deepseek-harness/packages/core/session/README.md

description, kind

description	kind
The event-sourced session log and in-memory store for users and maintainers building, inspecting, or extending the durable record behind every agent interaction.	package-reference

@deepseek-ai/dsh-session

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Summary

dsh-session records every model-visible fact in an append-only session log and derives model history from that record. Consumers can inspect, replay, fork, and flush sessions while preserving historical events; compaction hides superseded entries from the active conversation without deleting them. Sessions remain in memory unless a persistence backend is added, and durability checkpoints wait for configured backends. Choose this package wherever an agent needs a reconstructable session record; it does not call models.

Table of Contents

• Use this package
• Understand the implementation
• Further Exploration
• Model Experience
• Known Limitations and Deferred Work
• Dev Note

---

Use this package

Mount dsh-session wherever a session must exist. It creates and holds event-sourced Session instances in memory; durable storage is layered on by a persistence plugin that subscribes to the session/event feed.

Create and inspect sessions

ctx.sessions.create() builds a live session bound to the calling fiber; get(id) and list() find sessions, and fork() creates a child session from a stable prefix of a live one.

const session = ctx.sessions.create(sessionId, { meta: { cwd: '/workspace' } })
ctx.sessions.get(sessionId)      // the live session
ctx.sessions.list()              // every live session, in creation order

Append and derive

session.append(type, data, opts?) commits one typed event — it snapshots and freezes the payload, validates it as lossless JSON, and notifies observers. session.deriveMessages() projects the log into the Message[] the model sees, incrementally and cached:

session.append('user/message', { role: 'user', content: [{ type: 'text', text: 'hello' }], source: { kind: 'user' } },
  { surfaceOp: 'append' })
session.deriveMessages()         // the derived model history

Surface events (system/message, developer/message, user/message, assistant/message, tool/result) require surfaceOp in both typed events and append input. A replacement uses exactly { op: 'replace', startSeq, endSeq }, with inclusive SessionSeq endpoints in current surface order. An Assistant message embeds its exact compact provider stream and forbids sourceEventSeqs. Known log-only events forbid both metadata fields and never produce a message.

developer/message stores tool-addition names and a headerSeq reference to an earlier request/header. The reference is required exactly when additions are present, and each name must identify exactly one complete schema in that header. Removals name the tool without a header reference. Historical headers remain available across restart, fork, and surface replacement; the current registry and latest header do not select an earlier addition's definition. sourceEventSeqs continues to describe derivation and replaced nodes.

Plugins declare content-changing events with @messageProjection and register a pure definition through ctx.sessions.registerMessageProjection(). Session calls the definition before accepting an event and caches its immutable message updates. Missing definitions reject append and restore; unloading a used definition also blocks cached reads. Detached constructors and foldSurface(events, projections) require explicit definitions. Reconstructors pass the fold's projectedMessages to deriveEventMessage(); live instance methods apply the same projections automatically. Plugin-owned message projections defines ownership and offline assembly.

Append, seed/restore, and event adoption/snapshot reject any header.system and exactly empty optional request-header fields (tools: [], adapterDefaults: {}) instead of normalizing input. Tool-result data.error is allowed only when the first-class message has isError === true; failure identity remains optional. Rejected appends do not change the log, derived state, or event feed. Adoption validates event-local metadata but not referenced history or replacement membership.

system/message holds the rendered system prompt: the first one is surface node 0, the prepared call capability governs admission, with a non-empty rendering consolidated at the first system node on an incapable route or appended after cached history inside a continuing in-history series; empty system nodes project to no message, so clearing the prompt requires logged empty replacements of all active system nodes, not just the latest; the surface fold rejects a replacement covering node 0 while it is a system/message unless the replacing event is itself a system/message over exactly that node, while later system nodes carry no protection and a compaction range may shadow them (decision).

Read the log

session.seq reads the current log length without materializing an array, and session.eventAt(seq) reads one accepted, deeply frozen event by sequence number. session.snapshotEvents(fromSeq?, toSeqExclusive?) materializes a frozen, stable snapshot of a half-open range; a complete current snapshot is cached until the next append. eventAt(), snapshotEvents(), and ownEvents() are deprecated: existing logic may remain unmigrated for now, but new production calls are prohibited. Repository test files may use these three readers under their scoped lint allowance (policy). Callers that only need a length use seq.

Session log positions use two numeric types. SessionSeq identifies an existing event or inclusive event watermark; SessionLogOffset identifies a gap, prefix length, or read boundary and may equal the event count. SessionSeqCursor adds the -1 “no event yet” value, while OptionalSessionSeq uses null when absence is data. The constructors validate non-negative safe integers, and the brands disappear at runtime, so durable JSON and wire values remain ordinary numbers.

Fork a session

ctx.sessions.fork(source, boundary?, childSessionId?) copies an exact inclusive event prefix (default: the last event) from a live source. buildForkSeed in dsh-session/fork places an inherited marker after the copied events, adds missing error tool results only for the open step, and closes that step and turn with a forked reason. Closed steps and turns remain unchanged, including historical missing results. The marker and closers belong to the child; inheritedEventCount counts only the copied prefix.

The logical SessionHeader.isSeeded field reports whether fork history exists without exposing a positional integer. Session.inheritedEventCount retains the exact checked SessionLogOffset; ownEvents() returns events at and after that cut, and isOwnSeq(seq) accepts only an existing child-owned position. A low-level seeded constructor must supply an explicit seed and inheritedEventCount because the constructor seed can contain child-owned setup events after the inherited prefix.

Flush durable state

ctx.sessions.flush(session) dispatches the awaited durability checkpoint: every persistence listener flushes and the call settles after all of them. A producer that needs an immediate durability barrier awaits it instead of assuming the write-behind drained.

---

Understand the implementation

Implementation internals — click to expand

This section explains how the package realizes the behavior above; the observable contract is covered in Use this package.

Design concept

The package is built on event sourcing: a Session is an append-only log of typed SessionEvents, and everything else — model history, transcripts, telemetry, titles, persistence — derives from that stream. The surface is a derived projection: an incremental manager validates append candidates, advances the ordered view from committed events, and tracks replaceGeneration for positional replacements and contentGeneration for replacements and plugin-owned message changes. Model-visible means logged: anything that reaches a model request must be reconstructable from the log. Each model attempt that reaches settlement commits one event: assistant/message carries the assembled model-visible message plus its compact timed stream, while assistant/attempt retains a failed, retried, cancelled, or stream-error attempt without adding model history. A hard process loss before settlement leaves no durable attempt stream.

Request headers

request/header stores a full canonical snapshot of the non-history request envelope with reason initial, resume, change, or series. An explicit message-series start, a surface replacement, or a plugin-owned message change writes a series snapshot when the envelope is unchanged; a simultaneous change uses startsSeries: true. Same-series steps, retries, and ordinary later turns inherit the latest snapshot. adapterDefaults distinguishes values resolved by the adapter from explicit settings, and foldRequestHeader() selects the latest snapshot. This self-contained record supports partial-window rendering and exact reconstruction at the cost of growth per message series; the reconstructable-requests Agent Note owns the detail.

Source map

File	Role
src/index.ts	Plugin entry: SessionStore service, store lifecycle, fork, flush
src/types.ts	SessionEventMap, SessionEvent, UserMessage, SessionHeader, TurnEndReasonMap
src/surface.ts	Ordered surface projection, replacement validation, deriveEventMessage
src/request-header.ts	request/header folding and reconstruction
dsh-util-values	Shared lossless JSON validation and detached snapshots
src/repair.ts	Cold repair of crash-orphaned logs
src/invariant.ts	Invariant companion: seq, turn/step enclosure, tool call/result pairing

Append validation

Every append uses the shared iterative snapshotJsonValue() pass, which reads, validates, and copies each nested value once, so a stateful getter cannot supply one value to validation and another to storage. Non-lossless-JSON payloads (BigInt, cycles, sparse arrays, -0, exotic prototypes) are rejected at the append site, before any backend flush. The append path constructs each SessionSeq; surface events additionally validate marker shape, cited source-event sequences, and complete shadowed-node coverage for replacements.

Derived history

deriveMessages() caches deep-frozen projections and returns a fresh array per call. The surface event types (system/message, developer/message, user/message, assistant/message, tool/result) supply their recorded message identities and content; empty-content system and developer nodes project to no message. Plugin-owned projections change derived content without mutating recorded messages. Replacements and projection decisions invalidate the cache. Embedded Assistant streams and assistant/attempt events remain replay and diagnostic data only.

The request header

The loop logs a full canonical request/header snapshot (call config, adapter defaults, assembled tool schemas — the rendered system prompt is a system/message surface node, not header state) at each loop-instance boundary and on change; foldRequestHeader(events) reconstructs it by selecting the latest snapshot, making every conversation request a pure function of the log. Route metadata (request/context) is separate logged state appended only when the provider, model, capacity, or systemPromptUpdate mode differs; it records the actual prepared call's mode after prompt and user admission, rather than supplying that admission decision.

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Further Exploration

The package-level contract is enough for most consumers; read these when you need the surrounding domain.

• Session subsystem — the full event vocabulary, surface types, and generated service API.
• Persistence subsystem — how backends make this log durable.
• Core subsystem — the loop that writes and derives from sessions.
• Generated persistence catalog — every session event with its payload and declaration site.
• Core group map — how the core packages compose.

---

Model Experience

Derived message history

What the model sees

The model receives the complete messages from system/message, developer/message, user/message, assistant/message, and tool/result surface entries with logged projections applied, the system prompt first. Identities, roles, sources, and unmodified blocks retain their original values; projections never mint identities. Direct prompts and injected context remain separate user/message events whose sources preserve their attribution. Embedded streams, assistant/attempt, boundaries, and other log-only facts add no message.

Token effect

Appended surface entries are resent on later steps. A replace surface operation removes the shadowed entries from future inputs without deleting their raw log records.

KV Cache effect

Appended surface entries preserve reusable prefixes. A replace operation invalidates reuse from the first shadowed message even though the underlying event log stays append-only.

Crash-repair and fork results

What the model sees

If recovery finds an assistant tool request with no durable tool/call, its synthetic TOOL_NOT_STARTED result says The tool call was interrupted before the Harness recorded it as started. Retry it if it is still needed. If a durable tool/call has no result, its TOOL_OUTCOME_UNKNOWN result says The tool call was interrupted after it was recorded, but no result was durably recorded. Its outcome is unknown. Decide whether to retry from the tool semantics: retry only if the operation is read-only or idempotent; if it may have side effects, first verify external state or ask the user. Do not retry blindly. Fork-generated results describe only the inherited records: the parent may have started or completed a call after the selected event. TOOL_NOT_STARTED means the prefix contains no start record; TOOL_OUTCOME_UNKNOWN means it contains a start but no result. Both tell the model to retry only read-only or idempotent operations without further checks; operations with side effects require external verification or user input. See the fork decision.

Token effect

Zero tokens in an intact session. Each repaired call adds its retained risk-specific error text on resume.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Logged request header

What the model sees

The session reconstructs the tool schemas and call config that the loop actually sent; the system prompt is part of deriveMessages() as surface node 0 and, after an in-history update, as the latest system node. Header events add no message to history and hold no copy of the prompt.

Token effect

Zero duplicate tokens from logging. The system nodes and schemas still incur their normal per-request cost.

KV Cache effect

Logging causes no invalidation, and exact reconstruction preserves request-prefix identity. A later header with changed config or schemas may invalidate reuse from its first difference; a prompt change that replaces surface node 0 invalidates reuse from the first token, while an in-history append keeps the prefix through the cached history reusable.

Known Limitations and Deferred Work

These limits define when the session store needs special care. They are current package constraints, not a task backlog.

• Live-source Store API — persisted-but-unloaded sources use the Host observation path. Fork closes only the open tail and does not repair missing results in previously closed steps.
• SESSION_FORMAT_VERSION names the current logical representation — the current reader rejects retired header.system and validates system/message payloads and protected-head rewrites. Historical headers and events belong to adjacent format packages; the adjacent migration chain converts supported history before constructing Session, and the write-open path publishes only the current-format successor. Equal-version unknown events require the envelope's explicit ignorable marker, which does not promise safe structural migration (mechanism).
• TurnEndReasonMap omits the ACP-named refusal / max_turn_requests variants — producer-gated: they land when an adapter or the loop first emits them.
• No session tree beyond fork — a pi-style entry tree over branched sessions is deferred unless a consumer needs more than boundary-based forking.

Dev Note

Working context for maintainers — click to expand

None.