A/B test models in production

Sooner or later every team wants to answer the same question: is the new model actually better for our users compared to the current model? Not better on a benchmark but rather better on retention, thumbs-up rate, task completion, whatever your product actually measures.

Shadow traffic can't answer that question. Shadowing tells you the candidate is operationally sound with respect to latency, errors, throughput, but its responses are discarded; no user ever acts on them. Quality questions need real exposure to end users where a cut of your users get model B, and you compare what happens.

A feature flag or a hash-mod-100 on user ID in the client code.

Two endpoints (or two hardcoded model strings) the client switches between.

A spreadsheet somewhere explaining what group A vs B means.

It works, but it entangles your experiment with your infrastructure in ways that hurt later: the routing logic ships with your application, the cohort split can drift as clients cache decisions, and even after the experiment "ends" the branching code lives on long afterward because nobody's sure it's safe to remove.

The Together AI platform allows you to run A/B experiment logic at the endpoint level.

An A/B experiment attaches to an endpoint and declares members with exactly one control and one or more variants, each pointing at a deployment, each with a percent setting, that must sum to 100, controlling traffic routing.

How the endpoint router works is that whenever the base traffic sends a request to the control the experiment re-samples it among the arms and redistributes such that 95% stays on the control, 5% goes to the variant.

To be precise about the mechanism: the experiment subdivides the control's share of the base traffic split. Routing first resolves a request through the weight split; when the winner is the control of an A/B experiment, the request is re-sampled among the experiment's arms by their percents. With the control as the only entrypoint in the split member percents therefore are absolute traffic shares. Also worth noting is that a control whose split weight is zero gives the experiment nothing to subdivide and as a result the whole experiment receives no traffic.

Importantly variant deployments must not be in the endpoint's traffic split, the platform requires variants to carry zero weight; only the control lives in the base split. The experiment will own traffic routed to the variant entirely; its percentage is its traffic share. If a variant could also draw capacity-weighted traffic from the split, your measurements would be quietly wrong. One way to think about it is that you should set up the variant like a shadow deployment: created, READY, weight zero and then let the experiment percentage setting route to it.

Another important point here is that A/B percents are true fixed traffic shares summing to 100% and are independent of replica counts. We made this deliberately different from traffic-split weights (which are per-ready-replica and follow capacity). An experiment is a measurement instrument; you want the split to be constant while you measure and not drift with autoscaling.

Your clients won’t notice this experiment because on the surface the same endpoint name, API and keys persist. On the backend 5% of requests will now be answered by the variant candidate.

Updates are guarded by an etag because if a teammate ramped the experiment while you were composing your update, yours will be rejected instead of silently overwriting theirs.

With up to 20 variant members you can also run multi-way tests, lets say for example you want to try out a full-precision endpoint along with three other quantized variants (V1, V2, V3). This will work as expected as long as the percents still sum to 100% and there's exactly one control.

Every request is served by a specific deployment, and every platform metric is available per deployment — so the infra side of the comparison (latency, errors, throughput per cohort) is a filter, not a project. The product side is yours: log which deployment served each response (it's in the response metadata) alongside your quality signals — ratings, retries, task completions — and the join key is just the deployment ID. The platform deliberately doesn't guess at your quality metrics; it makes the attribution trivial so your analytics can do the judging.

Suppose the experiment shows the variant wins. Ending it is a two-step process:

Promote via rollout. Run a blue-green rollout from the control deployment to the variant deployment. Health gates, propagation waits, and rollback safety all apply.

Delete the experiment. All experiment routing disappears and 100% of the traffic then follows the endpoint's base traffic split which, post-rollout, points to your winner.

And if the variant loses, you can just delete the experiment and traffic will return entirely to the control. On the backend the variant deployment scales to zero or gets deleted.

The variant degrades mid-experiment. What's the blast radius?

Only its cohort. Deployments are independently monitored and independently autoscaled, which means that a struggling variant won’t drag the control down. To fix this you can resend the member set without the sick variant and its users will be back on control within a certain propagation time. This is also why starting at 5% is a good idea.

Do the observed shares actually match the configured percents?

Over meaningful volume, yes! For an example of this in action check out the experiment below. Over small windows you can expect sampling noise: a 5% share of 1,000 requests is a small sample. If your observed share is off by a lot and stays off, check the setup rule above.

Can an A/B experiment and a rollout run on the same endpoint?

Yes, and the composition order is defined as: routing resolves the base split first, then A/B experiments (subdividing the control's share), then rollouts (re-sampling between a source deployment and its rollout target). The platform still enforces one active rollout per endpoint but the stages are designed to compose if they overlap.

Are cohorts sticky per user?

Assignment uses the request's sampling key, for example a top-level prompt_cache_key or user field in the request body, so requests carrying the same key route consistently and a given user can stay in one test arm across a session. Requests without a key are assigned effectively at random per request (our experiment measurements below used key-less traffic, which is why observed shares match the percents so closely). If per-user consistency matters to your study, especially if you have multi-turn quality comparisons, send a stable user field.

What happens to autoscaling under a split?

Each member deployment scales on its own policy, sized by its own share of traffic. A variant at 5% with bounds 1-2 and a control at 95% with bounds 2-8 is a perfectly normal shape. You should watch each cohort's replicas independently, this is what we capture in the chart below.

We ran the full lifecycle against a live endpoint where we create at 95/5, ramp to 80/20, ramp to 50/50 then delete. We do this while maintaining a steady 3 RPS stream, with every request attributed to the deployment that served it. The graph below shows traffic seen at the variant with green dots capturing variant traffic share:

Each ramp really was just one call and you can resend the full member set with the current etag. The etag advances 1 → 2 → 3 across the two ramps; a stale etag would have been rejected rather than it silently overwriting a teammate's change.

Propagation is fast but not instant. We waited ~75 seconds after each update before measuring; the routing layer picks up experiment changes on the same 30–60s timescale as traffic-split changes.

Delete: After removing the experiment we sent 360 consecutive requests and they all landed on the control. There is no left over cohort logic besides the delete left over to unwind.

Try it yourself!

You need an endpoint with a control deployment serving traffic, plus a candidate deployment (created, READY, not in the traffic split). Then:

Create the experiment at 95/5.

Let it run until you have enough volume — the whole point of 5% is that exposure stays low while you collect data.

Ramp with a single update when the data says so. Promote with a rollout when it's decisive. Delete when you're done — there's nothing else to clean up.