diff --git a/static/ui.js b/static/ui.js
index ebeb7675..791288ea 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -1531,14 +1531,28 @@ function _reconcileModelDropdownSelection(sel,data,previousState,opts){
   // rebuild should preserve the loaded session model or the user's current
   // in-page selection when it still exists in the refreshed catalog.
   const shouldApplyBootDefault=!!(opts&&opts.preferProfileDefaultOnFreshBoot);
+
+  // Helper: apply the requested model, but if it is missing from the current
+  // catalog (cross-provider selection after a partial/timed-out rebuild), inject
+  // it as a custom option instead of returning null and letting the browser
+  // silently snap to the first <option>. _ensureModelOptionInDropdown already
+  // tries _applyModelToDropdown first, so delegate to it (single scan) and keep
+  // the plain-apply fallback for the unlikely case it is unavailable.
+  const _applyOrEnsure = function(modelId, providerId) {
+    if (typeof _ensureModelOptionInDropdown === 'function') {
+      return _ensureModelOptionInDropdown(modelId, sel, providerId);
+    }
+    return _applyModelToDropdown(modelId, sel, providerId);
+  };
+
   if(shouldApplyBootDefault && data&&data.default_model && !(activeSession&&activeSession.model)){
-    return _applyModelToDropdown(data.default_model,sel,data.active_provider||null);
+    return _applyOrEnsure(data.default_model, data.active_provider||null);
   }
   if(activeSession&&activeSession.model){
-    return _applyModelToDropdown(activeSession.model,sel,activeSession.model_provider||null);
+    return _applyOrEnsure(activeSession.model, activeSession.model_provider||null);
   }
   if(previousState&&previousState.model){
-    return _applyModelToDropdown(previousState.model,sel,previousState.model_provider||null);
+    return _applyOrEnsure(previousState.model, previousState.model_provider||null);
   }
   return null;
 }
diff --git a/tests/test_model_default_boot_precedence.py b/tests/test_model_default_boot_precedence.py
index b276bf71..beec74e8 100644
--- a/tests/test_model_default_boot_precedence.py
+++ b/tests/test_model_default_boot_precedence.py
@@ -196,9 +196,16 @@ def test_populate_model_dropdown_reconciles_selection_after_rebuild():
     assert "_reconcileModelDropdownSelection(sel,data,previousSelection,opts);" in UI_JS
     snippet = _reconcile_selection_snippet()
     assert "preferProfileDefaultOnFreshBoot" in snippet
-    assert "return _applyModelToDropdown(data.default_model,sel,data.active_provider||null);" in snippet
-    assert "return _applyModelToDropdown(activeSession.model,sel,activeSession.model_provider||null);" in snippet
-    assert "return _applyModelToDropdown(previousState.model,sel,previousState.model_provider||null);" in snippet
+    # #4363: each branch now routes through _applyOrEnsure, which delegates to
+    # _ensureModelOptionInDropdown so a cross-provider model missing from a
+    # partially-rebuilt catalog is injected as a custom option instead of the
+    # browser silently snapping the <select> to its first <option>. The
+    # branch ORDER + per-branch model/provider arguments are unchanged.
+    assert "_applyOrEnsure(data.default_model, data.active_provider||null)" in snippet
+    assert "_applyOrEnsure(activeSession.model, activeSession.model_provider||null)" in snippet
+    assert "_applyOrEnsure(previousState.model, previousState.model_provider||null)" in snippet
+    # the helper must fall back to injecting the missing option, not return null
+    assert "_ensureModelOptionInDropdown(modelId, sel, providerId)" in snippet
     assert "_readPersistedModelState()" not in snippet
     assert "localStorage.getItem('hermes-webui-model')" not in snippet