Short answer
This is urgent. Chives are toxic to dogs, just like onions and garlic, and can damage their red blood cells leading to anemia. Even a small amount from a party snack can be harmful. Watch for signs like weakness, pale gums, vomiting, or red-tinged urine over the next few days. Call your vet or the ASPCA Animal Poison Control Center at 888-426-4435 right away to discuss what he ate.
Chives — Food safety facts
Do not feed · Dogs · full breakdown on the linked food page
An allium, like onion and garlic. Damages red blood cells and causes anaemia.
The full verdict, portion guidance and serving notes are collected on the Chives food facts page.