fix(irc): preserve one-unit chunk limits

steipete · steipete · commit 44f75d394d63 · 2026-07-05T19:52:35.000+01:00
diff --git a/extensions/irc/src/client.surrogate.test.ts b/extensions/irc/src/client.surrogate.test.ts
@@ -142,6 +142,16 @@ describe("irc client PRIVMSG chunking", () => {
     client.close();
   });
 
+  it("preserves one-unit chunks for BMP text", async () => {
+    const { client, socket } = await connectReadyClient(1);
+
+    client.sendPrivmsg("#room", "ABC");
+
+    expect(privmsgBodies(socket)).toEqual(["A", "B", "C"]);
+
+    client.close();
+  });
+
   it("splits an all-emoji message into whole emoji when the budget is one code unit", async () => {
     const { client, socket } = await connectReadyClient(1);
     const text = "\u{1F642}\u{1F642}\u{1F642}";
diff --git a/extensions/irc/src/client.ts b/extensions/irc/src/client.ts
@@ -11,6 +11,17 @@ import {
   sanitizeIrcTarget,
 } from "./protocol.js";
 
+function sliceIrcPrivmsgChunk(text: string, end: number): string {
+  const sliced = sliceUtf16Safe(text, 0, end);
+  if (sliced || end <= 0) {
+    return sliced;
+  }
+  // A one-unit budget cannot contain an astral code point. Emit that point
+  // whole so chunking advances without changing one-unit behavior for BMP text.
+  const firstCodePoint = text.codePointAt(0);
+  return firstCodePoint !== undefined && firstCodePoint > 0xffff ? text.slice(0, 2) : sliced;
+}
+
 const IRC_ERROR_CODES = new Set(["432", "464", "465"]);
 const IRC_NICK_COLLISION_CODES = new Set(["433", "436"]);
 
@@ -108,8 +119,7 @@ export function buildIrcNickServCommands(options?: IrcNickServOptions): string[]
 
 export async function connectIrcClient(options: IrcClientOptions): Promise<IrcClient> {
   const timeoutMs = options.connectTimeoutMs != null ? options.connectTimeoutMs : 15000;
-  // Two UTF-16 code units are the smallest budget that can hold one full code point.
-  const messageChunkMaxChars = Math.max(2, Math.floor(options.messageChunkMaxChars ?? 350));
+  const messageChunkMaxChars = Math.max(1, Math.floor(options.messageChunkMaxChars ?? 350));
 
   if (!options.host.trim()) {
     throw new Error("IRC host is required");
@@ -221,7 +231,7 @@ export async function connectIrcClient(options: IrcClientOptions): Promise<IrcCl
         if (splitAt < Math.floor(messageChunkMaxChars / 2)) {
           splitAt = messageChunkMaxChars;
         }
-        chunk = sliceUtf16Safe(chunk, 0, splitAt).trim();
+        chunk = sliceIrcPrivmsgChunk(chunk, splitAt).trim();
       }
       if (!chunk) {
         break;
