The Sia Daemon provider API routes for all of its modules. Documentation for each module's API routes are found in the Sia GitLab repository.
All Siad API calls return either JSON or no content.
A few important notes to keep in mind when working with the API are:
curl -A "Sia-Agent" "localhost:9980/wallet/transactions?startheight=1&endheight=250"
curl -A "Sia-Agent" --data "amount=123&destination=abcd" "localhost:9980/wallet/siacoins"