The wiki doesn't have a dedicated Tyler Guyton player page, but the Dak Prescott page (June 4 update) covers the left tackle situation directly. Here's the answer:
It's not a lock. According to the Cowboys wiki (from the Dak Prescott page), Tyler Guyton allowed 31 pressures in just 10 games in 2025 — a concerning number. The coaching staff is openly exploring upgrades, including second-year option Nate Thomas and fourth-round rookie Drew Shelton, which signals they aren't certain Guyton can hold the job. Additionally, first-round rookie edge rusher Malachi Lawrence already beat Guyton for a sack during OTAs, which doesn't inspire confidence.
So Guyton will likely compete for the job in camp, but he's far from guaranteed to be the Week 1 starter at left tackle.