diff --git a/package.json b/package.json
index ab1950522..e68818651 100644
--- a/package.json
+++ b/package.json
@@ -93,7 +93,7 @@
     "moment-duration-format": "^2.3.2",
     "moment-timezone": "^0.5.33",
     "mui-color-input": "^9.0.0",
-    "openstack-uicore-foundation": "5.0.58",
+    "openstack-uicore-foundation": "5.0.62",
     "p-limit": "^6.1.0",
     "path-browserify": "^1.0.1",
     "postcss-loader": "^6.2.1",
diff --git a/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/__tests__/edit-cart-form.test.js b/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/__tests__/edit-cart-form.test.js
index 6482f2aee..2c421ee83 100644
--- a/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/__tests__/edit-cart-form.test.js
+++ b/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/__tests__/edit-cart-form.test.js
@@ -98,6 +98,7 @@ import configureMockStore from "redux-mock-store";
 import thunk from "redux-thunk";
 import showConfirmDialog from "../../../../../../../../components/mui/showConfirmDialog";
 import EditCartForm from "../edit-cart-form";
+import { buildGlobalQuantitySchema } from "../quantity-schema";
 /* eslint-enable import/first */
 
 const middlewares = [thunk];
@@ -577,4 +578,71 @@ describe("EditCartForm", () => {
       });
     });
   });
+
+  describe("EditForm - buildInitialValues (out-of-stock default_quantity)", () => {
+    const buildQuantityInitialValue = (item) => {
+      const hasStock =
+        !item.is_sold_out && item.remaining_quantity_sponsor !== 0;
+      return hasStock
+        ? item.quantity || item.default_quantity || 0
+        : item.quantity || 0;
+    };
+
+    it("ignores default_quantity when the item is sold out for the show", () => {
+      const item = { is_sold_out: true, default_quantity: 1 };
+      expect(buildQuantityInitialValue(item)).toBe(0);
+    });
+
+    it("ignores default_quantity when the sponsor's remaining quantity is 0", () => {
+      const item = { remaining_quantity_sponsor: 0, default_quantity: 1 };
+      expect(buildQuantityInitialValue(item)).toBe(0);
+    });
+
+    it("keeps the sponsor's own existing quantity even when out of stock", () => {
+      const item = { is_sold_out: true, quantity: 3, default_quantity: 1 };
+      expect(buildQuantityInitialValue(item)).toBe(3);
+    });
+
+    it("still applies default_quantity when the item has stock", () => {
+      const item = { is_sold_out: false, default_quantity: 1 };
+      expect(buildQuantityInitialValue(item)).toBe(1);
+    });
+  });
+
+  describe("EditForm - buildValidationSchema (quantity cap)", () => {
+    it("rejects a quantity above remaining_quantity_show when it is the tighter axis", async () => {
+      const schema = buildGlobalQuantitySchema({
+        remaining_quantity_show: 2,
+        remaining_quantity_sponsor: 5
+      });
+      await expect(schema.isValid(3)).resolves.toBe(false);
+      await expect(schema.isValid(2)).resolves.toBe(true);
+    });
+
+    it("rejects a quantity above remaining_quantity_sponsor when it is the tighter axis", async () => {
+      const schema = buildGlobalQuantitySchema({
+        remaining_quantity_show: 8,
+        remaining_quantity_sponsor: 3
+      });
+      await expect(schema.isValid(4)).resolves.toBe(false);
+      await expect(schema.isValid(3)).resolves.toBe(true);
+    });
+
+    it("applies no upper bound when both remaining quantities are null", async () => {
+      const schema = buildGlobalQuantitySchema({
+        remaining_quantity_show: null,
+        remaining_quantity_sponsor: null
+      });
+      await expect(schema.isValid(1000)).resolves.toBe(true);
+    });
+
+    it("rejects any positive quantity when remaining_quantity_show is 0 (boundary)", async () => {
+      const schema = buildGlobalQuantitySchema({
+        remaining_quantity_show: 0,
+        remaining_quantity_sponsor: 5
+      });
+      await expect(schema.isValid(1)).resolves.toBe(false);
+      await expect(schema.isValid(0)).resolves.toBe(true);
+    });
+  });
 });
diff --git a/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/index.js b/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/index.js
index 092d71263..53da95a94 100644
--- a/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/index.js
+++ b/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/index.js
@@ -25,6 +25,7 @@ import MuiFormItemTable, {
 } from "openstack-uicore-foundation/lib/components/mui/form-item-table";
 import { DISCOUNT_TYPES } from "../../../../../../../utils/constants";
 import showConfirmDialog from "../../../../../../../components/mui/showConfirmDialog";
+import { buildGlobalQuantitySchema } from "./quantity-schema";
 
 const parseValue = (item, timeZone) => {
   switch (item.type) {
@@ -146,8 +147,10 @@ const buildInitialValues = (form, timeZone) => {
     // add notes
     acc[`i-${item.form_item_id}-c-global-f-notes`] = item.notes || "";
     // if no quantity inputs we add the global quantity input
-    acc[`i-${item.form_item_id}-c-global-f-quantity`] =
-      item.quantity || item.default_quantity || 0;
+    const hasStock = !item.is_sold_out && item.remaining_quantity_sponsor !== 0;
+    acc[`i-${item.form_item_id}-c-global-f-quantity`] = hasStock
+      ? item.quantity || item.default_quantity || 0
+      : item.quantity || 0;
     // custom rate
     acc[`i-${item.form_item_id}-c-global-f-custom_rate`] =
       item.custom_rate || item.rates.custom || 0;
@@ -168,16 +171,7 @@ const buildValidationSchema = (items) => {
     // notes
     acc[`i-${item.form_item_id}-c-global-f-notes`] = yup.string();
     // validation for the global quantity input
-    let globalQtySchema = yup.number().min(0, " ");
-
-    if (item.quantity_limit_per_sponsor > 0) {
-      globalQtySchema = globalQtySchema.max(
-        item.quantity_limit_per_sponsor,
-        " "
-      );
-    }
-    globalQtySchema = globalQtySchema.required(" ");
-    acc[quantityKey] = globalQtySchema;
+    acc[quantityKey] = buildGlobalQuantitySchema(item);
     // custom rate
     acc[`i-${item.form_item_id}-c-global-f-custom_rate`] = yup.number();
 
diff --git a/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/quantity-schema.js b/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/quantity-schema.js
new file mode 100644
index 000000000..7f544cfdb
--- /dev/null
+++ b/src/pages/sponsors/sponsor-page/tabs/sponsor-cart-tab/components/edit-form/quantity-schema.js
@@ -0,0 +1,16 @@
+import * as yup from "yup";
+
+// The sponsor-facing quantity input is capped by whichever axis is tighter:
+// what's left for the show, or what's left for this sponsor specifically.
+// Either field being null means that axis has no cap.
+export const buildGlobalQuantitySchema = (item) => {
+  let schema = yup.number().min(0, " ");
+  const maxQty = Math.min(
+    item.remaining_quantity_show ?? Infinity,
+    item.remaining_quantity_sponsor ?? Infinity
+  );
+  if (Number.isFinite(maxQty)) {
+    schema = schema.max(maxQty, " ");
+  }
+  return schema.required(" ");
+};
diff --git a/yarn.lock b/yarn.lock
index a73dddca0..e8af2c069 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -9042,10 +9042,10 @@ open@^10.0.3:
     is-inside-container "^1.0.0"
     wsl-utils "^0.1.0"
 
-openstack-uicore-foundation@5.0.58:
-  version "5.0.58"
-  resolved "https://registry.yarnpkg.com/openstack-uicore-foundation/-/openstack-uicore-foundation-5.0.58.tgz#641aa67d2d231373dda1e5f5edaebbd68e908320"
-  integrity sha512-9p28r/arDv6Y6RF3KT/xppOyO8gvo+QdZqxGwsk5+6/AEBtcXVD9nRBV2jeMfO0cZKiEe8ocXFElptymSN7WWg==
+openstack-uicore-foundation@5.0.62:
+  version "5.0.62"
+  resolved "https://registry.yarnpkg.com/openstack-uicore-foundation/-/openstack-uicore-foundation-5.0.62.tgz#d7e680d5df00b9456b48186e156804c05826ddcf"
+  integrity sha512-oRtVrVNvvYGvfiwK8gU0mAiXMQJegaQ4mH6vMJtT2vmtAZAhsUf2P8aOT4ie7ER8BggLjYoYG8M2qNyz7Kk1qg==
   dependencies:
     use-sync-external-store "^1.6.0"