# Session Persistence

English | [中文](persistence.zh.md)

The **durability seam** for the event log. [session.md](session.md) describes the in-memory `Session` — the append-only `SessionEvent` log that is the source of truth. This page describes how that log is made durable: the abstract `SessionPersistence` service, its backends, the flush checkpoint, crash recovery, and the metadata header that travels alongside the log. The event vocabulary the log carries is enumerated, member by member, in the generated [persistence log event catalog](../persistence-catalog.md).

The seam is a [capability seam](../../.agents/notes/implemented/architecture/2026-06-13-capability-seams.md): one abstract service ([dsh-session-persistence](../../packages/session/session-persistence), `ctx.sessionPersistence`) defining locate/create/append, reusable Session preparation, logical load/inspect, physical suffix reads, and lightweight list/snapshot observation over the existing `SessionEvent` — **no parallel persisted event type** — and two interchangeable backends implementing the same contract. See the [session-persistence Agent Note](../../.agents/notes/implemented/architecture/2026-06-14-session-persistence.md).

## The flush checkpoint

`session/event` is a *synchronous* notification; persistence plugins copy the event into a per-session controller without blocking the producer. The first pending event starts a fixed batching window, and later events join without resetting its deadline. Expiry starts one durable batch; events admitted during that write receive their own deadline and form a follow-up batch. `session/flush` cancels the wait and drains through quiescence, so the loop still uses it as the ordering and error-observation checkpoint before claiming the next ordinary turn. A rejected background write retains its events and pauses automatic retry; a new event starts a fresh window, while explicit flush retries immediately and reports failure through `agent/error` and the logger, never as a session event past the closed turn. Disposal performs the same final drain. The configured maximum bounds only intentional batching wait, not event-loop scheduling or backend durability latency ([decision](../../.agents/notes/implemented/architecture/2026-08-08-bounded-session-persistence-write-batching.md)).

## Crash recovery preserves an interrupted turn

A backend that reloads a log crashed mid-turn finds an open `turn/start` with no `turn/end`. It does **not** truncate — a single turn can be huge in a long-horizon task (many steps, large tool output), and those events were durably appended before the crash. Instead it closes the orphaned turn with a synthetic `turn/end { reason: { kind: 'interrupted' } }`, keeping the interrupted execution balanced without changing any standalone events before or after it. `interrupted` is the one `TurnEndReason` no loop emits (see [session.md](session.md#why-a-turn-ended-turnendreasonmap)).

Repair applies only to cold sessions. For a live id, `SessionPersistence.load(id)` waits until the authoritative in-memory snapshot is durable and returns it only when balanced; an open live turn rejects rather than receiving synthetic interruption boundaries. HMR adopts a live prefix without closing its active turn.

`SessionPersistence.inspect(id)` constructs an immutable logical Session without publishing it or writing recovery. Cold inspection balances an interrupted turn in memory while leaving torn physical tails untouched; inspection of an already-live Session borrows its current immutable snapshot and may therefore contain an open turn. Coordinator-backed implementations retain the exact cold unpublished Session in a bounded LRU, so repeated history reads and a later `prepare(id)` share one read, decompression, validation, freeze, and Session construction. `prepare(id)` reserves the Session, commits pending repair, and returns a disposable publication handle; `load(id)` uses the same machinery to commit repair without publication. The [Session preparation decision](../../.agents/notes/implemented/architecture/2026-08-05-session-preparation.md) owns this lifecycle.

## `SessionLocation` — optional per-session artifact target

`SessionPersistence.locate(meta)` synchronously resolves a backend-owned independent artifact without reading, creating, or flushing it. JSONL returns the absolute transcript path inside its project/session directory; SQLite returns `undefined` because sessions share one database. A returned path can therefore name a file that does not yet exist or lacks the current unflushed turn; it is a location hint, not authorization or a freshness guarantee.

```ts type-equiv
/**
 * A backend-resolved, per-session local artifact location. The path is an
 * absolute target path and can name an artifact that has not materialized yet.
 * Consumers must treat it as a location hint, never as an authorization token.
 */
interface SessionLocation {
  /** Backend-specific artifact kind, for example `jsonl`. */
  readonly kind: string
  /** Absolute path to this session's backend-owned artifact. */
  readonly path: string
}
```

## `SessionHeader` — metadata beside the log

Per-session metadata travels **separately** from the event log: format version, cwd, lineage, and the seed boundary are storage concerns, not conversation events, so they stay out of `SessionEventMap` and never reach `deriveMessages()`. The header is attached to a `Session` via `session.header`.

Source: [`packages/core/session/src/types.ts`](../../packages/core/session/src/types.ts)

```ts type-equiv
/**
 * Immutable validated storage metadata, kept outside the conversation event log.
 */
interface SessionHeader {
  /**
   * On-disk format version, stamped from {@link SESSION_FORMAT_VERSION} when the
   * session is created. A persistence backend rejects any other version on load
   * (no migration — see the constant).
   */
  readonly version: number
  /** The session's id (mirrors the {@link Session}'s id). */
  readonly id: SessionId
  /** Non-negative safe-integer Unix epoch milliseconds when the session was created. */
  readonly createdAt: number
  /** Absolute working directory the session was created in (if any). */
  readonly cwd?: string
  /** The session this one was forked from (seed lineage), if any. */
  readonly parentSession?: SessionId
  /**
   * How many leading events were inherited through a seed. Persisting this
   * boundary lets resume and replay distinguish parent history from child work.
   */
  readonly seedLength?: number
  /**
   * Coarse product classification for a session created as a subagent child.
   * This is presentation metadata, not proof that the child is continuable.
   */
  readonly origin?: 'subagent'
  /**
   * Delegation depth: absent (zero) for a top-level session, parent depth + 1
   * for a subagent child. Persisted so a recursion budget survives restart and
   * resume — a runtime-only depth would reset a resumed child to top-level.
   */
  readonly delegationDepth?: number
  /**
   * Id of the agent preset this session's agent was composed from, when the
   * deployment composes per session. Durable because the preset decides the
   * session's tools and prompt: a resume that restored a different composition
   * would replay history the model can no longer act on.
   */
  readonly agentPreset?: string
}
```

## `CreateSessionOptions` — seeding and metadata

Creating a `Session` through the store takes a `seed` (initial replay or fork history) and `meta` (the storage-level fields the store folds into a `SessionHeader`). The store fills in `version`/`id` and defaults `createdAt`; the caller may supply the validated absolute `cwd`, the `parentSession` lineage, the `seedLength` seed boundary, the optional coarse `origin`, the `delegationDepth`, the `agentPreset` the agent was composed from, and an existing `createdAt`. `origin: 'subagent'` lets product navigation hide duplicate child rows; it does not prove that a descriptor is valid or that the child can resume.

```ts type-equiv
/**
 * Options for creating a {@link Session} via the store. `seed` replays/forks
 * an existing event log; `meta` carries the caller-supplied storage fields the
 * store folds into a {@link SessionHeader}.
 */
interface CreateSessionOptions {
  /** Initial replay or fork history supplied at construction. */
  readonly seed?: readonly SessionEvent[]
  /**
   * Storage metadata read once before publication. `seedLength` is explicit
   * because a resumed seed contains the full stored log, not only its inherited prefix.
   */
  readonly meta?: {
    readonly cwd?: string
    readonly parentSession?: SessionId
    readonly createdAt?: number
    readonly seedLength?: number
    readonly origin?: 'subagent'
    readonly delegationDepth?: number
    readonly agentPreset?: string
  }
}
```

Replay/fork is therefore `ctx.sessions.create(id, { seed: seedEvents })`; resuming a *persisted* session into a live agent is `ctx.agents.resume({ resumeSessionId })`.

## Preparation and restoration ownership

`SessionStore.prepare()` accepts ordinary creation options or fresh persistence graphs transferred through `RestoredSessionOptions`. The restoration branch validates and freezes the transferred header and events in place, so callers must retain no mutable aliases. `SessionPreparation` then owns the exact unpublished Session until publication or rollback; disposal is synchronous and idempotent. Persistence inspection exposes only `SessionInspection`, an immutable logical view borrowed from the same prepared Session.

```ts type-equiv
/**
 * Fresh storage values transferred to {@link SessionStore.prepare} without a
 * second serialization copy. Callers retain no mutable aliases.
 */
interface RestoredSessionOptions {
  /** Fresh detached storage events to validate and freeze in place. */
  readonly seed: SessionEvent[]
  /** Fresh detached storage metadata to validate and freeze in place. */
  readonly meta: SessionHeader
  /** Select the persistence ownership-transfer path. */
  readonly seedSource: 'persistence'
}
```

```ts type-equiv
/** Inputs accepted while constructing an unpublished Session. */
type PrepareSessionOptions =
  | (CreateSessionOptions & { readonly seedSource?: undefined })
  | RestoredSessionOptions
```

```ts type-equiv
/** Options for a preparation whose provider retains unpublished state. */
interface SessionPreparationOptions {
  /** Release provider-owned state when the Session was not published. */
  readonly release?: () => void
}
```

```ts public-api
/**
 * One exact unpublished Session and the provider state that keeps it usable.
 * Disposal is synchronous and idempotent. Providers decide whether release
 * returns the Session to a cache or discards it; publication may consume that
 * state before disposal, making the callback a no-op.
 */
declare class SessionPreparation implements Disposable {
  /** The exact Session to use for setup and publication. */
  readonly session: Session;
  /**
   * Wrap an unpublished Session in one preparation lifetime.
   * @param session - exact unpublished Session.
   * @param options - optional provider release behavior.
   * @returns a preparation disposed after publication or rollback.
   */
  static create(session: Session, options?: SessionPreparationOptions): SessionPreparation;
  /** Release provider state once when this preparation leaves its caller. */
  [Symbol.dispose](): void;
}
```

```ts type-equiv
/** Immutable logical session prepared from persistence or a live owner. */
interface SessionInspection {
  /** Validated immutable session metadata. */
  readonly meta: SessionHeader
  /** Validated contiguous logical event log. */
  readonly events: readonly SessionEvent[]
}
```

## Lightweight source revisions

Consumers of derived state compare a cheap opaque revision before loading a full event log. The persistence backend owns its representation and changes it transactionally with append or mutating load repair; callers compare it only for equality.

```ts type-equiv
/**
 * Backend-owned token that identifies both one storage source and one revision
 * of a persisted session log.
 */
type SessionPersistenceRevision = Branded<'SessionPersistenceRevision'>
```

```ts type-equiv
/** Lightweight immutable source identity returned without loading a full log. */
interface SessionPersistenceSnapshot {
  /** Detached metadata for one materialized session. */
  header: SessionHeader
  /** Opaque source-qualified token that changes whenever this stored log changes. */
  revision: SessionPersistenceRevision
}
```

## The backends

Both implement the same abstract `SessionPersistence` (locate/create/append/prepare/load/inspect/readFrom/list/listSnapshots over `SessionEvent`, with optional cancellation on observation methods) and pass the shared `runPersistenceContract` suite:

- **[dsh-session-persistence-jsonl](../../packages/session/session-persistence-jsonl)** — an append-only logical JSONL log per session, stored as checksummed concatenated Zstandard frames by default or raw lines by configuration, with crash-safe atomic writes, interrupted-turn recovery, and a read/replay path.
- **[dsh-session-persistence-sqlite](../../packages/session/session-persistence-sqlite)** — `node:sqlite`, one row per `SessionEvent`. The row fields `(session_id, seq, type, time, data, source_event_seqs, surface_op)` map 1:1 onto the event, including optional surface metadata, so there is no parallel persisted schema to keep in sync.

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## Cordis surface

Generated from source by `scripts/gen-cordis-catalog.ts` (verified fresh by `pnpm run verify-cordis-catalog` in doc-sync; regenerate with `pnpm run gen-cordis-catalog`) — this section is byte-identical in both language sides of the page. Signature blocks use a `ts cordis-catalog` fence and keep the original source JSDoc; dispatch modes are defined in the [primer](../cordis-primer.md#dispatch-modes), and the framework-inherited `ctx` surface lives in [cordis-api/inherited.md](../cordis-api/inherited.md).

<a id="ctxsessionpersistence--sessionpersistence-abstract-seam"></a>

### `ctx.sessionPersistence` — `SessionPersistence` (abstract seam)

Durable append-only session storage. Implementations preserve contiguous, losslessly JSON-serializable events; append resolves only after durability, and load balances a complete interrupted tail without rewriting committed events.

```ts cordis-catalog
/**
 * Resolve this backend's independent local artifact for a session without
 * reading, creating, flushing, or otherwise materializing it. Backends such
 * as SQLite that do not own one artifact per session return `undefined`.
 * @param meta - the immutable session header whose artifact is requested.
 * @returns the backend-specific absolute location, when one exists.
 */
abstract locate(meta: SessionHeader): SessionLocation | undefined

/**
 * Register a new session's metadata. A backend MAY defer the physical write
 * until the first {@link append} (lazy materialization), in which case a
 * created-but-never-appended session is absent from {@link list}
 * — abandoned sessions leave nothing behind.
 * @param meta - the immutable header (id, version, cwd, lineage) to record.
 */
abstract create(meta: SessionHeader): Promise<void>

/**
 * Durably persist a batch of events. Honors the append-only and contiguous-
 * seq contracts: the first event's `seq` MUST equal the stored next-seq
 * (after `load` has durably closed any interrupted turn). Rejects non-JSON-
 * serializable `event.data` with an error naming the offending event type.
 * @param id - the session the batch belongs to.
 * @param events - the contiguous batch to persist, in seq order.
 */
abstract append(id: SessionId, events: readonly SessionEvent[]): Promise<void>

/**
 * Prepare the exact unpublished Session used by resume. Implementations may
 * reuse object graphs retained by an earlier {@link inspect} after confirming
 * their durable revision is still current; disposal releases an unpublished
 * reservation. Revision retries require the durable log to remain unchanged
 * for one read/check round trip; continuous external writers may delay completion.
 * @param id - persisted session to prepare.
 * @param signal - optional cancellation for preparation work.
 * @returns one owned unpublished Session preparation.
 */
async prepare(id: SessionId, signal?: AbortSignal): Promise<SessionPreparation>

/**
 * Load an immutable balanced logical view and commit any required cold
 * recovery. A complete interrupted final turn is preserved and durably
 * closed with missing tool errors plus any open step and turn boundaries;
 * only a torn final record is discarded. Unknown versions and corruption in
 * the committed prefix reject. Implementations MUST NOT crash-repair an
 * identity still bound to a live Session: a balanced live log may return as a
 * durable snapshot, while an open live turn rejects. Returned values may be
 * shared with immutable live or prepared state and must not be mutated.
 * Revision-based implementations may wait for one stable read/check round trip.
 * @param id - the persisted session to reload.
 * @returns the header and a log ending on a balanced `turn/end`.
 */
abstract load(id: SessionId): Promise<SessionInspection>

/**
 * Inspect an immutable logical session without committing recovery or
 * publishing it. A cold complete interrupted turn receives synthetic closers
 * in memory and a torn physical tail remains untouched. An already-live
 * Session instead yields its current immutable snapshot, which may contain an
 * open turn and its `session/end-seed` boundary. Coordinator-backed
 * implementations retain the exact cold unpublished Session for bounded
 * reuse by a later {@link prepare}. A stale ready source is reloaded; a source
 * already committing or reserved for resume remains exclusive, and inspection
 * may borrow its immutable view. Callers borrow only the immutable header and
 * log. Continuous external writers may delay revision convergence.
 * @param id - the persisted session to inspect.
 * @param signal - optional cancellation for queued and backend read work.
 * @returns the validated header and current logical event log.
 */
abstract inspect(id: SessionId, signal?: AbortSignal): Promise<SessionInspection>

/**
 * Read the stored events from `fromSeq` onward — the read-from-seq
 * primitive for read models that resume from a watermark (e.g. a persisted
 * projection cache folding only the tail past its checkpoint). Unlike
 * {@link inspect}, it is a detached physical suffix read: no preparation
 * cache, torn-tail truncation, synthetic closers, or coordinator-state
 * publication. Only events from the valid contiguous stored prefix are
 * returned, so a torn fragment never reaches the caller. `fromSeq` at or
 * beyond the stored prefix returns an empty event list (never an error).
 * Backends whose medium can seek by seq
 * (SQLite) read only the suffix; sequential media (JSONL, both encodings)
 * still parse the whole artifact and skip forward — the primitive bounds
 * what is RETURNED and refolded, not every backend's physical read.
 * @param id - the persisted session to read.
 * @param fromSeq - first event seq to include; a non-negative safe integer.
 * @param signal - optional cancellation for queued and backend read work.
 * @returns the header and the stored events with `seq >= fromSeq`.
 */
abstract readFrom(id: SessionId, fromSeq: number, signal?: AbortSignal): Promise<{ meta: SessionHeader; events: SessionEvent[] }>

/**
 * Lightweight listing from metadata, without a full-log parse.
 * @param signal - optional cancellation for backend listing work.
 * @returns one header per materialized session.
 */
abstract list(signal?: AbortSignal): Promise<SessionHeader[]>

/**
 * List materialized sessions with cheap per-log change tokens.
 *
 * Repeated observations of an unchanged log return the same revision. A
 * successful mutating {@link load} repair changes the next listed revision.
 * Revisions also distinguish independently backed stores so backend-local
 * counters cannot compare equal across different persistence sources.
 * @param signal - optional cancellation for backend snapshot-listing work.
 * @returns one header and opaque revision per materialized session without loading full logs.
 */
abstract listSnapshots(signal?: AbortSignal): Promise<SessionPersistenceSnapshot[]>
```

Types: [SessionEvent](session.md) · [SessionId](core.md)

Source: [`packages/session/session-persistence/src/index.ts:72`](../../packages/session/session-persistence/src/index.ts)
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