diff --git a/packages/ai/src/usage/xai-oauth.ts b/packages/ai/src/usage/xai-oauth.ts
index 9a6e172a8..40aaa08ed 100644
--- a/packages/ai/src/usage/xai-oauth.ts
+++ b/packages/ai/src/usage/xai-oauth.ts
@@ -138,11 +138,14 @@ function parseWeeklyBillingConfig(raw: Record<string, unknown>): XaiWeeklyBillin
 	}
 
 	// Fresh weekly periods (or accounts with 0 usage) omit creditUsagePercent;
-	// default to 0 when a valid weekly period exists.
-	const creditUsagePercent =
-		raw.creditUsagePercent === undefined || raw.creditUsagePercent === null
-			? 0
-			: parsePercent(raw.creditUsagePercent);
+	// default to 0 only when the weekly period is active (end > now).
+	// Expired periods without explicit usage data are rejected to retain last good cache.
+	let creditUsagePercent: number | undefined;
+	if (raw.creditUsagePercent === undefined || raw.creditUsagePercent === null) {
+		creditUsagePercent = end > Date.now() ? 0 : undefined;
+	} else {
+		creditUsagePercent = parsePercent(raw.creditUsagePercent);
+	}
 	if (creditUsagePercent === undefined) return null;
 
 	const productUsage: XaiProductUsage[] = [];
diff --git a/packages/ai/test/xai-oauth-usage.test.ts b/packages/ai/test/xai-oauth-usage.test.ts
index 9be58536f..1330f2128 100644
--- a/packages/ai/test/xai-oauth-usage.test.ts
+++ b/packages/ai/test/xai-oauth-usage.test.ts
@@ -224,6 +224,26 @@ describe("xai-oauth usage provider", () => {
 		expect(report?.limits[0]?.id).toBe("xai-oauth:credits:1w");
 		expect(report?.limits[0]?.window?.resetsAt).toBe(Date.parse(periodEnd));
 	});
+	it("rejects an expired weekly period when creditUsagePercent is omitted", async () => {
+		const periodEnd = new Date(Date.now() - 60_000).toISOString();
+		const periodStart = new Date(Date.now() - 8 * 24 * 60 * 60 * 1000).toISOString();
+		const report = await xaiOauthUsageProvider.fetchUsage(
+			{ provider: "xai-oauth", credential: makeCredential() },
+			{
+				fetch: capturingFetch({
+					config: {
+						currentPeriod: {
+							end: periodEnd,
+							start: periodStart,
+							type: "USAGE_PERIOD_TYPE_WEEKLY",
+						},
+					},
+				}).fetch,
+			},
+		);
+
+		expect(report).toBeNull();
+	});
 
 	it("reports zero usage when weekly period is active but creditUsagePercent is omitted (fresh reset)", async () => {
 		const periodEnd = new Date(Date.now() + 6 * 24 * 60 * 60 * 1000).toISOString();