Sendblue vs Telnyx API + MCP

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.

Telnyx API + MCP

US long code SMS $0.004 a part outbound and inbound, published with carrier fees and as machine-readable pricing.md. No per-key scopes or read-only keys, and the MCP can send or buy numbers without confirmation.

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

Telnyx API + MCP

1. Register the 10DLC brand ($4.50) and campaign ($10 a month) before sending US long code traffic
2. Back off on error 10011 for as long as the Retry-After header says
3. Call get_api_endpoint_schema before invoke_api_endpoint, the schemas aren't in the tool list
4. Give each agent its own key, since keys can't be scoped
5. Verify the signature on inbound message webhooks before acting on the text