fix: fold session fork into session store
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Hypatia May
@@ -58,7 +58,7 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
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| [dsh-hooks-claude + dsh-hooks-codex — the Claude Code / Codex hook bridges](implemented/feature/2026-06-30-hook-bridges.md) | 2026-06-30 |
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| [dsh-hook-protocol — the shared Claude Code / Codex hook wire-protocol core](implemented/feature/2026-06-30-hook-protocol-lib.md) | 2026-06-30 |
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| [Interception seams — the typed-Decision surface a hook programs against](implemented/feature/2026-06-30-interception-seams.md) | 2026-06-30 |
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| [Session fork service](implemented/feature/2026-06-30-session-fork-service.md) | 2026-06-30 |
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| [SessionStore fork helpers](implemented/feature/2026-06-30-session-store-fork-helpers.md) | 2026-06-30 |
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| [Subagent lifecycle enrichment — lastAssistantMessage (observe-only)](implemented/feature/2026-06-30-subagent-observe-enrich.md) | 2026-06-30 |
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### Simplification
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@@ -1,53 +0,0 @@
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# RFC: Session fork service
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Status: implemented (proposed 2026-06-30, accepted 2026-06-30)
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## Context
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The event-sourced session log already has the primitive a fork needs: create a new session with a seed event prefix, then derive model history from that seeded log exactly as replay does. That primitive is intentionally low-level. It lives on `dsh-session` as `ctx.sessions.create(id, { seed })`, while durable metadata such as `parentSession` and `seedLength` is stored on the out-of-log `SessionHeader` introduced by [session persistence](../../implemented/architecture/2026-06-14-session-persistence.md). The same mechanics already support in-process subagent fork children and replay routing for forked child logs.
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What is missing is a reusable product service for ordinary session forking. Putting that directly on `dsh-session` would make a derived workflow part of the core log API, even though the core session package should stay focused on append-only storage, derived history, and lifecycle events. The harness architecture prefers optional capability plugins over widening the core spine; [event-sourced sessions](../../implemented/architecture/2026-06-11-event-sourced-sessions.md) provide the log semantics, and [capability seams](../../implemented/architecture/2026-06-13-capability-seams.md) provide the extension pattern.
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The main semantic hazard is the fork boundary. A session event log is only a valid seed when it is contiguous and balanced. Forking inside an active turn would copy an open `turn/start`, possibly an open `step/start`, and possibly dangling tool calls. That violates the turn-enclosure and provider-transcript invariants, and it creates a misleading child history that appears to have participated in an unfinished parent turn. The existing [subagent seam](../../implemented/feature/2026-06-21-subagent-capability-seam.md) deliberately solves a different problem: a tool-triggered subagent fork usually happens while the parent turn is open, so `dsh-subagent-fork` clips the seed to the parent's last completed-turn prefix. A general session fork should not silently clip; it should reject attempts made away from a boundary.
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## Decision
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The shipped design adds an optional product package, `@deepseek-ai/dsh-session-fork`, under `packages/session-fork/session-fork`. It registers `ctx.sessionFork` and depends only on `cordis` plus the `dsh-session` vocabulary/service. No new session event types, persistence methods, ACP methods, agent-loop hooks, or subagent behavior are added in the first cut.
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The service exposes two operations:
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```ts ignore-check
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type SessionForkSource = Session | SessionId
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interface SessionForkSeed {
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source: Session
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seed: SessionEvent[]
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meta: {
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parentSession: SessionId
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seedLength: number
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cwd?: string
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}
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}
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interface ForkSessionOptions {
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source: SessionForkSource
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sessionId?: SessionId
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}
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class SessionForkService extends Service {
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snapshot(source: SessionForkSource): SessionForkSeed
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fork(options: ForkSessionOptions): Session
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}
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```
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`snapshot()` is the reusable half. It resolves only live sessions from `ctx.sessions`; v1 does not load unloaded persisted sessions by id. It validates the source is at a turn boundary, deep-clones the source events, and returns the seed plus metadata a caller can pass to a later session or agent creation path. This shape keeps the fork computation reusable for future ACP or agent-facing consumers without coupling this service to `ctx.agents`.
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`fork()` is the convenience half. It calls `snapshot()`, then creates a live child session via `ctx.sessions.create(sessionId, { seed, meta })`. The child inherits the source session's `cwd`, stamps `parentSession` to the source id, and sets `seedLength` to the seeded prefix length. When `sessionId` is omitted, `SessionStore` generates one using its existing id policy.
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The boundary rule is structural: an empty source log is forkable, and any source whose last event is `turn/end` is forkable regardless of the turn-end reason (`completed`, `aborted`, `error`, `disposed`, `max-tokens`, `interrupted`, or a future merge-extensible reason). Any non-empty source whose last event is not `turn/end` is inside a turn or otherwise not at the boundary and is rejected with a typed `SessionForkError` code. The service also classifies non-live source ids (`SESSION_NOT_FOUND`), stale `Session` object references whose id is live on a different instance (`SESSION_NOT_LIVE`), and duplicate requested child ids (`SESSION_ALREADY_EXISTS`) instead of leaking raw store errors. This is intentionally stricter than `dsh-subagent-fork`, whose completed-prefix clipping remains unchanged because it serves tool-time delegation rather than user/session branching.
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## Consequences
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The feature is a small capability seam rather than a change to `dsh-session`: the core log keeps its low-level seed primitive, while `dsh-session-fork` owns policy, error taxonomy, and convenience creation. Persistence continues to work through existing `session/created` and `session/flush` behavior: a forked child starts life with seeded events, so existing backends persist that seed once and preserve `parentSession` / `seedLength` in the header.
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The v1 scope deliberately excludes ACP `session/fork`, unloaded persisted-session forking, model-facing tools, and subagent refactors. Those can consume `snapshot()` later. If a future ACP method is added, it should advertise the capability only after it has transcript/snapshot coverage; this RFC adds no editor-facing updates, so no ACP snapshot is required now. Fork-child replay remains covered by the existing [seed-boundary testing RFC](../../implemented/testing/2026-06-22-fork-child-replay-seed-boundary.md), while this service gets focused unit tests plus one persistence integration test.
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@@ -0,0 +1,59 @@
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# RFC: SessionStore fork helpers
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Status: implemented
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## Problem
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The event-sourced session log already has the primitive a fork needs: create a new session with a seed event prefix, then derive model history from that seeded log exactly as replay does. That primitive is intentionally low-level: `ctx.sessions.create(id, { seed, meta })` accepts any valid seed, but ordinary live-session branching needs policy around where the seed may be taken, which metadata is stamped on the child, and how errors are classified.
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The semantic hazard is the fork boundary. A session event log is only a valid user-visible fork seed when it is contiguous and balanced. Forking inside an active turn would copy an open `turn/start`, possibly an open `step/start`, and possibly dangling tool calls. That violates the turn-enclosure and provider-transcript invariants, and it creates a misleading child history that appears to have participated in an unfinished parent turn. The existing [subagent seam](../../implemented/feature/2026-06-21-subagent-capability-seam.md) deliberately solves a different problem: a tool-triggered subagent fork usually happens while the parent turn is open, so `dsh-subagent-fork` clips the seed to the parent's last completed-turn prefix. A general session fork should not silently clip; it should reject attempts made away from a boundary.
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## Decision
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`dsh-session` owns ordinary live-session fork helpers directly on `ctx.sessions`. There is no separate `dsh-session-fork` package or `ctx.sessionFork` service: the helpers have no independent backend, event vocabulary, lifecycle, or persistence behavior, and all durable work delegates to the existing session store and persistence backends.
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The store exposes two operations:
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```ts ignore-check
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type SessionForkSource = Session | SessionId
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interface SessionForkSeed {
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source: Session
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seed: SessionEvent[]
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meta: {
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parentSession: SessionId
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seedLength: number
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cwd?: string
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}
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}
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interface ForkSessionOptions {
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source: SessionForkSource
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sessionId?: SessionId
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}
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class SessionStore extends Service {
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snapshot(source: SessionForkSource): SessionForkSeed
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fork(options: ForkSessionOptions): Session
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}
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```
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`snapshot()` is the reusable half. It resolves only live sessions from `ctx.sessions`; v1 does not load unloaded persisted sessions by id. It validates the source is at a turn boundary, deep-clones the source events, and returns the seed plus metadata a caller can pass to a later session or agent creation path. This keeps the fork computation reusable for future ACP or agent-facing consumers without coupling `dsh-session` to `ctx.agents`.
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`fork()` is the convenience half. It calls `snapshot()`, then creates a live child session via `ctx.sessions.create(sessionId, { seed, meta })`. The child inherits the source session's `cwd`, stamps `parentSession` to the source id, and sets `seedLength` to the seeded prefix length. When `sessionId` is omitted, `SessionStore` generates one using its existing id policy.
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The boundary rule is structural: an empty source log is forkable, and any source whose last event is `turn/end` is forkable regardless of the turn-end reason (`completed`, `aborted`, `error`, `disposed`, `max-tokens`, `interrupted`, or a future merge-extensible reason). Any non-empty source whose last event is not `turn/end` is inside a turn or otherwise not at the boundary and is rejected with a typed `SessionForkError` code. The helpers also classify non-live source ids (`SESSION_NOT_FOUND`), stale `Session` object references whose id is live on a different instance (`SESSION_NOT_LIVE`), and duplicate requested child ids (`SESSION_ALREADY_EXISTS`) instead of leaking lower-level store errors.
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## Alternatives considered
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**Separate `ctx.sessionFork` service.** This was the first implementation, but review showed it overfit the capability-seam pattern. The code had no swappable backend, no extra event surface, no independent ownership lifecycle, and no durable behavior beyond `ctx.sessions.create({ seed, meta })`. Keeping a separate package would make callers discover and install a second service just to perform policy around a session-store primitive.
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**Only expose `fork()`.** A one-function API is simpler for immediate child-session creation, but it forces callers that need a seed for another creation path to create a detached child session just to get the seed. `snapshot()` keeps the seed/metadata computation reusable without importing `ctx.agents` into `dsh-session`; `fork()` remains the simple one-call convenience.
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**Silently clip open turns to the last completed boundary.** That is correct for `dsh-subagent-fork`, where delegation often starts while the parent turn is open and the child should inherit only the completed prefix. It is wrong for ordinary user/session branching because it hides that the requested fork point was not actually a valid boundary and silently drops the parent turn tail.
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## Consequences
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The public surface stays small and discoverable: live session branching is part of `ctx.sessions`, next to `create({ seed })`, rather than a standalone service. Persistence continues to work through existing `session/created` and `session/flush` behavior: a forked child starts life with seeded events, so existing backends persist that seed once and preserve `parentSession` / `seedLength` in the header.
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The v1 scope still excludes ACP `session/fork`, unloaded persisted-session forking, model-facing tools, and subagent refactors. Those can consume `snapshot()` later. If a future ACP method is added, it should advertise the capability only after it has transcript/snapshot coverage; this RFC adds no editor-facing updates, so no ACP snapshot is required now. Fork-child replay remains covered by the existing [seed-boundary testing RFC](../../implemented/testing/2026-06-22-fork-child-replay-seed-boundary.md), while these helpers get focused `dsh-session` unit tests plus JSONL persistence coverage.
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