Per a design revision, the per-scenario snapshot fixture becomes EXACTLY the
persisted session JSONL (<scenario>/session.jsonl) rather than a hand-authored
llm.json. The log already holds all LLM behavior (assistant/chunk carries every
StreamChunk) AND all harness behavior (tool/call, tool/result, turn/*, usage),
so one artifact drives replay and doubles as a behavioral golden.
llm-replay becomes replay-only (the record-tee is removed; recording is now
"run the real agent once and harvest the .jsonl", done by the harness in a
later commit). deriveReplayScript(events) groups assistant/chunk by (turn,step)
in log order — exact because the loop makes one ctx.llm.stream() call per step
and tags each chunk with the current (turn,step). The two failure modes the log
can't express (a thrown stream — no terminal finish; cancel/hang — timing) use
an optional replay.override.json sidecar.
Hardens against a Codex review finding: a derived group is only valid if it
ends in a `finish` chunk. A group without one is the fingerprint of a thrown
stream() and is NOT silently replayed as a clean stop — deriveReplayScript
throws, naming the (turn,step), so a missing sidecar override fails loud.
Updates the unit tests (parse/derive/load helpers, sidecar override, finish-
terminated grouping, HMR), the example README, and the RFC prose to the JSONL
format. Two goldens (stdout transcript + re-persisted JSONL) and the harness
wiring land in the next commit.
Introduces examples/acp-agent/src/llm-replay.ts, a function/namespace plugin
that installs a single llm/stream waterfall listener. In record mode it tees
the real model's StreamChunks into a per-scenario llm.json (flushed atomically
after EACH stream, since the snapshot subprocess is SIGKILLed and start.ts has
no disposal path). In replay mode it short-circuits the waterfall and serves
recorded streams back positionally — the Nth stream() call gets the Nth entry —
so a snapshot test can drive the real agent with no API key.
Each fixture entry is a discriminated record {chunks|throw|hang} so it can
replay BOTH branches of the LLM failure contract (throw from stream() vs a
finish-error chunk) plus cancellation. A throw entry carries the prefix chunks
emitted before the throw, replayed before the error, so a mid-stream failure
(partial output then STREAM_CLOSED) reproduces what the loop saw live.
Fail-loud on a missing or exhausted fixture (never a silent skip). Unit tests
drive the real LlmService waterfall (record tee, ordered replay, the three
entry kinds, partial-then-throw, fail-loud, event-driven abort, HMR-safety).
Broadens the unit vitest include to examples/*/tests and registers the plugin
+ snapshot tests as knip entries. Per docs/rfc/implemented/2026-06-19.
Tianyi Cui