Plivo API vs Sendblue

Plivo API

US long code SMS $0.0077 a message and numbers $0.50 a month, with carrier surcharges listed per carrier. No OpenAPI spec.

Sendblue

Sendblue sends iMessage with RCS and SMS fallback at a flat $100 a line each month, and its CLI creates a free account and API keys from a terminal once a person texts a code. Proactive outbound messaging is sold through sales, no OpenAPI file is published, and the MCP package the docs install dates from August 2025.

Plivo API

1. Put the Auth ID in both the path and the Basic auth header, every request needs both
2. Stay under 300 API requests per 5 seconds, a 429 carries no Retry-After
3. Give each agent its own subaccount to isolate credentials and spend
4. Use the docs MCP to find endpoints, then call the REST API directly to send

Sendblue

1. Send from_number on every request. Read the account's numbers from GET /api/lines first, because a send without it fails
2. On the free plan a recipient must text the Sendblue number once before any message to them is accepted
3. Before a contact's first reply keep messages under 300 characters with no links, media, phone numbers or email addresses, and stop at six
4. Branch on error_key in an HTTP 400 decline and don't retry it. Only HTTP 429 clears by waiting
5. Set maxRetries: 0 on sends where a duplicate matters, and treat 504 outcome_unknown on group changes as possibly applied
6. Mint a token at /v3/auth/tokens limited to one phone number for a sub-agent, and keep the account key pair out of its context