deepseek-harness/docs/rfc/implemented/testing/2026-06-22-fork-snapshot-scenarios.md

RFC: Record fork and mixed spawn+fork snapshot scenarios

Status: implemented

Problem

The seed-boundary RFC made fork-child replay route correctly: dsh-llm-replay derives a child's script from the events at or after its persisted seedLength boundary, so a fork child's inherited parent prefix is not replayed as the child's own model calls. But it shipped with no recorded fork scenario — the slice was exercised only by llm-replay's unit tests (a synthetic child fixture) and a persistence round-trip test. The full-transcript snapshot tier, the one net that boots the real acp-agent and replays an end-to-end nested transcript, had only spawn children (subagent-spawn, subagent-multi). A fork-routing regression that left the unit tests green would still have escaped the tier built to catch transcript regressions.

The snapshot infrastructure to express a fork scenario was already in place — both in-process backends are wired into cordis.yml / cordis.snapshot.yml as two model-facing tools (subagent → spawn, subagent_fork → fork), the harness harvests every child log, and replay forwards per-child fixtures keyed by seedLength. What was missing was a recorded scenario that drives a fork child through it.

Decision

Record two scenarios against the real API, both replayed keyless in the default gate:

• subagent-fork — the parent completes a turn that establishes a fact, then delegates one subtask via subagent_fork. The fork child inherits the conversation (its log carries a non-zero seedLength), so it can answer from the parent's context. This is the focused regression: the child fixture's seedLength is the boundary the replay slice depends on, recorded from a real fork rather than hand-synthesized.
• subagent-mixed — the parent completes a turn, then delegates once via subagent (a fresh spawn child, seedLength 0) and once via subagent_fork (a fork child, non-zero seedLength) in one transcript. This is the mixed spawn+fork scenario the seed-boundary and per-session-replay RFCs both named as a future addition: one transcript exercises both transports and both branches of the slice (seedLength 0 = no-op, seedLength > 0 = trim the inherited prefix), with the two children ordered spawn-then-fork by createdAt.

Why a completed turn-1 is required

The fork backend seeds the child with the parent's balanced completed-turn prefix. A parent that forks on its very first turn has no completed turn to inherit, so the seed is empty (≡ a fresh spawn, seedLength 0) — which would NOT exercise the slice. Both scenarios therefore use a two-prompt input: the first prompt completes a turn (establishing a codeword the child is later asked to recall), the second delegates the fork. The recalled codeword in the child's transcript is incidental to the model's behavior; the load-bearing artifact is the child fixture's recorded seedLength, which the replay slice consumes.

Consequences

• The fork-routing slice is now guarded at the full-transcript tier, not just by unit tests. Removing the slice(seedLength) (replaying the whole child log) turns both new scenarios red — the fork child receives the parent's recorded chunks instead of its own — proving the guard bites (verified red→green when the scenarios landed).
• subagent-mixed is the first snapshot scenario to drive two different subagent backends in one transcript, exercising the per-session replay keying across a spawn and a fork child simultaneously.
• Out-of-process (ACP) subagent replay remains a different shape (each child is its own process with its own replay) and is still tracked as TODO(acp-subagent-replay) — these scenarios are in-process only.
• Re-recording (pnpm run test:snapshot:record) regenerates all four fork/spawn fixtures from the live API; the two new scenarios self-skip without a key like every recorded scenario.