diff --git a/frontend/packages/swarm-ui/src/ui/dialog/Dialog.css b/frontend/packages/swarm-ui/src/ui/dialog/Dialog.css
index 4feeaef4..ee4d4b96 100644
--- a/frontend/packages/swarm-ui/src/ui/dialog/Dialog.css
+++ b/frontend/packages/swarm-ui/src/ui/dialog/Dialog.css
@@ -3,6 +3,12 @@
    column below its existing 16em-per-column threshold — see
    CreateAgentForm.css — so this doesn't need to special-case that). */
 .ui-dialog {
+  /* No sitewide `box-sizing: border-box` reset exists — without this,
+     the padding + border below add ON TOP of `width: 90vw` rather than
+     being carved out of it, overflowing a real phone-width viewport
+     (measured: 401px rendered against a 390px viewport). Caught via a
+     real screenshot at 390px, not assumed. */
+  box-sizing: border-box;
   position: relative;
   width: 90vw;
   max-width: 44em;