The one-bag travel movement - built on the premise that any journey can be accomplished with a single carry-on that fits under the seat, no checked baggage fees required - is facing an increasingly organised challenge from US budget carriers. A gear report on r/onebag has crystallised the problem with unusual precision.The report documents a Frontier Airlines gate where agents actively size-checked every passenger's personal item bag before boarding - catching the poster's Thule Aion 28L, a popular one-bag choice, in an uncomfortable position. The bag is listed as compliant with Frontier's personal item dimensions when empty. But when fully loaded, its expansion puts it over the edge of the size template. "My bag was not expanded but was very full," the poster wrote. "I was only able to avoid the oversized bag fee by fitting as many of my belongings as possible into my coat pockets, and even then the bag was a tight fit."This is not a one-off. Frontier, Spirit, and Allegiant have all intensified gate enforcement of personal item sizing as ancillary fee revenues become an increasingly critical component of budget carrier economics. According to Bureau of Transportation Statistics data, US airlines collected over $7 billion in baggage fees in 2024. For ultra-low-cost carriers, carry-on fees are a primary profit lever.The one-bag community has been tracking which packs reliably clear even aggressive enforcement. The consensus from frequent Frontier fliers: bags that comfortably fit within dimensions when fully packed - not just technically within spec when empty - are the only safe choice. The Aer Travel Pack 3, Tom Bihn Synik 30, and Peak Design Travel Backpack 30L are frequently cited as reliably compliant on the strictest carriers. Packs with significant expansion capability - including the Thule Aion 28L when stuffed - carry meaningful risk.The practical strategy for avoiding gate fees on budget carriers: pack before you measure. Fill your bag to your actual travel load, then compare it against the airline's stated personal item dimensions with the bag fully loaded. If it does not fit cleanly, it is a liability. Frontier's personal item limit is 18 x 14 x 8 inches; Spirit's is 18 x 14 x 8 inches; Allegiant's is 16 x 15 x 7 inches - the tightest of the three.One-baggers who regularly fly budget carriers also recommend wearing or hand-carrying the bulkiest items through the gate rather than packing them, and timing gate arrival to avoid peak enforcement windows - agents are most active in the first 15 minutes of boarding when bin space anxiety is highest.The broader question for the one-bag community: as enforcement tightens, is the personal item category still a reliable free tier, or has the economics of budget flying finally caught up with the movement? The data suggests checking luggage pricing on your specific route before assuming a carry-on strategy is always the cheapest option.