Index: lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp
===================================================================
--- lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp
+++ lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp
@@ -1412,7 +1412,10 @@
     return SendErrorResponse(69);
 
   StreamString response;
-  response.Printf("QC%" PRIx64, thread->GetID());
+  response.PutCString("QC");
+  if (bool(m_extensions_supported & NativeProcessProtocol::Extension::multiprocess))
+    response.Format("p{0:x-}.", m_current_process->GetID());
+  response.Format("{0:x-}", thread->GetID());
 
   return SendPacketNoLock(response.GetString());
 }
Index: lldb/test/API/tools/lldb-server/TestGdbRemoteFork.py
===================================================================
--- lldb/test/API/tools/lldb-server/TestGdbRemoteFork.py
+++ lldb/test/API/tools/lldb-server/TestGdbRemoteFork.py
@@ -817,3 +817,56 @@
             data = ret.get("data")
             self.assertIsNotNone(data)
             self.assertEqual(data, old_val[1])
+
+    @add_test_categories(["fork"])
+    def test_qC(self):
+        self.build()
+        self.prep_debug_monitor_and_inferior(
+            inferior_args=["fork",
+                           "thread:new",
+                           "trap",
+                           ])
+        self.add_qSupported_packets(["multiprocess+",
+                                     "fork-events+"])
+        ret = self.expect_gdbremote_sequence()
+        self.assertIn("fork-events+", ret["qSupported_response"])
+        self.reset_test_sequence()
+
+        # continue and expect fork
+        self.test_sequence.add_log_lines([
+            "read packet: $c#00",
+            {"direction": "send", "regex": self.fork_regex.format("fork"),
+             "capture": self.fork_capture},
+        ], True)
+        self.add_threadinfo_collection_packets()
+        ret = self.expect_gdbremote_sequence()
+        pidtids = [
+            (ret["parent_pid"], ret["parent_tid"]),
+            (ret["child_pid"], ret["child_tid"]),
+        ]
+        self.reset_test_sequence()
+
+        for pidtid in pidtids:
+            self.test_sequence.add_log_lines(
+                ["read packet: $Hcp{}.{}#00".format(*pidtid),
+                 "send packet: $OK#00",
+                 "read packet: $c#00",
+                 {"direction": "send",
+                  "regex": "^[$]T05thread:p{}.{}.*".format(*pidtid),
+                  },
+                 ], True)
+
+        self.add_threadinfo_collection_packets()
+        ret = self.expect_gdbremote_sequence()
+        self.reset_test_sequence()
+
+        pidtids = set(self.parse_threadinfo_packets(ret))
+        self.assertEqual(len(pidtids), 4)
+        for pidtid in pidtids:
+            self.test_sequence.add_log_lines(
+                ["read packet: $Hgp{:x}.{:x}#00".format(*pidtid),
+                 "send packet: $OK#00",
+                 "read packet: $qC#00",
+                 "send packet: $QCp{:x}.{:x}#00".format(*pidtid),
+                 ], True)
+        self.expect_gdbremote_sequence()