Best for
- Systems whose unit tests pass while the integrations burn
- Third-party dependencies that misbehave in ways you discover live
- Webhooks and queues that lose or double messages under stress
What you give it
- The integration points and their contracts (or the code, where contracts are implied)
- What must survive: the flows where a lost or doubled message costs money
What you get back
- Contract tests that catch breaking changes on both sides of every boundary
- Failure-mode coverage: timeouts, errors, malformed payloads, the dependency being down
- Idempotency and ordering proofs for the paths where twice or never means trouble
How it works
- Maps every boundary: external APIs, webhooks, queues, files, databases shared with others — and what each side believes the contract is.
- Writes contract tests from the consumer's real expectations, so breakage on either side fails a build instead of production.
- Simulates the dependency's bad days: slow, down, erroring, returning garbage — your system's behaviour under each is asserted, not hoped.
- Attacks the time-shaped bugs: duplicate delivery, reordering, retries, races — the ones unit tests structurally cannot see.
- Keeps the suite honest about what it mocks: recorded reality where possible, documented assumptions where not.
Example
You: Our payment webhooks sometimes double-fulfil orders and nobody can reproduce it.
Result: A test harness that replays the provider's real delivery behaviour: duplicates, out-of-order events, retries after timeout. The double-fulfilment reproduced in minutes (two deliveries racing past a check-then-act gap), the fix verified under the same harness, and a contract suite that now runs on every change — plus coverage for the three failure modes nobody had tested: provider down, malformed event, signature failure.
Limits — please read
- A sandbox is not production; it closes most of the gap with recorded traffic and says what gap remains.
- Third parties change without asking; contract tests detect it fast, they cannot prevent it.
- Truly concurrent races are made probable, not certain, in tests — it engineers the probability up and reports it.