In a good Thai chicken soup there will be whole herbs which are not meant to be eaten, such as ginger and lemongrass. They add wonderful flavour though.
Place the chilli, garlic, ginger and lemongrass in a saucepan with the water. Bring to a boil, take off the heat and let infuse 30 minutes.
Return to the boil and add coconut milk. Reduce heat. Add chicken, onion shallot and beans and simmer 12 minutes. Add zucchini and simmer a couple more minutes.
Remove the chicken, let cook a bit then shred. Return it to the soup with the lime, salt and pepper. Reheat and serve, garnished with coriander.