If you've ever shipped a landing page from a dist/ folder, you already have the hard parts of an A/B testing platform. You have build artifacts. You have branches. You have a CI pipeline that knows how to push them somewhere. The thing you're missing — the thing Unbounce and Instapage charge $200–$600/month for — is a router that picks one of those builds per visitor and remembers the choice. That's it. That's the whole feature.

This post is about wiring that router in front of your existing build pipeline. Each variant stays a normal git branch with a normal build artifact. BFFless splits traffic across them at the edge and sets a cookie so visitors stay sticky. No SDK, no template lock-in, no per-page pricing.

Two things this screenshot makes obvious that a client-side A/B testing tool can't:

• It's the whole site, not a page. Because each variant is a complete build artifact from its own git branch, the title, lede, section headings, meta description, OG card, and even the blog-index preview you see above all change together. You're not testing a button color — you're testing a whole positioning, end to end, across every URL the site serves.
• It's decided at build time and at the edge. There's no flash of the wrong content, no client-side flicker, no JavaScript swap-in mid-paint. By the time the first byte of HTML reaches the browser, the variant has already been chosen — what loads is what was in dist/, fully cached, fully fast, indistinguishable from a regular static page.

The "just give me a router" problem​

Drag-and-drop builders are great at what they do. But the moment your team has a real frontend stack — a design system, a component library, a React or Astro setup that already builds into dist/ — the same friction points show up every time you try to plug a builder into your workflow:

• A/B testing is a premium feature. Unbounce's Smart Traffic, Instapage's experimentation suite, and Leadpages' split testing all sit on plans that run $200–$600/month per seat before you're testing at any real volume.
• Templates cap what you can ship. Custom components, design systems, and anything your frontend team has already built in React or Astro tend not to survive the round-trip through a WYSIWYG editor.
• Client approval is its own product. Sending a stakeholder a preview of variant B without showing them the live split usually means upgrading again to the tier with share links.
• Page speed is the hidden tax. Google Ads Quality Score punishes slow LCPs. Every builder layers its own runtime on top of your page; static HTML doesn't.

None of these are fatal on their own. Stack them together and you end up paying a premium tool to do a job that mostly consists of serving HTML.

Why a build artifact is the right primitive​

Landing pages are the ideal case for "just ship the bundle". They're small, they're read-heavy, and the two metrics that matter most — Quality Score for paid traffic and conversion rate for everything else — both reward raw page speed. A pre-built bundle gives you the fastest TTFB and LCP you can physically achieve, and it scales to any traffic spike without an autoscaler.

The other reason it's the right primitive is that you already have one. Whatever your team builds in — React, Astro, Eleventy, Hugo, a folder full of HTML — produces a directory that can be served as-is. Each variant is just another version of that directory.

The missing piece is usually traffic splitting. That's what BFFless adds on top.

How A/B testing works on BFFless​

The primitive is simple: every variant is a deployment alias, and traffic splitting routes visitors across aliases by weight.

When a visitor lands on your domain, BFFless does roughly this:

1. Check for an existing __bffless_variant cookie. If it's set, route to that alias.
2. Otherwise, evaluate traffic rules (query parameters, cookies). If one matches, force that alias.
3. Otherwise, pick an alias using weighted random selection — for weights [50, 30, 20], random 0–50 routes to the first alias, 51–80 to the second, 81–100 to the third.
4. Resolve the alias to its current deployment SHA (and optional path override), fetch the file from storage, and serve it back.
5. Set the __bffless_variant cookie so the visitor sticks to that variant on return visits.

The variant cookie is intentionally readable from JavaScript (not HttpOnly), so your analytics layer can pick it up and attribute conversions without any server-side glue. The selected alias is also returned in the X-Variant response header if you prefer reading it from the network layer.

Visually, a single request from a first-time visitor looks like this:

Step by step:

1. Request hits BFFless. The visitor's browser sends a normal GET / to your landing-page domain. If they've been here before, the request also carries a __bffless_variant cookie naming the alias they were assigned last time.
2. Domain config lookup. BFFless resolves the Host header to a domain mapping and pulls the traffic config for it: the list of aliases, their weights, sticky-session settings, and any traffic rules. If the domain has only one alias, this whole flow short-circuits and serves it directly.
3. Cookie branch — sticky session. If sticky sessions are on and the cookie's alias is still valid (still in the weights list, or still referenced by an active rule), BFFless reuses that alias and skips selection entirely. This is why a returning visitor never flips between variants mid-test.
4. Rule branch — forced routing. If there's no usable cookie, BFFless evaluates traffic rules in order: query parameters first, then cookies, then headers. The first match wins and forces that alias. This is the path that powers ?version=headline-urgency preview links and segment overrides like Cookie: plan=enterprise.
5. Random branch — weighted pick. With no cookie and no matching rule, BFFless rolls a weighted random number across the configured splits. For weights [34, 33, 33] the math is exactly what you'd expect: 34% of new visitors land on the first alias, 33% on each of the other two.
6. Alias resolution. Aliases are mutable pointers — landing-headline-urgency resolves to whichever immutable deployment SHA is currently promoted to it, plus an optional path override if the variant lives in a subfolder of a single shared deployment instead of its own branch build.
7. Config returns SHA + path. The domain config hands back the resolved deployment coordinates: the commit SHA (e.g. abc123) and the path prefix (e.g. /dist). Together these form the storage key prefix for every file in that variant.
8. Storage fetch. BFFless requests the file from whichever storage backend you've configured — S3, GCS, Azure Blob, MinIO, or the local filesystem — using the key owner/repo/abc123/dist/index.html. The storage adapter is pluggable; the routing logic above doesn't care which one is wired up.
9. Bytes back. The storage backend returns the raw file contents. BFFless streams them through without rewriting HTML or injecting any runtime — what was in your dist/ folder is what the visitor gets.
10. Response with cookie + header. BFFless writes the body back to the visitor along with Set-Cookie: __bffless_variant=headline-urgency (so the next request is sticky) and X-Variant: headline-urgency (so your edge logs, analytics, and curl debugging can see which variant was served without parsing cookies).

That's the whole mechanism. Nothing you put in front of it — routing, caching, rules, previews — requires a new SDK or a new plan tier.

Shipping a headline test, end to end​

Here's a concrete walkthrough. Say you're testing three headline variants on landing.clientname.com.

1. Deploy each variant as its own alias​

Branch per variant, one GitHub Actions workflow, one alias per push. The action uploads whatever's in dist/ to BFFless under an alias named after the branch:

on:
  push:
    branches: [main, headline-outcome, headline-urgency]
jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: pnpm install && pnpm build
      - uses: bffless/upload-artifact@v1
        with:
          path: dist
          api-url: ${{ vars.BFFLESS_URL }}
          api-key: ${{ secrets.BFFLESS_API_KEY }}
          alias: landing-${{ github.ref_name }}

After three pushes you have landing-main, landing-headline-outcome, and landing-headline-urgency — three live static sites, each with its own preview URL. Git is your variant manager.

2. Configure the split​

In Admin → Settings → Domain Mapping → Traffic, attach those three aliases to landing.clientname.com:

landing-main:             34%
landing-headline-outcome: 33%
landing-headline-urgency: 33%

Turn on sticky sessions so return visitors stay on the same variant, and add a ?version= query-param rule for each alias:

Now anyone can preview a specific variant by appending ?version=headline-outcome to the URL — which is exactly the "share a preview with the client" workflow that enterprise builders charge extra for. Send the three links to your PM, let them pick, done.

/plugin marketplace add bffless/claude-skills
/plugin install bffless

3. Read the cookie and attribute conversions​

Your frontend doesn't need to know which variant it is at build time. At runtime, read the cookie:

function getVariant() {
  const match = document.cookie.match(/(?:^|; )__bffless_variant=([^;]*)/);
  return match ? decodeURIComponent(match[1]) : null;
}

Then attach that value to every conversion event your analytics tool sends. GA4 example:

document.querySelector('#cta').addEventListener('click', () => {
  gtag('event', 'cta_click', {
    variant: getVariant(),
  });
});

Same one-liner works for Mixpanel, Amplitude, Pendo, PostHog, or whatever internal pipeline you pipe events to. Segment by variant in your dashboard and you have per-variant conversion rates without paying for a second analytics product.

4. Pick a winner and roll out​

When the test is conclusive, flip the winner to 100% in the traffic splitting panel. Instant rollback, no redeploy. Keep the losing aliases around as a history of what didn't work, or delete them.

What you get that the builders don't​

This isn't a full replacement for Unbounce or Instapage — BFFless is not a drag-and-drop page builder, and if your team doesn't have a dev who can ship HTML, you probably want one of those tools. But if you do have a dev, you get a handful of things that stop being hypothetical:

• Any framework, any CI. React, Astro, plain HTML, an internal design system — whatever builds into a static folder works.
• Branches are variants. Git is your variant manager, code review is your QA process, git revert is your rollback.
• No per-page pricing. Experiments aren't billable events. Run twenty at once if you want to.
• Client preview links for free. ?version= rules give every stakeholder a stable URL per variant without another plan tier.
• Static-fast by default. LCP stays wherever your build tool puts it. Quality Score benefits compound into lower CPCs over the life of the account.
• Self-hostable. If your client's legal team has opinions about where landing page data lives, BFFless runs on your own infra.

Try it​

There's a live traffic-splitting demo running at demo.docs.bffless.app — open it in an incognito window a few times and watch the button color change. Our own homepage at bffless.app is currently running a four-way headline test using exactly the pattern above.

If you want to go deeper, the A/B testing recipe walks through the GitHub Actions setup, analytics integrations for GA4, Mixpanel, and Pendo, and the full cookie behavior. The traffic splitting reference covers weights, rules, and canary rollouts in detail.

Spin up an instance, point a client's landing page at it, and see what your experiment velocity looks like when variants are free.