Identity resolution used to be a checkbox in a vendor pitch and is now genuine engineering work, mostly because the easy signals went away.

The short version: session stitching that does not depend on signals you do not control is mostly a matter of doing the obvious thing carefully. Lean on first-party cookies, server-side hashed identifiers, and authenticated user IDs in that order. Treat fingerprinting as a last resort.

You will resolve fewer sessions than you used to. The ones you do resolve will be cleaner. That trade-off is the one most teams underweight when they plan the work, and it is the one that quietly determines whether the project ships on time.

If you want a longer treatment, we have a deeper post in the series. For now, the takeaway is small enough to fit on a sticky note: The teams getting this right in 2026 are the ones who built first-party identity foundations two years ago.