Use environment: "sandbox" with an anonymous br_anon_ session or a Test API
key that starts with br_test_. Send the request to
https://api.batchrelay.com. The stored credential selects Test Mode and the
sandbox provider lane.
Anonymous sessions follow the same public sandbox provider-selection and
compatibility rules as account-owned Test API keys. They add isolation,
expiration, and operation-specific quotas; they do not use a reduced fake API.
Sandbox compatibility and quotes are safe ways to test canonical validation,
provider availability, quote freezing, and idempotency. A provider must be
explicitly enabled with sandbox credentials and fulfillment switches before a
hosted order can reach a provider. Automated tests use mock transports only.
Treat a successful sandbox response as order acceptance, not provider
acceptance. Confirm provider status from the order and fulfillment events.
rpl) is currently preview-only: it may appear in a
compatibility report but is unavailable for hosted sandbox or production orders
until credentials and provider verification are complete. 