Both tools record real traffic and replay it as a mock. Hoverfly is the mature OSS proxy in this category, written in Go and at home in any pipeline that already speaks Go. Gostly is a Rust proxy with AI gap-fill layered on top of the same record-replay loop.
Hoverfly has been around long enough to be the reference implementation of "service-virtualization for HTTP" in the OSS world. SpectoLabs open-sourced it in 2016, it carries an Apache-2.0 license, and the middleware story — drop in a Python or JavaScript script to mutate captured responses — is genuinely powerful. If your shop is a Go shop, the operational fit is hard to beat: one binary, well-understood, no surprises. Hoverfly's commercial cloud arm was acquired by iOCO in 2023; the open-source Hoverfly project (Apache-2.0) remains actively maintained — latest v1.12.9, June 2026.
Gostly takes the same record-replay primitive and asks a different question: what happens when the request you need to serve in MOCK mode wasn't in the recording? Hoverfly's answer is "write middleware to synthesize one." Gostly's answer is "the self-hosted inference sidecar generates a response grounded by the shapes of the requests we did record." Same record-replay floor, different ceiling.
The other axis is the pipeline beyond the proxy. Hoverfly stops at "record, replay, modify in middleware." Gostly's paid tiers take the same traffic and feed it forward into a chaos-events log and, for teams, a self-hosted training-data corpus. If you only need the proxy, both are fine. If the recordings are also the input to something downstream, Gostly is built for that path.
| Feature | Hoverfly | Gostly |
|---|---|---|
| Free OSS proxy / server | Apache-2.0 | FSL-1.1 |
| Record-replay of real traffic | Yes | Yes |
| Multi-protocol (HTTP, gRPC bridges, MQ) | HTTP + WebSocket + a few via middleware | HTTP/HTTPS today, gRPC roadmap (Q4 2026) |
| Middleware / response shaping (custom code) | yes — JS, Python, any binary | yes — proxy plugins (Rust) |
| AI gap-fill for unrecorded routes | No | self-hosted, grounded by recordings (Pro+) |
| Stateful simulation modes | yes — multiple modes incl. modify, synthesize | shipping (statecharts) |
| Latency / failure injection | Yes | Yes |
| CLI for CI | Yes | Yes |
| Cloud team workspace | No | Team plan |
| Traffic → training-data pipeline | No | self-hosted LoRA (Pro+) |
| Self-hostable | single Go binary | self-hosted stack (Docker Compose) |
| Active maintenance | actively maintained (v1.12.9, Jun 2026) | Gostly team |
Roadmap items are explicitly labelled. Drift detection ships today — re-record and compare, emitting drift events and a freshness score; broader schema-fingerprint drift is on the roadmap.
Choose Hoverfly when
Choose Gostly when
| Tier | Hoverfly | Gostly |
|---|---|---|
| Free | Apache-2.0, unlimited, self-host only | Unlimited services, OSS proxy (FSL-1.1) |
| Pro | n/a — no managed offering | $10 / mo single user (unlimited services, AI on miss) |
| Team | n/a — coordinate sims yourself | $79 / seat / mo (min 3 seats) |
| Self-host / Enterprise | Free forever (Apache-2.0) | $499 / mo Self-host · $25K+ Enterprise |
Hoverfly is free at every tier — that is the deal Apache-2.0 buys you. The honest comparison is "free OSS that you operate" versus "open core proxy plus a managed cloud you pay for if you want the AI and the team workspace." Both are legitimate; pick the one that matches how your team wants to spend its time.
Drop in the proxy, record an hour of staging traffic, see whether replay-from-reality plus AI gap-fill changes the calculus for your team.
Evaluating for a team of 3+? We’d love to talk before you commit.