deepseek-harness/packages/compact/compact

@deepseek-ai/dsh-compact

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The compaction seam: an abstract CompactService (ctx.compact) defining WHAT compaction does — decide when history is too large and summarize an older range into a single surface node — without saying HOW.

This package is the interface tier of the compaction capability, split so each concern evolves (and swaps) independently:

Package	Role
@deepseek-ai/dsh-compact (this)	the interface: abstract service + compact/* events + CompactionResult + canonical checkpoint source + tool-pairing boundary helpers
@deepseek-ai/dsh-compact-basic	a backend: ctx.tokenMeter pressure + token-budget retention + llm.stream() summarization
@deepseek-ai/dsh-tool-compact (deferred)	the model-facing /compact tool over ctx.compact

Unlike the bash seam, this interface depends on @deepseek-ai/dsh-session and @deepseek-ai/dsh-llm — the contract's verbs are defined over a Session and its output is the ContentBlock vocabulary, so they cannot be expressed without naming those packages. That deviation from the "interface depends only on cordis" guidance is intentional and recorded in the compaction capability-seam Agent Note.

Service API (ctx.compact)

Both methods are abstract — the backend owns trigger policy, retention, event sequencing, and summarization. Reusable request measurement is a separate service, ctx.tokenMeter, rather than part of this interface.

Member	Semantics
compactIfNeeded(agent, trigger, signal)	Consider automatic compaction for trigger: 'pressure' | 'context-overflow'. A pressure trigger may apply the backend's threshold and retained-tail policy; a confirmed overflow may force a useful balanced reduction. Returns the CompactionResult, or null when no safe range exists. A backend's summarization request is a direct ctx.llm.stream() call (not a loop step), so per-call interception happens at llm/stream.
compactRegion(start, end, agent, signal?)	Forcibly summarize surface nodes [start, end] (inclusive seqs) from agent.session into a single replacement node whose source is COMPACT_CHECKPOINT_SOURCE. Throws if a compaction is already in progress, if start/end aren't surface nodes, or if start is positioned after end on the surface. The range is a SURFACE-POSITION span, not a numeric seq interval — after a prior replace lands a fresh high-seq summary node at the shadowed range's position, surface order no longer tracks seq order.

CompactionResult keeps the raw summary and bookkeeping-event seqs available to callers alongside the shadowed range and token accounting; its drift-checked shape lives in the compaction data-structure reference.

compactIfNeeded takes a required signal; compactRegion's is optional. A backend that summarizes via ctx.llm.stream() must forward it into the call's GenerateOptions.signal, so an abort or fiber dispose tears down the in-flight summarization instead of leaving an orphaned model call running past the cancellation. The turn that the compact/* events belong to is recoverable from the owned session's log (the currently-open turn), so the backend stamps it from the log rather than trusting a caller-supplied value.

Tool-pairing boundaries

The interface exports toolPairingBalancedBefore(session, seq) and toolPairingBalancedAfter(session, seq) for snapping and validating compaction edges. A safe edge has no unanswered assistant tool call crossing it. Each helper validates that the event sequence is in the current surface and answers from balances cached per cut in surface order.

The private per-session cache is keyed by session.surface.replaceGeneration and the processed surface-entry count. An unchanged generation extends the fold with unseen tail entries only; a log-only append with no new surface entry does no event reads, while a replacement generation rebuilds current membership and balances. Missing event seqs and a tool/result without a preceding open call reject as corrupt surface state.

Surface contract

SurfaceEventType is a closed union — only user/message, assistant/message, tool/result, and steering/message may carry surfaceOp. A compact/* event therefore cannot appear on the surface. A successful compaction instead:

1. appends compact/start (log-only) — acquires the lock,
2. summarizes the range,
3. appends compact/summary (log-only) — provenance: summary, range, shadowed seqs, token count, and provider/model call envelope,
4. appends a single user/message with source: COMPACT_CHECKPOINT_SOURCE and surfaceOp: { op: 'replace', start, end } carrying the summary — the only surface mutation in this operation,
5. appends compact/end (log-only) — releases the lock.

The surface mutation (step 4) sits inside the lock bracket: compact/end is the last event, so the lock is never released before the mutation lands. A crash between compact/start and compact/end therefore leaves a detectable orphaned lock (a compact/start with no matching compact/end) rather than a compact/end that falsely claims compaction finished while the surface was never shadowed.

deriveMessages() then renders the summary as a user-role message followed by the retained nodes. The shadowed events remain in the raw log, so replay is deterministic.

Blocking

Compaction is serialized via a log-recorded lock: compactRegion refuses to start if the last compact/start has no matching compact/end after it. The lock is the log (not an in-memory mutex), so it survives replay and a persistence backend can detect an orphaned compact/start on reload. The lock brackets the whole operation — summarization, the compact/summary provenance record, and the user/message surface replacement all happen before compact/end — so a session/event listener firing on compact/end never observes the lock free while the surface mutation is still pending. The basic backend revalidates the selected surface after summarization: a surface change rejects, while an unrelated log-only append does not invalidate the replacement. compact/end is appended even when summarization throws, so a failure can never wedge the lock.

Events

The compact/* events extend SessionEventMap (merge-extensible) via declaration merging — they are session events, not cordis Events, and all three are log-only (no surfaceOp). Per-event payloads and semantics are in the generated persistence log event catalog.

Implementing a backend

Subclass CompactService, implement compactIfNeeded and compactRegion, and load the subclass as a plugin — it registers as ctx.compact. Every successful backend uses COMPACT_CHECKPOINT_SOURCE on its replacement user message; isCompactCheckpointSource() recognizes the marker after persistence or cloning without depending on backend identity. A template- or model-backed implementation can live as a sibling package without changing callers or the shared token meter.

Model Experience

Conversation history, when a backend is invoked

What the model sees

A successful implementation replaces an older surface range with one user-role summary checkpoint — a user/message carrying surfaceOp: { op: 'replace', start, end }; the raw events stay logged but stop appearing in derived model messages. The seam itself performs no rewrite.

Token effect

Zero direct tokens from this interface. A backend trades many retained history tokens for one summary and leaves the recent tail unchanged.

KV Cache effect

A successful backend replacement invalidates reuse from the first shadowed history token; the seam itself does not alter a request.

Known Limitations and Deferred Work

• No model-facing consumer tier yet — @deepseek-ai/dsh-tool-compact (the /compact tool) is deferred; compaction is reachable only via direct ctx.compact calls or a backend's auto listener.
• Some single-unit overflow is out of contract — balanced summary compaction cannot split one indivisible unit. The optional pruning companion can still repair a closed tool pair when text-bearing tool-result bulk is removable; a large non-tool node or a tool unit whose non-prunable remainder is oversized cannot be compacted.
• An envelope that alone approaches the window is not surface-compaction work — compaction shrinks derived history, never the system prompt, tools, or session prefix.