Send iMessage programmatically: curl, TypeScript, Python

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 short answer

You cannot call Apple directly, so you call a provider. The flow is the same everywhere:

1. Get an API key.
2. Receive an event when someone writes to your line.
3. POST a reply into the same conversation.

The sections below show the Flow send loop in curl, TypeScript and Python. For what an iMessage API is and why Apple has none, start with the iMessage API guide.

Get a key and a first message

One thing to know first. Flow’s self-serve test key works on a Telegram sandbox bot. For iMessage, signing in at flow.engineer gives you a free trial on Flow’s shared iMessage line for 25 people, and there are dedicated lines. The handler you write against the sandbox is the same one that answers on iMessage, so you can build and test now.

Create a test key with no account. From your project folder:

npx @flow-engineer/messaging init

That writes FLOW_MESSAGING_KEY to .env and prints a sandbox link. Open the link on your phone and tap Start. Or use plain HTTP:

curl -X POST https://api.flow.engineer/v0/sandbox/keys

A test key allows one contact and 50 outbound messages and expires after seven days. Sign in with a Flow account to keep the app and get a larger allowance.

Now read an inbound message and reply to it. This is curl:

# Read the newest inbound messages (oldest first).
curl -G https://api.flow.engineer/v0/events \
  -H "Authorization: Bearer $FLOW_MESSAGING_KEY" \
  --data-urlencode "type=message.received"

# Reply into the conversation named in the event.
curl https://api.flow.engineer/v0/conversations/conv_01JB8ZC3K5M7P9R1T3V5X7Z9B1/basic \
  -H "Authorization: Bearer $FLOW_MESSAGING_KEY" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: $(uuidgen)" \
  -d '{"text": "Hello from my agent"}'

TypeScript:

// npm install @flow-engineer/messaging
// node --env-file=.env agent.ts   (Node 22.18+; older Node: npx tsx --env-file=.env agent.ts)
import { FlowMessaging, contentText } from "@flow-engineer/messaging";

const flow = new FlowMessaging();
for await (const event of flow.events.stream({ types: ["message.received"] })) {
  const said = contentText(event.data.message.content);
  await event.conversation.reply("You said: " + said, { idempotencyKey: event.id });
}

Python:

# pip install flow-messaging
from flow_messaging import Flow, content_text

flow = Flow()
for event in flow.events.stream(types=["message.received"]):
    said = content_text(event.data.message.content)
    flow.messages.send(
        event.conversation.id,
        f"You said: {said}",
        idempotency_key=event.id,
    )

A send is queued first. Watch message.sent, message.delivered or message.failed to learn what happened. These samples come from Flow’s quickstart.

Receive replies with a signed webhook

A stream is fine for scripts. In production you register an HTTPS endpoint with POST /v0/webhook_endpoints, keep the whsec_... secret it returns, and verify every delivery. Flow sends the signature in a Flow-Signature header as t=<unix>,v1=<hex HMAC-SHA256 of "t.<t>.<raw body>">.

This is the verification from Flow’s docs, in a Next.js route:

import { createHmac, timingSafeEqual } from "node:crypto";

function verify(body: string, header: string, secret: string): boolean {
  const parts = header.split(",").map((p) => p.trim().split("="));
  const t = parts.find(([k]) => k === "t")?.[1];
  if (!t || Math.abs(Date.now() / 1000 - Number(t)) > 300) return false;
  const want = createHmac("sha256", secret).update(`t.${t}.${body}`).digest();
  return parts.some(([k, v = ""]) => k === "v1" && /^[0-9a-f]{64}$/.test(v) && timingSafeEqual(want, Buffer.from(v, "hex")));
}

export async function POST(req: Request) {
  const body = await req.text(); // the raw body: a re-serialized one will not verify
  if (!verify(body, req.headers.get("Flow-Signature") ?? "", process.env.FLOW_MESSAGING_WEBHOOK_SECRET!)) {
    return new Response(null, { status: 400 });
  }
  const event = JSON.parse(body);
  if (event.type !== "message.received") return Response.json({});
  return Response.json({ reply: `You said: ${event.data.message.content.text ?? ""}` });
}

Three rules matter more than the code:

• Use the raw body. If a framework parses and re-serializes it, the signature will not match.
• Deduplicate on the event id. Deliveries are at least once. Flow retries for up to three days, so a slow endpoint can see the same event again.
• Answer fast, work later. Return quickly and do slow model calls in a queue, then send the reply with the event ID as the idempotency key.

If your server is down, nothing is lost: the same events are in a log you can read with GET /v0/events?after=evt_... to catch up. The log keeps events for 30 days by default.

Send media

On Flow, upload the file first. The files endpoint checks the real file type from its first bytes, scans documents for malware, and returns a file_id you send in a basic message’s media, one file per message with an optional caption, or as a voice note. iMessage takes images, video, audio and documents, and voice notes from mp3, wav, m4a, caf or aac files. Text can be up to 9,999 characters.

Two details that surprise people. Flow cuts reply bubbles at about 400 characters on iMessage, at a sentence end, because people read iMessage as short lines. And iMessage has no buttons. Flow converts nothing, so a buttons message to an iMessage conversation is refused with 422 unsupported_content and nothing is sent: list the options in the text instead, and read the person’s answer as text.

Handle a recipient without iMessage

On Flow, when a customer cannot get iMessage the same conversation continues over SMS or RCS from the same number, so your handler does not change. That fallback is included on every plan. Ask any provider what your app sees when a message changes channel, and store the real channel. Customers text the line first, so you only reply to someone who is already on iMessage. Writing first comes with the $40 a month Outbound add-on on a dedicated line, and the conversations it starts ramp from 2 a day to 40 a day over 21 days; from any other line, a start is refused with 403 permission.

Handle errors and retries

A good send path is mostly error handling. Flow answers refusals with a typed error that carries a type, a hint and a doc_url, so you can branch on the type instead of parsing text. The ones you will meet on iMessage:

Retry only what clears by itself. rate_limited and sender_throttled carry retry_after. outside_window does not clear until the person writes again.

Idempotency keys are kept for 24 hours. A repeat returns the first answer with an Idempotent-Replayed: true header, so a retry never sends twice. Answers worth retrying (429, 500, 502, 503 and 504) are not kept, so the retry runs again. One case the key cannot cover is a channel that takes a send and never answers. Flow then reports channel_timeout_unknown_outcome, and you should check the conversation before you send again.

The DIY route: a Mac and a relay

You can also run a Mac that stays signed in to Messages and relay requests to it. That can work for a personal project. For a product you then own the hardware, the Apple ID, the uptime and every Apple change. A hosted provider takes that on, which is most of what you pay for.

Frequently asked questions

Keep reading

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