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A world is one isolated universe of state: its own objects, its own webhook endpoints, its own clock. The rule that makes ProdBreak simple to reason about:
One running instance = one world.
The isolation boundary is the process, not a key lookup inside a shared server. Two worlds never share objects, webhooks, or time — because they’re two different running instances.

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The key authenticates, it doesn’t select

Each instance mints its own key at startup (high-entropy, printed to the boot log, pinnable via env for reproducible CI). Your test reads that key; it’s never user-chosen. That key authenticates you to this one world. It does not select among worlds — there’s only one. So two test suites can never collide on a shared key, and a wrong key is simply a 401. 
npx prod-break run deck
# ▸ url=http://localhost:8801   key=pbw_3f9a…   ← this instance's one world

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reset() between tests

You don’t get a fresh instance per test — that would be slow. Instead the world is reset between tests: state emptied, clock rewound to t0. It’s the beforeEach / DB-transaction-rollback pattern, and your test plugin does it for you. 
// the plugin calls this between tests — you rarely call it by hand
await sandbox.reset();

t0 is a fixed, reproducible constant

Every fresh world starts at the same canonical t0 (e.g. 2025-01-01T00:00:00Z), not system-now — so time-derived fields are byte-identical across runs. Override it per world to test date-sensitive logic like month-end billing. See The virtual clock.

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Parallelism: one instance per worker

Because one instance is one world, parallel test workers must not share an instance. Each worker gets its own instance (its own world + key); reset() keeps tests independent within a worker. The CLI provisions this for you: 
prod-break run deck --workers 4
# prints 4 url/key pairs; the plugin assigns worker i the i-th pair
See the Parallel workers guide for the env-var contract.
A future “many worlds out of one shared instance” model (session-ids) is on the roadmap, but today the mental model is dead simple: a fresh world is a fresh instance.