# Sendblue (slim)

> Sendblue is a hosted API from Round One, Inc. for sending and receiving iMessage, RCS, SMS and MMS from dedicated or shared phone numbers. Agents use a REST API, TypeScript and Python SDKs, a CLI and an MCP server.

- Full: https://www.anchorterminal.com/tools/sendblue.md (~8,250 tokens) · this version ~2,030 tokens · JSON https://www.anchorterminal.com/tools/sendblue.json · canonical https://www.anchorterminal.com/tools/sendblue
- Index: https://www.anchorterminal.com/llms.txt · API: https://www.anchorterminal.com/api/v1/index.json · Updated: 2026-10-08

**C · 61.7/100 · rank #358 of 722 · #9 in Messaging APIs · not agent-ready · confidence medium**


Assessment: 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.

## Facts

- Kind: HTTP API · vendor: Round One, Inc. · category: Messaging APIs · legal entity: Round One, Inc. · provenance 86/100
- Endpoint: `https://api.sendblue.com` (HTTP, stdio)
- Auth: API key · pricing: Freemium · x402: no · licence: Proprietary service under Round One, Inc.'s terms of service. The TypeScript and Python SDKs and the MCP server are Apache-2.0, and the CLI is MIT
- Probe metrics: not measured yet (probes haven't run)
- Channels: iMessage, RCS, SMS and MMS, with group messages, tapback reactions, typing indicators, read receipts, carousels, App Cards and voice notes on iMessage. FaceTime audio and Twilio voice are separate add-ons. No WhatsApp
- Fallback: Ordinary messages fall back from iMessage to RCS and then SMS at no extra charge. App Cards and inline replies never fall back
- Sender registration: Sendblue says no A2P registration is needed. Each line is a phone number Sendblue assigns, shared on the free plan and dedicated on paid plans
- Plans: Free Sandbox $0 (shared number, up to 10 verified contacts). AI Agent $100 a month per line (inbound-first). Enterprise or Blue Ocean for outbound, price on request
- Rate limits: 10 messages a second per line, HTTP 429 above that. Blue Ocean 50 new contacts a day and 15 an hour per line. AI Agent 1,000 inbound contacts a day and 200 follow-ups a day per line. Contacts API 100 requests per 10 seconds. Lookups 30 an hour and 100 a day per line
- Before the first reply: At most six messages of up to 300 characters with no media, links, phone numbers or email addresses, until the contact replies. One `pre_reply_override` per contact per line per day
- Inbound: Seven webhook types (receive, outbound, typing_indicator, call_log, line_blocked, line_assigned, contact_created), managed at `/api/account/webhooks`, three retries on 5xx, a shared secret sent in the `sb-signing-secret` header
- Credentials: `sb-api-key-id` and `sb-api-secret-key` headers for the account. Temporary bearer tokens from `/v3/auth/tokens`, account-wide or limited to named phone numbers, 900 seconds by default, revocable
- MCP server: `sendblue-api-mcp` on npm (stdio, 2.0.1, 5 August 2025), 18 tools, `--operation=read` and `--resource` filters and a `--tools=dynamic` mode with three meta-tools. Not in the official MCP registry
- SDKs and CLI: `sendblue` on npm 3.19.1 and on PyPI 1.31.1, both 7 October 2026, Apache-2.0, generated by Stainless. `@sendblue/cli` 0.10.0 (17 August 2026, MIT). Community Go, Rust and Ruby clients
- Verify: An inverted one-time code under `/api/v2/verify`. The user texts a code to a Sendblue number and the verification turns `approved`
- Sandboxes: Cloud Linux machines under `/v3/sandboxes`, a separate product on the same keys, with $100 of compute for new free accounts per the docs
- Status: status.sendblue.com on UptimeRobot, five monitors (API health, API ingestion health, API root, Dashboard, Datacenter) with 90 days of daily ratios and no written incidents
- Certifications: Sendblue says it is SOC 2 Type 2 audited, with the report and penetration test reports on request, and HIPAA on a dedicated instance
- Prices: AI Agent plan, dedicated line $100 per month (plan)

- Scores: Reliability 71, Performance pending, Schema & documentation 66, Agent ergonomics 76, Security & auth 46, Payments & pricing 40, Task success pending, Maintenance & community 72, Transparency & trust 58 · total over the 7 assessed categories
- Why: Reliability, Read with the hosted lines. · Schema & documentation, We graded the REST API. · Agent ergonomics, Message lists take `limit` from 1 to 100, and the MCP server adds a `jq_filter` on every tool, a read-only filter and a three-tool dynamic m… · Security & auth, One key pair per account in two headers, never in the URL, plus temporary bearer tokens that can be limited to named phone numbers, expire a… · Payments & pricing, No x402, MPP or L402 (0). · Maintenance & community, `sendblue` 3.19.1 on npm and 1.31.1 on PyPI on 7 October 2026 (30). · Transparency & trust, Closed service with clear terms from Round One, Inc., and Apache-2.0 SDKs (15 of 30).
- Sources: 29, open questions: 10, both in the full twin
- Capabilities: messaging.sms, messaging.mms, messaging.rcs, messaging.inbound, messaging.verify
- JSON: https://www.anchorterminal.com/api/v1/tools/sendblue.json
- Verify (for the vendor): the badge `https://www.anchorterminal.com/badges/sendblue.svg` or a link to https://www.anchorterminal.com/tools/sendblue from a page on sendblue.com or one of its subdomains, or the README of github.com/sendblue-api/sendblue-ts, then `POST https://www.anchorterminal.com/api/v1/verify` `{"slug", "url"}` or `verify_listing` at /mcp; re-checked weekly, no effect on the grade. Snippets in the full twin.

## Before you call it

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

## Connect

```bash
npm install sendblue
```

```bash
curl -X POST https://api.sendblue.com/api/send-message \
  -H "sb-api-key-id: $SENDBLUE_API_KEY_ID" -H "sb-api-secret-key: $SENDBLUE_API_KEY_SECRET" \
  -H "Content-Type: application/json" \
  -d '{"number":"+19998887777","from_number":"+18887776666","content":"Hello from Sendblue!"}'
```

```bash
claude mcp add sendblue_api --env SENDBLUE_API_API_KEY=your-api-key --env SENDBLUE_API_API_SECRET=your-api-secret -- npx -y sendblue-api-mcp --client=claude-code --tools=all
```
Full config and headless snippets are in the full page. Through letme (picks today, calling later): https://letme.dev/sendblue

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| Tool | Grade | Score | Shared capabilities | Slim |
| --- | --- | --- | --- | --- |
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| AWS End User Messaging | BB | 74.3 | messaging.sms, messaging.mms, messaging.rcs, messaging.verify, messaging.inbound | https://www.anchorterminal.com/tools/aws-end-user-messaging.min.md |
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