The ACP snapshot suite kit: the shared machinery behind the keyless snapshot tier (pnpm run test:snapshot, testing policy). An example gets a full snapshot suite from a scenario table plus a fixtures directory; every compare/guard mechanic lives here, under the per-file coverage gate, instead of being copied per example.

Four layers, importable separately:

launchAcpTestAgent (launcher) — boots an unbuilt ACP agent from a temp cwd, pins tsx to the repo tsconfig, connects the SDK client over a raw-byte stdout tee, collects session updates and stderr, surfaces asynchronous spawn failures through its startup lifecycle, fails closed on unhandled permission requests, and owns graceful or signalled shutdown. Shutdown waits for process exit, inherited stdio closure, and ACP parser exhaustion before resolving or propagating a child error, so captures are complete and callers can remove owned paths after either outcome. Snapshot and ordinary e2e suites share this process boundary; a test supplies only agent paths, cwd, environment overrides, and any permission policy.
runScenario (harness) — boots the real agent bin as a subprocess via tsx (unbuilt, Loader path), drives it over ACP JSON-RPC stdio from a deterministic input.json script, tees raw stdout for the golden + purity check, and harvests every persisted session JSONL (parent + subagent children, primary-first) after a graceful stdin-EOF shutdown. Parameterized by AgentUnderTest (binScript, configPath, tsconfigPath — absolute paths; the subprocess cwd is a temp dir outside the repo).
Normalizers — pure functions turning the two captured surfaces into stable text: normalizeStdout (JSON-RPC ids → first-seen sequence; UUIDs/cwd → tokens; doubles as the stdout-purity check), normalizeSessionLog (times zeroed, seq kept), scrubSystemPrompts (prompt text → {{system}} in every JSONL), and scrubRequestHeaders (the remaining header bulk → {{tools}}/{{messagePrefix}} outside each pin, structure kept — pinned-header RFC).
defineAcpSnapshotSuite (factory) — registers the whole describe/it tree for a scenario table: per-scenario golden + re-persisted-log compares, record/refresh fixture write-back, the per-header-class pin (system-prompt.golden.md plus the JSONL's full tool schemas) with its live uniformity guard, and the fixture guard block (no orphan scenario dirs, required files present, exactly one pin per class, every JSONL prompt-scrubbed, non-pinning fixtures fully header-scrubbed). Each scenario directory's session.jsonl plus contiguous session.<n>.jsonl siblings are the ordered primary/child inventory; the scenario table does not duplicate their count. Must be called at vitest collection time.

A consuming *.snapshot.ts is the scenario table plus one factory call:

import { dirname, join } from 'node:path'
import { fileURLToPath } from 'node:url'
import { defineAcpSnapshotSuite, type Scenario } from '@deepseek-ai/dsh-acp-snapshot'

const SCENARIOS: Scenario[] = [
  { name: 'text-turn', hasModelTurn: true, recorded: true, pinsHeader: true },
]

defineAcpSnapshotSuite({
  agent: { // absolute paths, resolved from the suite's own location
    binScript: fileURLToPath(new URL('../../../packages/ui/acp-agent/src/bin.ts', import.meta.url)),
    configPath: fileURLToPath(new URL('../cordis.yml', import.meta.url)),
    tsconfigPath: fileURLToPath(new URL('../../../tsconfig.json', import.meta.url)),
  },
  snapshotsDir: join(dirname(fileURLToPath(import.meta.url)), 'snapshots'),
  scenarios: SCENARIOS, // exactly one entry per header class sets pinsHeader
  mode: process.env.DSH_SNAPSHOT === 'record'
    ? 'record'
    : process.env.DSH_SNAPSHOT === 'refresh'
      ? 'refresh'
      : 'replay',
})

A scenario booting a differently-composed tree sets its own configPath (an overlay whose basename still ends in cordis.yml, so the bin's replay swap finds the sibling *cordis.snapshot.yml) and, when that composition changes the request header, its own headerClass with its own pinning scenario — the acp-agent example's Code Mode scenarios are the template. Each pinning directory's generated system-prompt.golden.md is the reviewable snapshot of the normalized composed prompt; session.jsonl stores "system":"{{system}}" while retaining the complete tool list. A pin whose scenario legitimately changes its header mid-run declares expectedHeaderChanges; the Markdown snapshot then records each later full prompt under a request/header change marker.

The example also ships a cordis.snapshot.yml replay overlay next to its cordis.yml (the bin swaps them under DSH_SNAPSHOT=replay — single-source replay config RFC); replay fixtures are served by dsh-llm-replay, which this package points at via the DSH_SNAPSHOT_* env vars it sets on the child. pnpm run test:snapshot:record calls the live LLM and rewrites the recorded scenarios' model fixtures; pnpm run test:snapshot:refresh stays keyless, runs the replay overlay, and rewrites stdout, comparable session-log goldens, and each pin's Markdown prompt snapshot from the committed model scripts. Fixture roles, record/replay/refresh semantics, and scenario-table fields are documented on Scenario and in the snapshot RFC.

Constraints: suite.ts imports vitest, so the package entry is importable only inside a vitest run (the launcher, harness, and normalizers have no such dependency but ship from the same entry). ACP-specific by design — the launcher speaks the SDK's ClientSideConnection. Permission round-trips are scriptable: InputScript.permissionAnswers is a FIFO queue of option-kind selections (allow_once, reject_once, …) the client maps to the agent-issued optionId at answer time; an absent or exhausted queue answers cancelled, and a kind the request never offered rejects the run (the agent is answered cancelled, so a tolerant agent cannot absorb the scenario bug). Session config options are scriptable too: the setConfigOption step switches a knob over session/set_config_option, and setConfigOptionExpectError asserts the bridge rejects an unknown id or out-of-vocabulary value (the error frame stays in the transcript).