deepseek-harness/packages/support/llm-replay/README.md

@deepseek-ai/dsh-llm-replay

A replay LLM plugin for keyless snapshot tests. It installs a single llm/stream waterfall listener that short-circuits the waterfall (never calls next()) and yields model streams reconstructed from a recorded session JSONL fixture — so a test can boot the real agent against a fixed model transcript with no API key.

Its consumer is the ACP snapshot harness in examples/acp-agent, which loads this plugin (via cordis.snapshot.yml) in place of a real LLM adapter. The package exists so its derive/parse/replay logic falls under the per-file 100% coverage gate on packages/*/src (the same logic, while it lived under examples/, was outside the gate).

How the fixture works

The fixture IS the persisted session log (<scenario>/session.jsonl). Its assistant/chunk events carry every StreamChunk, so grouping them by (turn, step) reconstructs each stream() call's chunk sequence (one model call per loop step). Recording is therefore "run the real agent once and harvest the .jsonl", done by the snapshot harness — this plugin does not record.

Two failure modes are not reconstructable from assistant/chunk alone — a pure throw before any chunk (e.g. an HTTP 401, where the log holds only a turn/end {error} and no chunks) and a cancel/hang (timing, not chunk content). A scenario that needs those supplies an optional sidecar (<scenario>/replay.override.json: a ReplayEntry[]) that REPLACES the derived script.

Nested agents: per-session keying

A scenario where a parent agent delegates to in-process subagents records more than one log: the parent (session.jsonl) plus one per child (session.1.jsonl, …). Each agent runs as its own Session on the same context, so replay must serve each one its own script.

Replay keys every call by its calling session id (GenerateOptions.sessionId, stamped by the agent loop). Live session ids are freshly random each run and never equal the recorded ones, so a live session binds to a recorded script by first-call order: scripts are ordered by header createdAt (parent first — it streams before it can delegate), and the first live session to make any call claims the first script, the next new session the next, and so on. Each session then advances its own cursor. A call with no sessionId is one anonymous session bound to the primary script, so single-session scenarios behave exactly as before. More distinct live sessions than recorded scripts fails loud.

Config

Key	Type	Default	Notes
file	string	$DSH_SNAPSHOT_FILE	Path to the primary (parent) session.jsonl fixture. Required (config or env).
overrideFile	string	$DSH_SNAPSHOT_OVERRIDE	Optional path to a ReplayEntry[] sidecar that replaces the PRIMARY session's derived script.
childFiles	string[]	$DSH_SNAPSHOT_CHILD_FILES (path-delimited)	Recorded subagent child-session logs for a nested scenario; empty for a single-session scenario.

- id: llm-replay
  name: '@deepseek-ai/dsh-llm-replay'
  # file/overrideFile/childFiles default to $DSH_SNAPSHOT_FILE /
  # $DSH_SNAPSHOT_OVERRIDE / $DSH_SNAPSHOT_CHILD_FILES, set by the snapshot
  # harness per scenario.

Exports

• installLlmReplay(ctx, config) — install the llm/stream listener; returns the disposer (HMR safety). Use this in tests to drive replay without the Loader or env vars.
• loadSessionScripts(config) — resolve the ordered SessionScript[] (primary + children) for a scenario, ready to bind to live sessions in first-call order.
• loadReplayScript(config) — resolve the ReplayEntry[] for the PRIMARY session only (sidecar override if present, else derived from the JSONL; fail-loud if the fixture is missing).
• deriveReplayScript(events) / parseSessionLog(text) / parseSessionHeader(text) — the pure helpers that turn a recorded session log into a script and read its header id/createdAt. A derived group must end in a finish chunk; a group without one is the fingerprint of a thrown stream() and must instead be expressed via an override sidecar.
• Types ReplayEntry / SessionScript / ReplayConfig / Config.

Plugin export shape

Named name / inject / Config / apply, with no default export: the cordis Loader's unwrapExports does exports.default ?? exports, so a stray default would collapse the module to the bare function and drop the inject namespace (see docs/postmortem/0001).