message replacement all land, and only then compact/end. Releasing the lock last turns a crash mid-operation into a detectable orphaned lock (a compact/start with no matching compact/end) rather than a compact/end that falsely claims compaction finished.

These variants are merged inside a declare module '@deepseek-ai/dsh-session' block, so — unlike the top-level types on the other sub-pages — they are not pasted as a drift-checked ```ts type-equiv block (the verify-type-equiv extractor matches only top-level declarations by name). The payload table above is the catalog entry; follow the source link for the authoritative shapes.

CompactionResult

What a successful compaction returns to its caller: the shadowed range and seqs plus the estimated token count. The durable compact/summary event owns the raw summary and bookkeeping-event identity.

interface CompactionResult {
  /**
   * The surface-boundary pair that was shadowed: the seqs of the first
   * (`start`) and last (`end`) surface nodes of the replaced range. A
   * surface-POSITION span, not a numeric seq interval — after a prior replace
   * lands a fresh high-seq summary node at an older range's position, `start`
   * can be GREATER than `end`. {@link CompactionResult.shadowedSeqs} is the
   * authoritative set of shadowed nodes, in surface order.
   */
  shadowedRange: { start: number; end: number }
  /** The seqs of all shadowed surface nodes, in surface order. */
  shadowedSeqs: number[]
  /** Estimated token count of the shadowed content. */
  shadowedTokenCount: number
}

The service

CompactService exposes compactIfNeeded(...) for pressure-triggered compaction, returning null when no compaction is needed, and compactRegion(...) for an explicit inclusive surface range. The pre-step caller supplies the agent, full prompt, session prefix, and abort signal; implementations must forward that signal to summarization. Estimation, retention, event sequencing, and summarization remain backend policy.

Auto-compaction runs at serial agent/pre-step, before the step and request derivation, so it can replace surface nodes while keeping trace events outside the step. Region boundaries preserve tool-call/result pairing but do not preserve whole turns, allowing early closed steps of one oversized turn to compact. dsh-compact-basic owns the retention and failure details.