iMessage vs WhatsApp for AI agents

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.

The big difference: how a conversation opens

This is from Flow’s sending rules. On WhatsApp, the conversation carries window_open_until, and you can subscribe to conversation.window_closing to hear about it one hour before. On iMessage there is no timer to watch. If the person has written to the line, you can reply.

The design lesson is shared: reply, don’t broadcast. Agents that answer people who wrote first rarely meet the limits. Other iMessage providers differ on who can write first, so see the provider comparison.

What each can show

From Flow’s content types docs:

If your agent leans on buttons, WhatsApp is the better stage. If it leans on a personal feel, with tapbacks and quick corrections, iMessage is. Flow converts nothing, so one agent serving both chooses its content by the conversation’s channel: buttons on WhatsApp, the same choices written out as text on iMessage. Tapbacks, typing and effects goes into the iMessage side.

Setup and fees

• WhatsApp. You connect a number in your own WhatsApp Business Account, under your own Meta app, with a live key. You create a Meta app, add the number with an approved display name, add a payment method in WhatsApp Manager and generate a system-user token. Meta bills its fees to you directly. The Flow team can also connect a number. There is no WhatsApp sandbox yet, so build on Telegram first with the same code.
• iMessage. A US number, shared or dedicated: a free trial on the shared line for 25 people, a $25 a month shared line for 300 people a month (200 a day), or your own number for $100 a month (1,000 people a day; a new line starts at 250 a day and ramps to 1,000 over its first 14 days). Flow reviews a new dedicated line, then sets it up, usually within a day.

iMessage is priced on its own: a free trial on the shared line, a $25 a month shared line for 300 people a month, and a $100 a month dedicated line. Writing first comes with the $40 a month Outbound add-on on dedicated lines, and the conversations it starts ramp from 2 a day to 40 a day over 21 days. iMessage API pricing has worked examples.

Voice notes and identity

WhatsApp users send many voice notes. Each arrives as voice content with the audio file. Flow makes no transcripts on any channel, so fetch the audio and transcribe it yourself if your agent needs the words. On iMessage, a voice note also carries the transcript iMessage itself made, when it made one. iMessage supports voice notes both ways, from mp3, wav, m4a, caf or aac files.

Identity differs. WhatsApp numbers are normalised to E.164. An iMessage handle can be a phone number or an email address, so do not assume a number. Do not assume the two are the same person either. Map them only through a verified account lookup. One agent on three channels explains.

Build one agent for both

The loop is the same on both: an inbound event, a model call with your tools, a reply. Three habits keep it portable:

1. Choose by channel. event.conversation.channel says where the person is. Each endpoint’s docs page lists the channels it works on.
2. Handle 422 unsupported_content. Flow sends nothing it cannot deliver exactly, and never converts markdown or buttons, so send a substitute (plain text) with a new idempotency key.
3. Retry only what clears. rate_limited carries retry_after, and outside_window clears only when the person writes again, or when you send a template on WhatsApp.

Flow gives you one project that answers on iMessage, WhatsApp and Telegram, and works as an MCP server and CLI.

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