From 15ad3e35d16d0bc94bb52aeb960c4f5a7bff24f1 Mon Sep 17 00:00:00 2001
From: b84330808 <b84330808@gmail.com>
Date: Mon, 2 Nov 2026 01:38:30 +0900
Subject: [PATCH] fix(sio-client): do not treat the ack as an error when
 retries is used without ackTimeout

With the `retries` option, the queue wraps the user callback in a handler
whose first argument is the error. Without `ackTimeout` (or `timeout()`),
that handler was registered as-is, so it received the server response
directly: `cb(42)` from the server was read as an error, the packet was
sent again until the retries were exhausted, and the user callback got
`42` as the error. `emitWithAck()` rejected with the response value.

The queue handler is now flagged with `withError`, so the response is
always prepended with `null`, like when `ackTimeout` is set.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 packages/socket.io-client/lib/socket.ts |  7 +++--
 packages/socket.io-client/test/retry.ts | 35 +++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/packages/socket.io-client/lib/socket.ts b/packages/socket.io-client/lib/socket.ts
index cc8ff87df0..0d57a61928 100644
--- a/packages/socket.io-client/lib/socket.ts
+++ b/packages/socket.io-client/lib/socket.ts
@@ -544,7 +544,7 @@ export class Socket<
       flags: Object.assign({ fromQueue: true }, this.flags),
     };
 
-    args.push((err, ...responseArgs) => {
+    const onResponse = (err, ...responseArgs) => {
       if (packet !== this._queue[0]) {
         return debug("packet [%d] already acknowledged", packet.id);
       }
@@ -570,7 +570,10 @@ export class Socket<
       }
       packet.pending = false;
       return this._drainQueue();
-    });
+    };
+    // the first argument is always the error (or null), even without the "ackTimeout" option
+    onResponse.withError = true;
+    args.push(onResponse);
 
     this._queue.push(packet);
     this._drainQueue();
diff --git a/packages/socket.io-client/test/retry.ts b/packages/socket.io-client/test/retry.ts
index 9ceca208b1..13368877ed 100644
--- a/packages/socket.io-client/test/retry.ts
+++ b/packages/socket.io-client/test/retry.ts
@@ -111,6 +111,41 @@ describe("retry", () => {
     });
   });
 
+  it("should not treat the acknowledgement as an error without ackTimeout", () => {
+    return wrap((done) => {
+      const socket = io(BASE_URL, {
+        forceNew: true,
+        retries: 3,
+      });
+
+      const sent: string[] = [];
+
+      socket.io.engine.on("packetCreate", ({ data }) => {
+        sent.push(data);
+      });
+
+      socket.emit("echo", 1, (err, val) => {
+        expect(err).to.be(null);
+        expect(val).to.eql(1);
+        expect(sent).to.eql(["0", '20["echo",1]']);
+
+        success(done, socket);
+      });
+    });
+  });
+
+  it("should resolve emitWithAck() without ackTimeout", async () => {
+    const socket = io(BASE_URL, {
+      forceNew: true,
+      retries: 3,
+    });
+
+    const val = await socket.emitWithAck("echo", 2);
+    expect(val).to.eql(2);
+
+    socket.disconnect();
+  });
+
   it("should not emit a packet twice in the 'connect' handler", () => {
     return wrap((done) => {
       const socket = io(BASE_URL, {