ProdBreak delivers real HTTP webhooks that fall out of real state transitions. The fixture stands up a local sink, registers it on your world, and exposes the signing secret and an inbox to assert against.

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Receive and verify

const inbox = await sandbox.webhooks.sink();                 // local HTTP receiver
await sandbox.webhooks.register(inbox.url, { subscribe: ["task_run.completed"] });

// author the out-of-control payload value in the vocabulary you think in
await sandbox.exogenous.on("task_run.completed", "output.balance", 980.5);

const deck = sandbox.client();
const run = await deck.taskRuns.create({ taskId: "task_rent" });

await sandbox.clock.setDurations({ run_settle: "0s" });
await sandbox.clock.advance("0s");                           // drains the cascade, fires the webhook

const delivered = await inbox.waitFor("task_run.completed");
expect(delivered.verify(inbox.secret)).toBe(true);          // HMAC with the deterministic secret
expect(delivered.payload).toMatchObject({ output: { balance: 980.5 } });

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Signing secrets are deterministic

Each endpoint’s secret is derived from (world key, endpoint url), so re-registering the same URL after a reset() yields the same secret — signature-verification tests are reproducible across runs. The fixture hands you inbox.secret; no copying values from a dashboard.

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Webhook and GET always agree

The webhook payload and a later GET are two views of one frozen value (see set once, then frozen) — so this holds by construction: 
const got = await deck.taskRuns.retrieve(run.id, { include: ["output"] });
expect(got.output?.balance).toBeCloseTo(980.5); // same value the webhook carried

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Test your app’s handler, not just the inbox

To exercise your webhook handler instead of asserting on the sink, register your app’s endpoint URL instead of inbox.url: 
await sandbox.webhooks.register("http://localhost:3000/webhooks/deck", {
  subscribe: ["task_run.completed"],
});
Under the MVP wall-clock mode, delivery happens via the scheduler in real time — collapsing durations to 0 makes it fire inline so waitFor resolves immediately. See Deterministic CI.