WHAT IS THE RECIPE?

Jack in the Box seems to understand that you don't mess with a good thing. When a product is popular and you sell it in mass, obviously something's working and there's no point in introducing changes that could screw things up. And that's exactly the case with the Jack in the Box tacos. An article in Mental Floss points out that the recipe hasn't really changed in about 50 years, describing it as follows, "seasoned meat lump, two triangles of American cheese, some shredded iceberg lettuce, and a little sauce."