diff --git a/src/ui/windows/chat_tab.cpp b/src/ui/windows/chat_tab.cpp
index b04ae8b..7e26819 100644
--- a/src/ui/windows/chat_tab.cpp
+++ b/src/ui/windows/chat_tab.cpp
@@ -1653,7 +1653,10 @@ void RenderChatTab(App* app)
                         if (cbhov) {
                             ImGui::SetMouseCursor(ImGuiMouseCursor_Hand);
                             if (ImGui::IsMouseClicked(ImGuiMouseButton_Left)) {
-                                ImGui::SetClipboardText(m.body.substr(a, b2 - a).c_str());
+                                // Offsets can outlive a thread change (an older message inserted above shifts
+                                // indices): never let substr throw std::out_of_range from the render loop.
+                                if (a >= 0 && static_cast<std::size_t>(a) <= m.body.size())
+                                    ImGui::SetClipboardText(m.body.substr(a, b2 - a).c_str());
                                 anyMsgClicked = true;
                             }
                         }