Is the Cirriform good for running with a hydration vest?

Yes. The seamless shoulders and flat-locked seams are designed to sit flat under pack and vest straps without rubbing. It is one of the main reasons to pick it over a plain long-sleeve tee.

How breathable is it in warm weather?

Mesh panels under the arms and along the sides add airflow where you sweat most, and the fabric wicks quickly. Long sleeves still run warmer than a short-sleeve tee in high heat.

Does it control odor on multi-day efforts?

The fabric has a SILVADUR antimicrobial treatment meant to keep it fresher between washes. It helps, but it does not replace washing after heavy use.

What sun protection does it offer?

UPF 20+ blocks most UV rays through the fabric. That is fine for shorter runs but modest for long days of direct sun.

Should I size up to layer under a vest?

It is cut as a fitted running layer, so it sits close to the body. If you want room for a base layer underneath, consider a size up.

Is it warm enough on its own in cold weather?

It is a light, breathable layer, not an insulated top. It works well as a base layer or on mild days, but you will want another layer in cold conditions.