diff --git a/imgui.cpp b/imgui.cpp
index c1fff701ffd9..ba7bf994b4e3 100644
--- a/imgui.cpp
+++ b/imgui.cpp
@@ -11163,15 +11163,18 @@ bool ImGui::InputTextEx(const char* label, char* buf, int buf_size, const ImVec2
                 callback_data.EventFlag = ImGuiInputTextFlags_CallbackResize;
                 callback_data.Flags = flags;
                 callback_data.Buf = buf;
-                callback_data.BufTextLen = edit_state.CurLenA;
+                callback_data.BufTextLen = apply_new_text_length;
                 callback_data.BufSize = ImMax(buf_size, apply_new_text_length + 1);
                 callback_data.UserData = callback_user_data;
                 callback(&callback_data);
                 buf = callback_data.Buf;
                 buf_size = callback_data.BufSize;
+                apply_new_text_length = ImMin(callback_data.BufTextLen, buf_size - 1);
+                IM_ASSERT(apply_new_text_length <= buf_size);
             }
-            IM_ASSERT(apply_new_text_length <= buf_size);
-            ImStrncpy(buf, edit_state.TempBuffer.Data, apply_new_text_length + 1);
+
+            // If the underlying buffer resize was denied or not carried to the next frame, apply_new_text_length+1 may be >= buf_size.
+            ImStrncpy(buf, edit_state.TempBuffer.Data, ImMin(apply_new_text_length + 1, buf_size));
             value_changed = true;
         }
 
diff --git a/misc/stl/imgui_stl.cpp b/misc/stl/imgui_stl.cpp
index c504e98ae412..d3d5e35d6754 100644
--- a/misc/stl/imgui_stl.cpp
+++ b/misc/stl/imgui_stl.cpp
@@ -34,6 +34,7 @@ static int InputTextCallback(ImGuiInputTextCallbackData* data)
 
 bool ImGui::InputText(const char* label, std::string* str, ImGuiInputTextFlags flags, ImGuiInputTextCallback callback, void* user_data)
 {
+    IM_ASSERT((flags & ImGuiInputTextFlags_CallbackResize) == 0);
     flags |= ImGuiInputTextFlags_CallbackResize;
 
     InputTextCallback_UserData cb_user_data;
@@ -45,6 +46,7 @@ bool ImGui::InputText(const char* label, std::string* str, ImGuiInputTextFlags f
 
 bool ImGui::InputTextMultiline(const char* label, std::string* str, const ImVec2& size, ImGuiInputTextFlags flags, ImGuiInputTextCallback callback, void* user_data)
 {
+    IM_ASSERT((flags & ImGuiInputTextFlags_CallbackResize) == 0);
     flags |= ImGuiInputTextFlags_CallbackResize;
 
     InputTextCallback_UserData cb_user_data;