Structures made of carbon fiber-reinforced polymer (CFRP) can be assembled using adhesive bonding. However, such bonding is prone to brittle delamination, and a method to improve delamination resistance is desirable. Here, we propose a technique to introduce crack-arrest features that increase the R-curve response by engineering the adhesive bondline/interface. We specifically designed a wavy net-like thermoplastic insert that was embedded into the thermoset adhesive bondline where the new mechanisms of energy dissipation were generated. We demonstrate that the technique is effective at improving mode I fracture toughness of secondary bonded carbon/epoxy by more than 400%. The hybrid thermoset/thermoplastic bondline architecture was carefully tailored to achieve its best performance. We demonstrate that introducing porosities in the adhesive bondline (by adding a limited amount of thermoset adhesive) further improves the fracture toughness. This toughness improvement originates from the extrinsic toughening of the crack-arrest feature, which is enabled by the insert ductility and microstructures (via strand formation, anchoring and stretching).