diff --git a/docs/architecture.md b/docs/architecture.md
index bfbf2b8c1..4794a6ec9 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -34,7 +34,9 @@ including tests, 2026-10-10.
    enabled, run, journal, undoable }`; ids follow Excel's ribbon (`home.bold`, `insert.chart`,
    `data.sortAscending`) plus primitives (`cell.set`, `selection.set`, `range.setValues`).
    `Session::execute` catches escaped panics and restores the workbook, its selection and its
-   undo and redo history as they were before the command (never-crash guard).
+   undo and redo history as they were before the command (never-crash guard); a group that was
+   open before the command and that the command ended or rolled back stays ended and keeps a
+   step from its start to the restored workbook, so Undo still returns to the state before it.
 2. **Edits** change the model through copy-on-write bands; undo keeps the previous snapshot.
    Undo retains at most 100 steps and trims older steps when its estimated retained cell storage
    exceeds 128 MiB. The newest step remains undoable even if that step alone exceeds the budget.