About
Fly is a platform for running full stack apps and databases.
Categories: PaaS
Quick facts
Detection methodology for PaaS
Platform-as-a-Service offerings show up as response headers and deployment domains. Heroku's `Server: Cowboy` plus `*.herokuapp.com`; Render's `*.onrender.com`; Fly.io's `*.fly.dev`. We pair the host detection with framework signals so you can tell what's running on the platform — a Heroku dyno serving Rails looks very different from a Heroku dyno serving Node, even though both fit the same hosting fingerprint.
How we detect Fly.io
Each signal alone is rarely conclusive — Sourcemap Explorer cross-references all of them and weights by confidence. You can reproduce any of these checks yourself in Chrome DevTools.
Server-side fingerprint: the response header reveals the technology behind the page. Visible in DevTools → Network → response headers.
fly-request-id:
server: ^Fly/[\w]+\s\(.*\)$
via: ^.*\sfly\.io$
Cookies set by the platform. Visible in DevTools → Application → Cookies.
_fly
FAQ
How do I check if a website is using Fly.io?
Open the page in Chrome, click the Sourcemap Explorer toolbar icon, and read the Stack tab. The popup matches Fly.io's fingerprint signals (response headers, asset URL prefixes, runtime globals, sourcemap paths) and flags it whenever any combination is found. The same checks can be reproduced manually in DevTools — see the "How we detect" section above.
What Fly.io version can Sourcemap Explorer detect?
Fly.io ships as a hosted paas rather than a bundled npm package, so version-specific detection isn't always possible. Where the platform leaks a version in response headers (`X-Powered-By`, `Server`, generator meta tags) we surface it; otherwise we report presence only.
How much does Fly.io cost?
Fly.io's pricing model is documented as: freemium, payg. Detection is independent of the pricing tier — Sourcemap Explorer flags the technology by its fingerprints, regardless of which paid plan a site is on. See the official site for the current pricing page.
Where can I read more about Fly.io?
Official site: https://fly.io. For Sourcemap Explorer's detection guide, see the deep-dive link below or the related guides in the cross-link section.
Keep reading on Sourcemap Explorer
Alternative tools
Detected by Sourcemap Explorer
Open the popup on any page running Fly.ioand you'll see the exact version pulled from the bundled package.json when sourcemaps are exposed.