diff --git a/server/tests/unit/billing/filter-customer-products-by-processor-type.spec.ts b/server/tests/unit/billing/filter-customer-products-by-processor-type.spec.ts
new file mode 100644
index 000000000..3932f7926
--- /dev/null
+++ b/server/tests/unit/billing/filter-customer-products-by-processor-type.spec.ts
@@ -0,0 +1,138 @@
+/**
+ * Unit tests for filterCustomerProductsByProcessorType.
+ *
+ * Critical invariant: an unset `processor` field on a cus product MUST be
+ * treated as Stripe. Legacy Stripe-managed cus products do not tag the
+ * processor field; only RevenueCat-managed products explicitly set it.
+ */
+
+import { describe, expect, test } from "bun:test";
+import {
+	filterCustomerProductsByProcessorType,
+	type FullCusProduct,
+	ProcessorType,
+} from "@autumn/shared";
+import chalk from "chalk";
+
+const baseCusProduct = (id: string): FullCusProduct =>
+	({
+		id,
+		product: { name: id } as FullCusProduct["product"],
+	}) as FullCusProduct;
+
+describe(
+	chalk.yellowBright("filterCustomerProductsByProcessorType"),
+	() => {
+		test("includes cus product with unset processor when filtering for Stripe", () => {
+			const cp = baseCusProduct("legacy_stripe");
+			// processor is undefined — legacy data shape
+			const result = filterCustomerProductsByProcessorType({
+				customerProducts: [cp],
+				processorType: ProcessorType.Stripe,
+			});
+			expect(result).toHaveLength(1);
+			expect(result[0].id).toBe("legacy_stripe");
+		});
+
+		test("includes cus product with null processor when filtering for Stripe", () => {
+			const cp = {
+				...baseCusProduct("null_stripe"),
+				processor: null,
+			} as unknown as FullCusProduct;
+			const result = filterCustomerProductsByProcessorType({
+				customerProducts: [cp],
+				processorType: ProcessorType.Stripe,
+			});
+			expect(result).toHaveLength(1);
+		});
+
+		test("includes cus product with explicit Stripe processor when filtering for Stripe", () => {
+			const cp = {
+				...baseCusProduct("explicit_stripe"),
+				processor: { type: ProcessorType.Stripe },
+			} as FullCusProduct;
+			const result = filterCustomerProductsByProcessorType({
+				customerProducts: [cp],
+				processorType: ProcessorType.Stripe,
+			});
+			expect(result).toHaveLength(1);
+		});
+
+		test("excludes cus product with unset processor when filtering for RevenueCat", () => {
+			const cp = baseCusProduct("legacy_stripe_excluded");
+			const result = filterCustomerProductsByProcessorType({
+				customerProducts: [cp],
+				processorType: ProcessorType.RevenueCat,
+			});
+			expect(result).toHaveLength(0);
+		});
+
+		test("includes cus product with explicit RevenueCat processor when filtering for RevenueCat", () => {
+			const cp = {
+				...baseCusProduct("rc"),
+				processor: { type: ProcessorType.RevenueCat },
+			} as FullCusProduct;
+			const result = filterCustomerProductsByProcessorType({
+				customerProducts: [cp],
+				processorType: ProcessorType.RevenueCat,
+			});
+			expect(result).toHaveLength(1);
+		});
+
+		test("excludes RevenueCat cus product when filtering for Stripe", () => {
+			const cp = {
+				...baseCusProduct("rc"),
+				processor: { type: ProcessorType.RevenueCat },
+			} as FullCusProduct;
+			const result = filterCustomerProductsByProcessorType({
+				customerProducts: [cp],
+				processorType: ProcessorType.Stripe,
+			});
+			expect(result).toHaveLength(0);
+		});
+
+		test("mixed list: filtering for Stripe keeps unset + explicit Stripe, drops RevenueCat", () => {
+			const legacyStripe = baseCusProduct("legacy_stripe");
+			const explicitStripe = {
+				...baseCusProduct("explicit_stripe"),
+				processor: { type: ProcessorType.Stripe },
+			} as FullCusProduct;
+			const rc = {
+				...baseCusProduct("rc"),
+				processor: { type: ProcessorType.RevenueCat },
+			} as FullCusProduct;
+
+			const result = filterCustomerProductsByProcessorType({
+				customerProducts: [legacyStripe, explicitStripe, rc],
+				processorType: ProcessorType.Stripe,
+			});
+			expect(result.map((cp) => cp.id).sort()).toEqual(
+				["explicit_stripe", "legacy_stripe"].sort(),
+			);
+		});
+
+		test("mixed list: filtering for RevenueCat keeps only explicit RC", () => {
+			const legacyStripe = baseCusProduct("legacy_stripe");
+			const rc = {
+				...baseCusProduct("rc"),
+				processor: { type: ProcessorType.RevenueCat },
+			} as FullCusProduct;
+
+			const result = filterCustomerProductsByProcessorType({
+				customerProducts: [legacyStripe, rc],
+				processorType: ProcessorType.RevenueCat,
+			});
+			expect(result).toHaveLength(1);
+			expect(result[0].id).toBe("rc");
+		});
+
+		test("empty list returns empty list", () => {
+			expect(
+				filterCustomerProductsByProcessorType({
+					customerProducts: [],
+					processorType: ProcessorType.Stripe,
+				}),
+			).toEqual([]);
+		});
+	},
+);
diff --git a/server/tests/unit/billing/stripe/subscriptions/build-subscription-items-update-multi-product.spec.ts b/server/tests/unit/billing/stripe/subscriptions/build-subscription-items-update-multi-product.spec.ts
index 00aadb6a3..029e67120 100644
--- a/server/tests/unit/billing/stripe/subscriptions/build-subscription-items-update-multi-product.spec.ts
+++ b/server/tests/unit/billing/stripe/subscriptions/build-subscription-items-update-multi-product.spec.ts
@@ -1247,7 +1247,9 @@ describe(
 					options: stripeAddOn.allOptions,
 					status: CusProductStatus.Active,
 					subscriptionIds: [],
-					// processor defaults to Stripe via cusProductToProcessorType
+					// IMPORTANT: processor is intentionally unset here — legacy Stripe-managed
+					// cus products have no `processor` field. The filter must treat unset as
+					// Stripe (so this product's items get included). RevenueCat is always tagged.
 				});
 
 				const ctx = contexts.create({ features: [] });
diff --git a/shared/utils/cusProductUtils/filterCustomerProducts/filterCustomerProductsByProcessorType.ts b/shared/utils/cusProductUtils/filterCustomerProducts/filterCustomerProductsByProcessorType.ts
index c412b02bd..6cd0ba703 100644
--- a/shared/utils/cusProductUtils/filterCustomerProducts/filterCustomerProductsByProcessorType.ts
+++ b/shared/utils/cusProductUtils/filterCustomerProducts/filterCustomerProductsByProcessorType.ts
@@ -1,13 +1,14 @@
 import type { FullCusProduct } from "@models/cusProductModels/cusProductModels";
 import { ProcessorType } from "@models/genModels/genEnums";
-import { cusProductToProcessorType } from "../convertCusProduct.js";
 
 /**
  * Filter customer products by processor type.
  *
- * `cusProductToProcessorType` treats an unset `processor` as Stripe (default),
- * so passing `ProcessorType.Stripe` keeps both legacy unset rows and explicit
- * Stripe-tagged rows.
+ * IMPORTANT: a customer product with no `processor` set (or `processor.type` unset)
+ * is treated as Stripe. Historically, Stripe-managed cus products were created
+ * without explicitly tagging the processor field, so a null/undefined processor
+ * defaults to Stripe. RevenueCat-managed products always have
+ * `processor.type === ProcessorType.RevenueCat` explicitly set.
  *
  * @param customerProducts - The customer products to filter
  * @param processorType - The processor type to keep (e.g. `ProcessorType.Stripe`)
@@ -20,8 +21,10 @@ export const filterCustomerProductsByProcessorType = ({
 	customerProducts: FullCusProduct[];
 	processorType: ProcessorType;
 }): FullCusProduct[] => {
-	return customerProducts.filter(
-		(customerProduct) =>
-			cusProductToProcessorType(customerProduct) === processorType,
-	);
+	return customerProducts.filter((customerProduct) => {
+		// Default unset processor to Stripe — RevenueCat is always explicitly tagged.
+		const cusProductProcessorType =
+			customerProduct.processor?.type ?? ProcessorType.Stripe;
+		return cusProductProcessorType === processorType;
+	});
 };