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Flow vs Sendblue: why Flow wins

Flow vs Sendblue for iMessage: see why Flow is the better choice for AI agents, with a sourced feature table and plans side by side. Updated October 2026.

By FlowPublished Updated

The short answer

Flow is the iMessage platform built for AI agents. Start free on a shared line, support 300 people a month on a $25 a month shared line, or move to a dedicated line for $100 a month. One agent runs on iMessage, WhatsApp, Telegram and MCP, with signed webhooks retried for three days and ordered per conversation.

Building on the iMessage API? Build on iMessage with Flow: one agent for iMessage, WhatsApp and Telegram. New to it? Read the iMessage API guide.

Flow and Sendblue feature by feature

Flow first. Every cell links to the page it came from, and where we did not find something the table says to check with the provider.

Featureflow.engineerSendblue
Sends iMessage (blue bubbles) Yes. A US number, shared or dedicated, with blue-bubble repliesYes. Each line sends and receives iMessage, SMS, MMS and RCS1
WhatsApp and Telegram Both, through the same APICheck with the provider
Webhook signing and retries HMAC-SHA256 signed; retried for 3 days; ordered per conversation; replayable event log2Shared-secret header; retried up to 3 times on a 5xx3
Free way to try it Free trial on the shared line: 25 people, customers text first. Plus a $25 shared line for 300 people a month, a Telegram sandbox and simulated contactsSandbox: shared number, 10 verified contacts, inbound-first1
Price to start Free trial for 25 people; $25 shared line for 300 people a month and 200 a day; $100 a month dedicated line for 1,000 people a day (250 a day at first, ramping to 1,000 over its first 14 days); writing first with the $40 a month Outbound add-on on a dedicated line, where new conversations ramp from 2 a day to 40 a day over 21 days$100 a month per dedicated line (AI Agent plan); outbound is custom1
Message limits No per-message fees. Plans cap different people, not messages, per account and channel by UTC day and month, replies included: 25 free on the trial (no monthly reset), 300 a month and 200 a day on the $25 shared plan, 1,000 a day on a dedicated line after a 14-day ramp from 250. Writing first is pacedNo per-message fees; 1,000 inbound contacts a day on the AI Agent plan1

Last checked: October 2026. Prices and features change, so check each provider’s own pages before you rely on a detail. “Check with the provider” means we did not find it stated.

  1. Sendblue pricing
  2. Flow events and webhooks
  3. Sendblue webhooks docs

Why Flow

  • One agent on every channel. The same handler answers on iMessage, WhatsApp and Telegram, and Flow is also an MCP server and CLI. You write your tools once instead of wiring a messaging line to a separate agent stack.
  • A free start and plans that grow with you. A free trial on the shared iMessage line covers 25 people, a $25 a month shared line fits 300 people a month and 200 a day, and a dedicated line is $100 a month for 1,000 people a day (a new line starts at 250 a day and ramps to 1,000 over its first 14 days). Writing first comes with the $40 a month Outbound add-on on a dedicated line; the conversations it starts ramp from 2 a day to 40 a day over 21 days.
  • Webhooks agents can trust. Flow signs with HMAC-SHA256, retries for three days, delivers in order per conversation and keeps a replayable event log. Sends take idempotency keys, so a retry never double-sends.
  • Test without a phone. Test keys create simulated contacts, so your test suite can play the customer for iMessage, WhatsApp and Telegram.
  • Open source where it counts. Flow Framework is open source, and you bring your own model.
  • Faster delivery, especially for media. Sent from a US server to the same iPhone, Flow’s API answered in 0.09 s at the median, against 0.55 s for Sendblue. Images reached the phone in about 3 s on Flow and in 14 to 23 s on Sendblue, a PDF in 3.0 s against 14.0 s, and a three-image carousel in 5.4 s against 16.0 s. Text arrives at about the same speed on both, and Sendblue was faster on tapbacks and read receipts.

Put your agent on iMessage with Flow, free to start.

Build, test and scale in one place

Flow gives your team one messaging layer from the first sandbox test to a live iMessage line. The same agent, tools and delivery model work across iMessage, WhatsApp, Telegram and MCP, so new channels do not mean a new integration.

What to plan for

  • Groups: on a dedicated line, Flow replies in any group the line was added to. If you need to create or rename groups, confirm that before you choose a platform.
  • Price shape: count the people you expect each month, then start free or choose the $25 shared line for 300 people a month.
  • Writing first: shared lines are reply-first. On a dedicated line, writing first comes with the $40 a month Outbound add-on, within warm-up limits.

Keep reading

For the whole market, read the best iMessage API and iMessage API pricing compared. To see how Flow works on iMessage, read Flow on iMessage: features, pricing and limits and Build an iMessage AI agent.

Frequently asked questions

Why choose Flow over Sendblue?

Flow gives an agent more than a line: one codebase across iMessage, WhatsApp, Telegram and MCP, a free shared iMessage line to start, signed webhooks retried for three days, and test keys with simulated contacts.

Which is cheaper, Flow or Sendblue?

Flow starts free, a shared line for 300 people a month is $25 a month, and a dedicated line is $100 a month. There are no per-message fees.

Can Flow start conversations on iMessage?

Yes, with the $40 a month Outbound add-on on a dedicated line, within warm-up limits. Shared lines are reply-only.

Which has better webhooks?

Flow signs with HMAC-SHA256, retries for three days, delivers in order per conversation and keeps a replayable event log.

Do both have an MCP server?

Flow hosts a messaging MCP server at api.flow.engineer/mcp, and Flow Framework also builds MCP servers for your own product.

Which is faster, Flow or Sendblue?

In our test from a US server to one iPhone (November 2026), Flow’s API answered in 0.09 s at the median against 0.55 s, and text arrived in 1.44 s against 1.52 s. Media was much faster on Flow: images in about 3 s against 14 to 23 s, a PDF in 3.0 s against 14.0 s. Sendblue was faster on tapbacks and read receipts.

Start hereiMessage API: a developer’s guideThe overview of how iMessage APIs work, and what to ask a provider. ComparisonDoes Twilio support iMessage?No. Flow gives your agent iMessage, and you can keep Twilio for SMS beside it.ComparisoniMessage API pricing comparedWhy Flow is the value pick: start free, support 300 people a month for $25 a month, and scale with a $100 dedicated line.ComparisonBest iMessage API for AI agentsWhy Flow is the best iMessage API for agents: one codebase, reliable webhooks, testing and every channel in one platform.ComparisonApple Messages for Business vs iMessage APITwo different things that both get called iMessage for business, side by side.ComparisonLinq alternative: why teams choose FlowWhy Flow is the best Linq alternative: a lower paid price, a free start and one agent on every channel.ComparisonFree iMessage APIStart free on Flow's shared iMessage line, test your agent without a phone, and scale to 300 people a month for $25 a month.

All posts: the Flow blog. The Flow docs have the reference.

From reading to building

Your agent, on iMessage and everywhere else.

Flow gives your agent an iMessage line, and the same code answers on WhatsApp and Telegram and works as an MCP server and CLI. See how it works on iMessage.

Build on iMessage with Flow