diff --git a/server/__tests__/pty-attach.test.js b/server/__tests__/pty-attach.test.js
index 8c65654..dc08a76 100644
--- a/server/__tests__/pty-attach.test.js
+++ b/server/__tests__/pty-attach.test.js
@@ -5,10 +5,11 @@
  * or PTY spawned).
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
-const { describe, it, beforeEach } = require("node:test");
+const { describe, it, beforeEach, afterEach } = require("node:test");
 const assert = require("node:assert/strict");
 const { EventEmitter } = require("node:events");
 const ptyAttach = require("../lib/pty-attach");
+const tmux = require("../lib/tmux");
 
 function makeFakePty() {
   const emitter = new EventEmitter();
@@ -41,6 +42,10 @@ describe("pty-attach", () => {
   beforeEach(() => {
     fakePty = makeFakePty();
     ptyAttach.__setSpawnImpl(() => fakePty);
+    tmux.__setExecImpl(() => "");
+  });
+  afterEach(() => {
+    tmux.__reset();
   });
 
   it("rejects a runId that doesn't match ccam-lane-<digits>", () => {
@@ -52,6 +57,25 @@ describe("pty-attach", () => {
     assert.doesNotThrow(() => ptyAttach.validateRunId("ccam-lane-42"));
   });
 
+  it("turns tmux mouse mode on for the session it attaches to", () => {
+    const calls = [];
+    tmux.__setExecImpl((args) => {
+      calls.push(args);
+      return "";
+    });
+    ptyAttach.attach(makeFakeWs(), "ccam-lane-1", { cols: 80, rows: 24 });
+    assert.deepEqual(calls[0], ["set-option", "-t", "ccam-lane-1", "mouse", "on"]);
+  });
+
+  it("still attaches even if the mouse-mode set-option call fails", () => {
+    tmux.__setExecImpl(() => {
+      throw new Error("no such session: ccam-lane-1");
+    });
+    assert.doesNotThrow(() =>
+      ptyAttach.attach(makeFakeWs(), "ccam-lane-1", { cols: 80, rows: 24 })
+    );
+  });
+
   it("wires PTY data to binary WS frames and WS binary frames to PTY writes", () => {
     const ws = makeFakeWs();
     ptyAttach.attach(ws, "ccam-lane-1", { cols: 80, rows: 24 });
diff --git a/server/lib/pty-attach.js b/server/lib/pty-attach.js
index 1d8fe34..60cfc0d 100644
--- a/server/lib/pty-attach.js
+++ b/server/lib/pty-attach.js
@@ -10,6 +10,8 @@
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
+const tmux = require("./tmux");
+
 const RUN_ID_RE = /^ccam-lane-\d+$/;
 
 /**
@@ -38,6 +40,14 @@ function __setSpawnImpl(fn) {
  */
 function attach(ws, runId, { cols, rows }) {
   validateRunId(runId);
+  // Retroactively covers sessions started before mouse mode was wired into
+  // newSession (or before a server restart picked that change up) — see
+  // tmux.js's enableMouse.
+  try {
+    tmux.enableMouse(runId);
+  } catch {
+    /* session gone or tmux unavailable — attach below will surface that */
+  }
   const pty = spawnImpl("tmux", ["attach-session", "-t", runId], {
     name: "xterm-256color",
     cols: cols || 80,
diff --git a/server/lib/tmux.js b/server/lib/tmux.js
index b6fe237..677b470 100644
--- a/server/lib/tmux.js
+++ b/server/lib/tmux.js
@@ -40,10 +40,17 @@ function hasSession(name) {
  */
 function newSession({ name, cwd, argv }) {
   execImpl(["new-session", "-d", "-s", name, "-c", cwd, "--", ...argv]);
-  // Without this the pane has no scrollbar/drag scroll at all — wheel events
-  // just pass through to the running program instead of entering tmux's own
-  // copy-mode scrollback. xterm.js forwards tmux's mouse-tracking escapes
-  // automatically once mouse mode is on, so no client-side change is needed.
+  enableMouse(name);
+}
+
+// Without this the pane has no scrollbar/drag scroll at all — wheel events
+// just pass through to the running program instead of entering tmux's own
+// copy-mode scrollback. xterm.js forwards tmux's mouse-tracking escapes
+// automatically once mouse mode is on, so no client-side change is needed.
+// Called on every attach (not just at creation) so a session started before
+// this option existed — or before a server restart picked up the change —
+// still gets it; `set-option` is idempotent, so re-running it is harmless.
+function enableMouse(name) {
   execImpl(["set-option", "-t", name, "mouse", "on"]);
 }
 
@@ -112,6 +119,7 @@ function isTmuxAvailable() {
 module.exports = {
   hasSession,
   newSession,
+  enableMouse,
   paneCommand,
   sendCommand,
   killSession,