Traffic weights per variant

Every variant carries a weight, and the weights decide how eligible traffic is shared. They are stored as basis points out of 10,000 rather than percentages, which is what lets a three-way split be exact instead of rounding to 33 percent three times and losing a slice.

The invariant is that a stored distribution sums to exactly the total. The console preview, the API validation and the start handler all run the same arithmetic, because a split that three parts of the system compute differently is how variants get silently starved of traffic or visitors get dropped on the floor.

A variant left without an explicit weight shares whatever remains equally with the other unweighted ones, and any rounding residue is spread a basis point at a time so the sum still lands exactly. That means the common case, an even split, needs no configuration at all.

Weights can be changed while an experiment runs. What the report shows you afterwards is the share each variant actually received, measured from sessions, rather than the share that was configured, because those two drift the moment a split is edited mid-flight.