If your cookies are pale on top but dark on the bottom, your pan is likely too dark or too thin, or your rack is too low. Move the rack up one notch, switch to a lighter sheet, and consider a double-pan hack: place your cookie sheet on top of a second empty sheet to insulate the bottom.
If your cookies brown on top before they spread, your rack is too high or the top element is running aggressively. Drop the rack and, if your model allows it, choose bake rather than broil-like modes.
If one side browns faster, rotate the tray 180 degrees halfway through—most toaster ovens have a clear hot corner. When people say baking cookies in a toaster oven is unpredictable, it's usually because they're changing multiple variables at once.
About foil: it's tempting, but it tends to over-brown the bottoms and can reflect heat unpredictably in a small oven. If you must use foil, dull side up is a common approach, and keep it tightly wrapped to the pan so it can't flutter near elements. Parchment is usually more forgiving.
If your toaster oven has convection, decide early whether you'll use it for cookies. Convection can be excellent for evenly browning multiple items, but cookies are delicate: too much airflow can set edges quickly and make centers rise oddly.
My practical rule is simple: start with the fan off for your first batch, especially for drop cookies, then experiment with the fan on once you've nailed timing. If you do bake with convection, lower the set temperature by roughly 25°F and begin checking for doneness earlier than the recipe suggests.
Sugar type changes toaster-oven behavior. Higher brown sugar ratios generally keep cookies softer and help with browning; higher white sugar ratios can increase spread and crisp edges. If your toaster oven runs hot and you keep getting lace-thin cookies, you may just need colder dough, a slightly lower temperature, or a dough with a touch more flour structure.
Performance problems often look like bad recipes until you clean or recalibrate the basics. Crumbs on the bottom of a toaster oven can smoke and create bitter odors that cling to baked goods. Keep the crumb tray clean, wipe the interior once it's fully cool, and don't let burnt sugar accumulate on pans.