The first build
The sandbox lists 50 messages daily with API and SDK access, webhooks, receipts, typing and reactions. That gives a developer a useful starting point for planning an inbound-to-response loop. Confirm account and recipient restrictions during signup; the daily number is not necessarily the entire test agreement.
The paid build
A dedicated Line plan is $99 monthly with managed infrastructure. Enterprise quotes volume, custom integrations and SLAs. Ask for new-contact, request and fair-use limits before treating unlimited-message language as a capacity target.
The acceptance record
Connect the received event to an internal action and then to the provider’s resulting message identifier. Exercise a duplicate event in your own application and verify that the response is not repeated. The public product description supports investigating this workflow; it does not mean Message Works has tested live delivery or certified the SDK.