A client-callback throw only becomes a JSON-RPC error RESPONSE to the
agent's session/request_permission — runScenario itself kept going, so a
tolerant agent could treat the error as a denial and the scenario would
pass, or worse, record: the impossible click baked into fixture and
golden, green on every replay. The mismatch is now captured as a harness
error while the agent is answered plain cancelled (a well-defined path
it cannot reinterpret), and the step loop rejects the run on it as soon
as the in-flight step settles. The spec asserts the rejection instead of
the agent-side error echo.
InputScript gains an optional permissionAnswers queue, consumed FIFO by
the harness's requestPermission handler. Each entry selects by option
KIND (allow_once, reject_once, …): option ids are agent-issued randoms a
committed script cannot know, while kinds are the ACP-stable vocabulary,
so the client maps kind → the offered optionId at answer time. An absent
or exhausted queue answers cancelled — existing scenarios and goldens
are untouched — and a scripted kind the request never offered throws,
surfacing as a JSON-RPC error on the permission request: the scenario
scripted an impossible click.
This is what lets an approval-flow suite (the sandbox composition) drive
allow/reject round-trips deterministically from input.json, per the
shared-acp-snapshot RFC.
A scripted fake ACP agent bin (tests/fixtures/fake-acp-agent.ts) speaks
real newline JSON-RPC through the REAL runScenario spawn path (tsx
loader, temp cwd, env plumbing); every behavior — prompt outcome,
session/new rejection, persisted logs, filesystem noise — comes from a
behavior.json beside the fixture, so specs script whole subprocess runs
from data. harness.spec.ts drives every step op, both expect-error arms,
the permission-stub default, env forwarding, workspace seeding, and the
harvest ordering/noise/fallback branches. suite.spec.ts runs the factory
for real at collection time: a replay suite over committed synthetic
fixtures and a record suite over a temp copy (write-back never touches
the committed tree; ACP_SNAPSHOT_SPEC_BOOTSTRAP=1 re-bootstraps it),
plus direct cases for the exported pure helpers. The suite factory's
pure helpers (childFixturePaths, fixtureContext, normalizedHeaders,
headerDeltaCount) are exported for those direct specs.
Two branches carry justified v8 ignores, both structurally unreachable:
the waiter in-bounds guard (noUncheckedIndexedAccess) and waitForExit's
already-exited race guard (both call sites sit one synchronous frame
after stdin.end()/kill()). The fake bin substitutes the session/new cwd,
not process.cwd(), into scripted logs — the realpath difference
(/private on darwin) is exactly what the real bin's header carries.
packages/support/acp-snapshot/src is at 100% statements, branches,
functions, and lines under the per-file gate.
The snapshot tier's machinery leaves examples/acp-agent/tests for
packages/support/acp-snapshot (@deepseek-ai/dsh-acp-snapshot), where the
coverage gate measures it and a second example can consume it instead of
forking it: harness.ts (runScenario, parameterized by an AgentUnderTest
{binScript, configPath, tsconfigPath} instead of module constants),
normalize.ts (moved verbatim), and suite.ts (defineAcpSnapshotSuite — the
per-scenario golden/log compares, record write-back, per-suite header pin
with its uniformity guard, and the fixture guard block, lifted from
acp.snapshot.ts). The example file collapses to its scenario table plus
one factory call; env reading (DSH_SNAPSHOT) stays at that edge.
The exactly-one-pin meta-test generalizes from the hardcoded text-turn
name to "exactly one per suite" — which scenario pins is the scenario
table's reviewable choice (per-suite pinning per the proposal RFC).
Extraction parity: pnpm run test:snapshot is 36 passed + fs-policy-reject
failing BEFORE AND AFTER (BSD-sed environment failure, reproduced at the
base commit in a clean worktree — the recorded golden's sed -i syntax is
GNU-only), with zero byte changes under examples/acp-agent/tests/snapshots/.
Coverage for the new src files lands in the next commit.
