Onions might be the best carrot companion; they protect carrots from carrot fly, and carrots can protect onions from onion thrips.
Radishes and carrots grow in the same conditions, and radishes will grow quickly to mark carrot rows. They'll be ready to harvest before carrots need that space.
Chives can help deter carrot root fly, aphids, and mites. They're ideal for spring and winter sowings of carrots.
Marigolds and calendula repel aphids, mites, whiteflies, and other carrot pests. They're two of the best flowers for a veg garden.
Sage likes poorer soil than basil, but both will grow well in well-draining soil with carrots. They may also help repel pests, and basil supports beneficial insects.
for 3 more companion planting ideas + what not to plant with carrots