Comparison

Gostly vs Prism

Spec is your contract. Traffic is reality. When they drift, traffic wins.

Prism is Stoplight’s open-source mock server for OpenAPI. You point it at a spec, it serves stubbed responses that match the schema. For design-first work where the spec is genuinely the contract, it’s a clean choice.

The friction shows up later. Specs drift. Polymorphic schemas withoneOfandanyOfdon’t round-trip cleanly. Body examples are Lorem ipsum unless someone hand-authorsx-fakerextensions. Stateful flows aren’t the model.

Gostly starts from the other end. Point the proxy at your real upstream for an hour. It records the traffic. Then it replays the same bytes back, on demand, against your tests. Spec is the contract; traffic is what actually happened.

At a glance

Feature	Prism	Gostly
Source of truth	OpenAPI spec	Real recorded traffic
Drift detection	Manual — diff the spec by hand	Automatic — re-record and compare
Polymorphism (oneOf / anyOf / discriminator)	Open issues against the matcher	Returns the recorded shape
Body realism	Lorem ipsum unless you author x-faker	Real recorded bodies, PII-stripped
Stateful flows (POST then GET)	Stateless by design	Stateful via resource store
OSS / self-host	OSS (Apache 2.0)	OSS proxy (FSL)
Language / runtime	Node.js	Single Rust binary
Pricing	Free	Free · $10/mo Pro · $79/seat Team

Prism is OSS and free; Gostly’s OSS proxy is free too. Paid tiers cover gap-fill, drift detection, and multi-seat.

When Prism is the right choice

↳Greenfield API where the spec genuinely is the source of truth and there’s no upstream to record yet.
↳Design-first workflow where the spec is the contract you want the front end to develop against.
↳No PII concern — the spec carries no production payloads and your team isn’t trying to match real body shapes.
↳You don’t need stateful flows. POST and GET are independent for what you’re testing.

When Gostly is the right choice

↳Existing API where the spec has drifted from reality, and you need tests to assert on what production actually returns.
↳Polymorphic responses (oneOf,anyOf, discriminator) where the recorded shape is the only authoritative answer.
↳Stateful flows — POST creates a resource, the next GET needs to return it.
↳Tests need real response shapes — header sets, error envelopes, idempotency-key behavior — not Lorem ipsum.

Migration path

You don’t have to rip out Prism. Run Gostly in LEARN mode against your live upstream for an hour. The proxy captures the traffic.

Compare the captured traffic to what Prism is returning today. Most teams ship the diff as a spec PR, switch to Gostly for tests, and keep Prism for design-time contract validation.

The two tools answer different questions. Prism: “does the spec say this is allowed?” Gostly: “is this what the upstream actually returned last Tuesday?” You can keep both if you want both answers.

Record one hour. Replay forever.

Free to start. Self-hosted. Same proxy serves OpenAPI design teams and traffic-driven test teams without picking sides.

Try Gostly Stay on Prism Become a design partner

No shame in staying on Prism. It’s a good tool for the case it was built for.