The Reinforced Knife is an unlockable gadget upgrade in Shadow of the Tomb Raider (SOTTR). It allows you to cut through rope barriers with the knife.
Getting Reinforced Knife: It is sold by a merchant at the market square of Paititi “The Hidden City”. It costs 2900 Gold Pieces. What makes this easy to overlook is that it’s simply called “Knife Upgrade” in the merchant’s inventory.
Item Description: Knife Upgrade – Sharpen your blade to unlock new abilities. Upgrade basic blades to cut through BRAIDED ROPES. Already upgraded blades will be enhanced to receive more resources from harvested animals.
See the location in the images below:
This is halfway through the story. By the time you reach this place, you should easily have more than 2900 Gold. The reinforced knife is extremely useful and should be high up on your purchase list. Without it you can’t reach certain optional places. There’s braided ropes to the left of this merchant, if you want to test out your new upgrade right away.