Agent Onboarding

POST /v1/signup with { "user": { "email", "password" } }. Returns a 15-day JWT in the Authorization response header.

POST /v1/my/api_keys with that JWT. The response includes the full nskey_live_... secret once, in secret_key. Store it as a secret then. Later reads mask it; reveal it again with GET /v1/my/api_keys/{id}?reveal=true.

Get the brand SID from GET /v1/my/brands, then call POST /v1/my/brands/{sid}/prepare_sending with { "subdomain": "<name>" } (optionally "local_part", which defaults to hello). It reserves <name>.nitrosend.net for the brand, permanently, and materializes its local Domain and sending-identity rows synchronously under Nitrosend's verified shared root. The brand's brand_subdomain.suggested_subdomain, when present, is a safe starting name; if the call returns subdomain_taken, choose another. Without a subdomain, the call derives the name from the company name instead and refuses with brand_identity_required until the whole Brand Kit is complete. Once a name is reserved, repeat calls return it and ignore the body. The sender can send when the response status is ready. It creates no per-brand DNS, DKIM, provider identity, tenant resource, or background job. When no sender is selected, the same transaction selects this sender; otherwise it preserves the existing selection.

POST /v1/my/messages with { "channel": "email", "to", "subject", "html" } and an Idempotency-Key header. If the previous step was skipped, an eligible first request performs the company-derived materialization before admission, which needs the complete Brand Kit. If the shared root is unavailable, the API returns retryable hosted_sender_root_unavailable, stores no message, reservation, charge, or provider capability, and the same request is safe to retry.

A customer-owned sending subdomain builds portable reputation and never consumes the reserved Nitrosend brand-subdomain sender. Paid plans and checkout remain optional; $0 plans need no card.