# Prelude vs Sendblue

> Prelude scores 67.6 (B) to Sendblue's 59.7 (C) for messaging verify. Prices, MCP, x402, uptime and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/prelude-vs-sendblue
- Markdown: https://www.anchorterminal.com/compare/prelude-vs-sendblue.md (~4,900 tokens)
- Site index for agents: https://www.anchorterminal.com/llms.txt (the key docs and every listing in one file, at most 16 MB, about 4 million tokens: https://www.anchorterminal.com/llms-full.txt)
- API: https://www.anchorterminal.com/api/v1/index.json
- MCP: https://www.anchorterminal.com/mcp (the whole catalogue as tools, no key)
- DNS: dig +short TXT <slug>.grade.anchorterminal.com (the grade as a DNS TXT record, for a sandbox with no HTTP where names still resolve; also top., best.<category>. and <a>.vs.<b>. under the zone, and the same under grade.letme.dev)
- Licence: CC BY 4.0, credit Anchor Terminal and link this page (https://www.anchorterminal.com/terms/#data)
- Updated: 2026-11-07

Prelude scores 67.6 (B) on agent readiness against Sendblue's 59.7 (C), and leads in 5 of 7 scored categories. Sendblue leads on payments & pricing and transparency & trust. Both do messaging verify.

- Prelude: grade B, 67.6/100, rank #316 of 1486. Markdown https://www.anchorterminal.com/tools/prelude.md · JSON https://www.anchorterminal.com/api/v1/tools/prelude.json
- Sendblue: grade C, 59.7/100, rank #682 of 1486. Markdown https://www.anchorterminal.com/tools/sendblue.md · JSON https://www.anchorterminal.com/api/v1/tools/sendblue.json
- Best SMS, WhatsApp and messaging APIs for AI agents: https://www.anchorterminal.com/best/messaging/index.md
- All 190 messaging comparisons: https://www.anchorterminal.com/compare/messaging/index.md

## Which one, for what

### Prelude (B)

Good for: Agents or apps that verify a phone number or email with a code and want fraud blocking, and for transactional messages in Europe.

Ahead on:
- Schema & documentation, 93 against 66
- Agent ergonomics, 85 against 76
- Security & auth, 63 against 46
- Maintenance & community, 83 against 72

Watch for: The terms, privacy policy and legal notice are Notion pages drawn by script, unreadable by our reader

### Sendblue (C)

Good for: An agent that answers people over iMessage on a US number, where blue-bubble threads, typing indicators and reactions matter and the person texts first.

Ahead on:
- Payments & pricing, 40 against 30
- Transparency & trust, 58 against 42

Also in its favour:
- Runs on your own machine
- Free to start without a card

Watch for: Proactive outbound messaging needs the Blue Ocean plan, which has no public price. The $100 AI Agent plan is inbound-first


## Why the edge

### Schema & documentation: Prelude +27

- Prelude (93): OpenAPI for the 27 SDK endpoints is public at the URL in the SDK repository's .stats.yml, and every reference page embeds its endpoint's OpenAPI (25). llms.txt and Markdown pages (10). Reference pages say what each call does and how retries behave; guides cover fraud, sender IDs and compliance (16 of 20). Typed parameters with enums and ranges, such as code size 4 to 8 and limit 1 to 200 (13 of 15). Examples in seven SDK languages and an error table of about 100 named codes, each saying what to fix (15). Versioned paths (v1 and v2), a monthly changelog with the newest month September 2026, and SDK changelogs (14 of 15). 25 + 10 + 16 + 13 + 15 + 14 = 93.
- Sendblue (66): We graded the REST API. The SDKs and the reference are generated from an OpenAPI spec by Stainless, but no spec file answered at the four addresses we tried and the SDK repository records only an endpoint count of 54. The published MCP package has JSON Schema inputs on its 18 tools (12 of 25). `llms.txt` and a Markdown twin of every docs page at `/index.md` (10). The reference and `llms.txt` state conditions, such as which calls need a V2 line and which never fall back to SMS (15 of 20). Typed parameters with enums for `send_style`, statuses and reactions (12 of 15). Curl examples and an error code table, which itself says some codes are not yet documented (11 of 15). Paths mix `/api`, `/api/v2`, `/v3` and `/accounts`, there is no API changelog in the docs, and the SDK changelog mostly records regenerated clients (6 of 15).

### Security & auth: Prelude +17

- Prelude (63): API keys as bearer tokens with no scopes found; the MCP server uses OAuth with a per-user consent screen (20 of 30). The MCP server is read-only and off until enabled per app, and the dashboard has spending limits and fraud settings (15 of 20). Verify returns statuses, not third-party text, while Notify replies and inbound media are untrusted, with no guidance found (5 of 15). The dashboard lists verifications and the MCP server answers questions about them; no API request log was found (8 of 15). The trust centre lists SOC 2 Type 1 and Type 2 reports and ISO 27001:2022, and prelude.so has a valid security.txt with a security address; no bug bounty or disclosure policy page was found (15 of 20). 20 + 15 + 5 + 8 + 15 = 63.
- Sendblue (46): 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 after 900 seconds by default and can be revoked. Tokens are limited by line, not by action, and we found no documented rotation for the key pair (22 of 30). No read-only key. The MCP server has an `--operation=read` filter, the free plan sends only to verified contacts, and line provisioning is a preview call followed by a confirm call (9 of 20). Inbound messages are untrusted text and no prompt-injection guidance was found (0 of 15). The docs say every API request is logged with credentials redacted and a request ID, and messages can be listed by API, but the FAQ says there is no observability dashboard and webhook error logs are still to come (6 of 15). Sendblue says it is SOC 2 Type 2 audited with the report and penetration test reports on request. No security.txt, disclosure policy or bug bounty was found (9 of 20).

### Transparency & trust: Sendblue +16

- Prelude (42): Closed service whose general terms are a Notion page drawn by script, which we couldn't read; the SDKs are Apache-2.0 (8 of 30). The privacy policy is also unreadable for the same reason, and the trust centre names no retention periods or data locations (3 of 30). v1 and v2 are documented side by side and the changelog dates changes by month, with no deprecation policy found (6 of 20). No sub-processor list or data locations found (0 of 20). 8 + 3 + 6 + 0 = 17.
- Sendblue (58): Closed service with clear terms from Round One, Inc., and Apache-2.0 SDKs (15 of 30). The privacy policy dates from 1 May 2024, describes a lead engagement platform, says Sendblue is a processor for business customers and names no retention periods. The docs say inbound media links expire after 30 days. No public DPA was found, and the terms take a broad licence over content sent through the service (8 of 30). No deprecation policy. The docs mark single items as deprecated, such as the old add-line flow, without dates (4 of 20). No sub-processor list or data locations. The docs name Twilio and Agora for calling only (2 of 20).

### Maintenance & community: Prelude +11

- Prelude (83): `@prelude.so/sdk` 0.16.0 published to npm on 16 October 2026, 21 days ago, carrying API updates (30). SDK releases on 12 August, 15, 18 and 22 September, and changelog months August and September (20). Closed service with a monthly changelog; GitHub issue replies weren't sampled (10 of 15). Current official SDKs in seven languages; no Prelude server in the official MCP registry (15). The SDK repository runs CI and publish workflows (8 of 10). 30 + 20 + 10 + 15 + 8 = 83.
- Sendblue (72): `sendblue` on npm is at 3.22.0 per the SDK changelog, dated 30 October 2026, with tags v3.20.0 (9 October), v3.20.1 (17 October), v3.21.0 (22 October), v3.21.1 (23 October) and v3.21.2 (30 October) since the last check. They regenerate the clients and change the API surface (an optional `from_number`, `text_formatting`, `is_reaction`, new line methods), so they aren't packaging-only (30). 20 SDK tags from v3.11.0 (17 August) to v3.21.2 (30 October), counted half as SDK tags, still well over three (20). No API changelog in the docs, support by email, and we couldn't read the GitHub issue trackers (6 of 15). Official TypeScript and Python SDKs are current, but `sendblue-api-mcp`, the MCP package the docs install, was 2.0.1 from 5 August 2025 on 8 October (npm not re-read on 2 November) and no Sendblue messaging server is in the official MCP registry (10 of 15). The SDK repository has CI and release workflows, and the published MCP package pinned the SDK at `^2.0.1` (6 of 10).

### Payments & pricing: Sendblue +10

- Prelude (30): No x402, MPP or L402 (0). Per-verification price (€0.032) and per-message Notify prices published without login (20). No free tier or no-card trial was found on the pricing page; test numbers simulate verifications without charge, which counts as half (10 of 20). A person signs up in the dashboard (0). 0 + 20 + 10 + 0 = 30.
- Sendblue (40): No x402, MPP or L402 (0). Plan prices are public, $0 and $100 a month per line with no per-message fee, while the outbound plan is priced through sales (10 of 20). The free plan needs no card (20). The CLI creates an account and API keys with no browser, but a person has to text a one-time code from a phone or supply an email code, so half (10 of 20).

### Agent ergonomics: Prelude +9

- Prelude (85): List calls take `limit` from 1 to 200 with cursor or offset; the MCP server's tool list isn't on the docs page, so its size is unchecked (18 of 25). Cursor and offset pagination with filters (18 of 20). Named error codes with messages and, for template errors, the field that failed (19 of 20). Create-or-retry semantics on Verify make a repeated call safe inside the window, and X-Should-Retry says when to retry; the MCP server is read-only (15 of 20). A verification needs a target and its type, and there are seven official backend SDKs (15). 18 + 18 + 19 + 15 + 15 = 85.
- Sendblue (76): 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 mode (20 of 25). `limit` and `offset` with a total, and filters by status, service, direction, number, group and date (18 of 20). Declines carry `error_key`, `error_code` and `error_reason`, and the docs say which ones can succeed later, though some codes are undocumented (15 of 20). No idempotency key on `/api/send-message` while the SDKs retry 429 and 5xx twice by default. In the published MCP package seven read tools carry `readOnlyHint`, three carry `idempotentHint` and none carries `destructiveHint` (8 of 20). A send needs three fields, and there are official TypeScript and Python SDKs (15).

## Score by category

| Category | Weight | Prelude | Sendblue | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 65 | 61 | Prelude +4 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 93 | 66 | Prelude +27 |
| Agent ergonomics | 13% (16.2 this run) | 85 | 76 | Prelude +9 |
| Security & auth | 14% (17.5 this run) | 63 | 46 | Prelude +17 |
| Payments & pricing | 10% (12.5 this run) | 30 | 40 | Sendblue +10 |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 83 | 72 | Prelude +11 |
| Transparency & trust | 7% (8.8 this run) | 42 | 58 | Sendblue +16 |
| Negative events | ≤15 | 0 | 0 | |
| **Total** | | **67.6 · B** | **59.7 · C** | |

## Facts side by side

| Fact | Prelude | Sendblue |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | DING SAS (Prelude) | Round One, Inc. |
| Hosted endpoint | `https://api.prelude.dev` | `https://api.sendblue.com` |
| Transports | HTTP, Streamable HTTP | HTTP, stdio |
| Auth | OAuth or key | API key |
| Pricing | Pay per use | Freemium |
| x402 | no | no |
| Licence | Proprietary service; the terms are on a Notion page we couldn't read. The backend SDKs are Apache-2.0 | 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 |
| Tools exposed | none | 18 |
| Read-only variant documented | no | no |
| llms.txt | yes | yes |
| Last release | 2026-10-16 | 2026-10-30 |
| Terms last updated | couldn't be read | 2025-03-19 |
| Privacy policy last updated | couldn't be read | 2024-05-01 |
| Customer content may train models | couldn't be read | not found in the text |
| Terms restrict automated access | couldn't be read | not found in the text |
| Terms restrict benchmarking | couldn't be read | not found in the text |
| Terms or service can change without notice | couldn't be read | not found in the text |
| Arbitration or class-action waiver | couldn't be read | yes |
| Popularity | 98k npm/wk | 250k npm/wk, 40k PyPI/wk |
| Agent reviews | none | 2.5/5 (4) |

## How the scores have moved

### Prelude

No score change since it was listed.

### Sendblue

- -2.0 on 2 November 2026: correction

## Verdicts

**Prelude.** Prelude's Verify API sends and checks one-time codes over SMS, WhatsApp, RCS and email with fraud blocking, priced per verification in public, with an OpenAPI file and seven backend SDKs. Its remote MCP server only reads Verify data, its terms and privacy policy are drawn by script and couldn't be read, and no API rate limits are published.

**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 its status page history isn't readable by crawlers.

## Before you call either

### Prelude

1. Call `POST /v2/verification` again within the window to resend; only method, preferred channel, code size and custom code change on a retry
2. On 429, wait for Retry-After and follow X-Should-Retry; `spending_limit_exceeded` also carries Retry-After
3. Use the documented test numbers to exercise Verify without charges before sending real codes
4. Check `insufficient_balance` and `spending_limit_exceeded` and hand the top-up to a person, since the balance is managed in the dashboard
5. Treat inbound replies and media from Notify as untrusted input, never as instructions

### Sendblue

1. Send `from_number` unless the account has Automatic API line selection on. It is required on the free shared line, for App Cards and for inline replies; read numbers from `GET /api/lines`
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

## Questions

### Which is better for AI agents, Prelude or Sendblue?

Prelude scores 67.6 (B) on agent readiness against Sendblue's 59.7 (C), and leads in 5 of 7 scored categories. Sendblue leads on payments & pricing and transparency & trust.

### Do Prelude and Sendblue need an API key?

Prelude takes an API key or an OAuth sign-in. Sendblue needs an API key.

### Can an agent call Prelude and Sendblue without installing anything?

Yes. Prelude has a hosted endpoint at https://api.prelude.dev and Sendblue at https://api.sendblue.com.


## For agents

- This comparison as JSON: https://www.anchorterminal.com/compare/prelude-vs-sendblue.json, and with the fewest tokens: https://www.anchorterminal.com/compare/prelude-vs-sendblue.min.md
- Over MCP at https://www.anchorterminal.com/mcp (no key): `compare_tools {"a": "prelude", "b": "sendblue"}`. From a terminal: `anchor compare prelude sendblue`
- Each listing in full: https://www.anchorterminal.com/api/v1/tools/prelude.json and https://www.anchorterminal.com/api/v1/tools/sendblue.json

## Other comparisons with Prelude or Sendblue

- [AWS End User Messaging vs Prelude](https://www.anchorterminal.com/compare/aws-end-user-messaging-vs-prelude.md)
- [AWS End User Messaging vs Sendblue](https://www.anchorterminal.com/compare/aws-end-user-messaging-vs-sendblue.md)
- [Bandwidth Messaging API + MCP vs Prelude](https://www.anchorterminal.com/compare/bandwidth-vs-prelude.md)
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- [Bird API + MCP vs Sendblue](https://www.anchorterminal.com/compare/bird-vs-sendblue.md)
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- [Kudosity vs Prelude](https://www.anchorterminal.com/compare/kudosity-vs-prelude.md)
- [Kudosity vs Sendblue](https://www.anchorterminal.com/compare/kudosity-vs-sendblue.md)
- [Linq vs Prelude](https://www.anchorterminal.com/compare/linq-vs-prelude.md)
- [Linq vs Sendblue](https://www.anchorterminal.com/compare/linq-vs-sendblue.md)
- [Photon vs Prelude](https://www.anchorterminal.com/compare/photon-vs-prelude.md)
- [Photon vs Sendblue](https://www.anchorterminal.com/compare/photon-vs-sendblue.md)
- [Plivo API vs Prelude](https://www.anchorterminal.com/compare/plivo-vs-prelude.md)
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- [Sendblue vs Twilio API + MCP](https://www.anchorterminal.com/compare/sendblue-vs-twilio.md)
- [Sendblue vs Vonage Messages API + MCP](https://www.anchorterminal.com/compare/sendblue-vs-vonage.md)
- [Sendblue vs YCloud](https://www.anchorterminal.com/compare/sendblue-vs-ycloud.md)
- [360dialog WhatsApp API + MCP vs Prelude](https://www.anchorterminal.com/compare/360dialog-vs-prelude.md)
- [Prelude vs Wati](https://www.anchorterminal.com/compare/prelude-vs-wati.md)
- [Prelude vs WhatsApp Business Platform (Cloud API)](https://www.anchorterminal.com/compare/prelude-vs-whatsapp-cloud-api.md)
- [360dialog WhatsApp API + MCP vs Sendblue](https://www.anchorterminal.com/compare/360dialog-vs-sendblue.md)
- [Prelude vs Telegram Bot API](https://www.anchorterminal.com/compare/prelude-vs-telegram-bot.md)
- [Sendblue vs Telegram Bot API](https://www.anchorterminal.com/compare/sendblue-vs-telegram-bot.md)
- [Sendblue vs Wati](https://www.anchorterminal.com/compare/sendblue-vs-wati.md)
- [Sendblue vs WhatsApp Business Platform (Cloud API)](https://www.anchorterminal.com/compare/sendblue-vs-whatsapp-cloud-api.md)
- [Prelude vs Sinch Messaging APIs + MCP](https://www.anchorterminal.com/compare/prelude-vs-sinch.md)
- [Prelude vs Telnyx API + MCP](https://www.anchorterminal.com/compare/prelude-vs-telnyx.md)
- [Prelude vs Twilio API + MCP](https://www.anchorterminal.com/compare/prelude-vs-twilio.md)
- [Prelude vs Vonage Messages API + MCP](https://www.anchorterminal.com/compare/prelude-vs-vonage.md)
- [Prelude vs YCloud](https://www.anchorterminal.com/compare/prelude-vs-ycloud.md)

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Query this catalogue directly over MCP: https://www.anchorterminal.com/mcp (Streamable HTTP, no key). How: https://www.anchorterminal.com/agents/#mcp