Cut a long text message between characters, not inside an emoji

CHANGELOG.md

@@ -78,6 +78,12 @@ Newest first. `Unreleased` is what is on `main` and not yet tagged.
   `COMPUTER_MEMORY_HIGH_PERCENT=0` and `COMPUTER_TAB_IDLE_MS=0` (Helm: `computers.memoryHighPercent`
   and `computers.tabIdleMs`) keep the old behaviour.
 
+### A long text message is cut between characters
+
+A message too long for one SMS is cut and ends with "Open OpenBot to read the full message.". When
+the cut landed inside an emoji, the person received a "�" before the "…". The cut now drops the
+half character instead.
+
 ### A computer closes browsers nobody is using when its memory runs high
 
 A computer runs one browser for each Bot that is browsing, and the only limits were a count and half

server/src/delivery/twilio.ts

@@ -1,4 +1,5 @@
 import { validateRequest } from "twilio";
+import { cutAtCodeUnits } from "../channels/text";
 import { DELIVERY_REQUEST_TIMEOUT_MS } from "../timeouts";
 import {
   DeliveryProviderError,
@@ -119,7 +120,7 @@ export function createTwilioTransport(config: TwilioConfig) {
           Body:
             input.text.length <= 1500
               ? input.text
-              : `${input.text.slice(0, 1400)}…\nOpen OpenBot to read the full message.`,
+              : `${cutAtCodeUnits(input.text, 1400)}…\nOpen OpenBot to read the full message.`,
           ...(config.statusUrl ? { StatusCallback: config.statusUrl } : {}),
         },
       );

server/tests/delivery-providers.test.ts

@@ -182,6 +182,27 @@ describe("real HTTP provider adapters", () => {
       new URLSearchParams(String(wire.requests[2]?.init?.body)).get("To"),
     ).toBe("+15551234567");
   });
+  test("Twilio cuts a long SMS between characters, not inside an emoji", async () => {
+    const wire = http([{ sid: "SM1", status: "queued" }]);
+    const twilio = createTwilioTransport({
+      accountSid: "AC1",
+      authToken: "secret",
+      verifyServiceSid: "VA1",
+      from: "+15550000000",
+      webhookUrl: "https://o.test/sms",
+      fetch: wire.fetch,
+    });
+    // 1,399 letters and then an emoji: the 1,400-unit cut lands between its two halves.
+    const text = `${"a".repeat(1399)}😀${"b".repeat(200)}`;
+    await twilio.send({ address: "+15551234567", text, id: "out1" });
+    const body =
+      new URLSearchParams(String(wire.requests[0]?.init?.body)).get("Body") ??
+      "";
+    expect(body).not.toContain("\uFFFD");
+    expect(body).toBe(
+      `${"a".repeat(1399)}…\nOpen OpenBot to read the full message.`,
+    );
+  });
   test("Expo sends canonical deep links and reads receipts including invalid devices", async () => {
     const wire = http([
       { data: { status: "ok", id: "ticket1" } },