Flow iMessage API: blue-bubble chat for AI agents

Flow Messaging is a REST API (with a WebSocket stream, a TypeScript and a Python SDK, and an MCP server for development) that connects an AI agent to iMessage, Telegram and WhatsApp. People text a Flow phone line; each message reaches your agent as a signed webhook or stream event, and the agent replies with one endpoint per message kind: text and media, buttons, reactions, typing indicators, effects, edits and unsends. How it reaches iMessage: Apple has no public server API for iMessage (its developer routes are Messages app extensions and the separate Messages for Business service), so Flow runs hosted iMessage lines, which are US phone numbers, either a line shared with other Flow customers or one dedicated to you. Flow says you never need a Mac, and its terms say the lines run through an “iMessage line provider” that it does not name. It does not say whether that provider uses Mac relays, Apple IDs or other hardware, so we could not verify the mechanism. People who are not on iMessage get SMS or RCS from the same number. Flow says it is not affiliated with or endorsed by Apple.

Documented request, not run by us: needs a Flow key, and iMessage needs a signed-in Flow account (we did not create one). The response below is what we did run on 2026-10-27.

# Run without a key on 2026-10-27: shows the auth scheme
curl -sS https://api.flow.engineer/v0/app

# Not run: reply in an iMessage conversation (needs a key from a Flow account)
curl -sS -X POST https://api.flow.engineer/v0/conversations/$CONVERSATION_ID/basic \
  -H "Authorization: Bearer $FLOW_MESSAGING_KEY" \
  -H "Idempotency-Key: evt_123:reply" \
  -H "Content-Type: application/json" \
  -d '{"text":"Your table for two is booked for 7 pm."}'

Response (trimmed)

GET /v0/app (no key), run 2026-10-27: HTTP 401
{"error":{"type":"authentication","message":"No valid API key was given.",
 "hint":"Send the header Authorization: Bearer fk_test_... (or fk_live_...); ...",
 "doc_url":"https://api.flow.engineer/docs/errors/authentication", ...}}

Prompt to give your agent

You are a support agent reached over iMessage through Flow Messaging. When a message.received event arrives, answer only the person who wrote, in plain text under 600 characters. Never ask for or repeat card numbers, bank details, government IDs or health information. If the person writes STOP or asks to stop, confirm once and send nothing more. Send each reply with an Idempotency-Key built from the event id.

Agent / MCP-style tool snippet

// Claude Code, for building and testing (Flow says not to use MCP at runtime):
//   claude mcp add --transport http flow https://api.flow.engineer/mcp \
//     --header "Authorization: Bearer $FLOW_MESSAGING_KEY"
// Runtime: verify Flow-Signature, then reply in the webhook answer
import express from "express";
const app = express();
app.post("/webhook", express.raw({ type: "application/json" }), (req, res) => {
  // check Flow-Signature (HMAC-SHA256 of "t.{t}.{raw body}" with your whsec_ secret) first
  const event = JSON.parse(req.body);
  if (event.type !== "message.received") return res.sendStatus(200);
  res.json({ reply: { kind: "basic", text: "Thanks, looking into it now." } });
});
app.listen(3000);

Put Flow iMessage API side by side with other free agent tools on keys, limits, MCP support, licensing and CORS. Open the comparison grid →

1. 1flow.engineer/pricingVerified 2026-10-27
2. 2api.flow.engineer/llms.txtVerified 2026-10-27
3. 3flow.engineer/termsVerified 2026-10-27
4. 4flow.engineer/acceptable-useVerified 2026-10-27
5. 5flow.engineer/blog/imessage-api-without-macVerified 2026-10-27

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