Hand-written fixtures drift. Shared staging environments flake. Tools like WireMock and Specmatic force you to describe the API before you can mock it. Here's what Gostly does differently — and why it matters for teams shipping quickly.
Every engineering team that depends on external APIs eventually reaches the same inflection point: the real API is too slow, too flaky, or too expensive to call in tests. So you reach for a mock. And then you spend the next year maintaining it.
The mock diverges from reality. A field gets renamed upstream. A new error code appears that nobody thought to stub. The test suite stays green while the product breaks. You know this story.
The tools that exist today ask you to describe the API before you can mock it. That description — the stub definition, the contract, the OpenAPI spec — becomes a second source of truth that you have to keep in sync with the real API forever.
WireMock is powerful and battle-tested. It's been in production at serious companies for over a decade, and it earns that reputation. But every stub is a piece of code or configuration that someone wrote by hand, describing what they thought the API returned at the time they wrote it.
When the upstream API changes — and they always change — your WireMock stubs don't. They silently continue to return the old shape. Tests pass. Production fails. And the gap between your stubs and reality widens with every sprint that nobody explicitly goes back to update them.
Specmatic takes a contract-first approach: you write an OpenAPI or AsyncAPI spec, and Specmatic generates mocks and validates that both sides honour the contract. It's a disciplined, principled approach that works well when you own both sides of the API boundary.
The problem is the "when." Most of the APIs teams want to mock are third-party services — Stripe, Twilio, Salesforce, an internal microservice owned by a different team. You don't write the spec. You don't control when it changes. You're stuck either maintaining a spec you didn't author, or discovering the contract changed when your test suite breaks in CI.
Express servers returning hardcoded JSON. A __mocks__ directory full of fixture files. A Python script that someone wrote eighteen months ago and nobody fully understands anymore. Every team does some version of this. It works until it doesn't, and then it's archaeology.
Gostly doesn't ask you to describe the API. It learns from real traffic — and that traffic can come from any environment.
You drop in a proxy between your application and an upstream service. The proxy records every request and response automatically — no SDK to integrate, no annotations to add, no schema to write. Your application keeps talking to the same address. Nothing changes from its perspective.
That upstream can be a local dev server, a staging environment, a shared integration environment, or a public third-party API. Production access is never required. Many teams record entirely on their laptops against a locally running service and never touch a shared environment at all.
When you're ready to mock, you flip a switch. From that point, all requests are served from the recorded library — instant, deterministic, and offline. The mock library is committed to your repository. CI mounts it and runs fully air-gapped, with no upstream calls on any run.
When the upstream API changes, you run in LEARN mode for one pass to record the new responses. No fixture files to update. No spec to regenerate. You record truth and serve it back.
Recorded mocks have one weakness: they only cover paths you've actually exercised. A test that hits /users/99won't find a match if you only recorded /users/42.
Gostly addresses this in two ways. Smart swap normalises path parameters and matches recordings by template — so a recording of /users/42 serves /users/99 with the ID swapped in the response body. For requests with no structural match at all, the AI inference layer generates a response that fits the shape of the API, using your recorded interactions as few-shot examples.
The important distinction: the AI doesn't invent behaviour from thin air. It extrapolates from real, observed behaviour for this specific API. The generated responses are grounded in your recordings — not in a model trained on some generic API dataset.
| Capability | Gostly | WireMock / Specmatic / DIY |
|---|---|---|
| Mock authoring | Zero — learned from real traffic | Manual — you write every stub |
| Production access required | Never — any env works (local, staging, CI) | N/A — you describe it yourself |
| Keeps up with API changes | Record again and reload | Manual update of stubs or spec |
| Coverage of unexercised paths | Smart swap + AI generation | Only what you explicitly stub |
| Response accuracy | Verbatim observed data from real traffic | What you remembered to encode |
| Setup time | ~5 min (docker compose up) | Hours to days of spec/stub writing |
| Third-party APIs | Works — you don't need to own the spec | Needs spec or manual stubs |
| Offline / air-gapped dev | Full stack runs on a laptop, no connectivity needed | Depends on the tool |
| Multi-step flows | Record once, replay sequence in tests | Custom state machine per flow |
| Data residency | Everything stays on your machine | Varies (often cloud-dependent) |
Gostly is the right tool when:
Gostly isn't the best fit for every situation:
The fastest way to understand the difference is to try it. The free tier supports unlimited upstream services with exact match — enough to replace a hand-written mock in most teams' setups. You can run the entire stack on your laptop in under five minutes, record some traffic against a local service, and see mock mode serving it back before your coffee's cold.