From 9a229ce132a0e7a66ef2e2e5253ee1a2b2f0a79e Mon Sep 17 00:00:00 2001
From: Taesu <166604494+bytaesu@users.noreply.github.com>
Date: Fri, 6 Mar 2026 13:24:16 +0900
Subject: [PATCH] fix(client): preserve stale session data on network or server
 errors (#8437)

---
 packages/better-auth/src/client/query.test.ts | 141 ++++++++++++++++++
 packages/better-auth/src/client/query.ts      |   7 +-
 2 files changed, 146 insertions(+), 2 deletions(-)
 create mode 100644 packages/better-auth/src/client/query.test.ts

diff --git a/packages/better-auth/src/client/query.test.ts b/packages/better-auth/src/client/query.test.ts
new file mode 100644
index 0000000000..4ef796913f
--- /dev/null
+++ b/packages/better-auth/src/client/query.test.ts
@@ -0,0 +1,141 @@
+// @vitest-environment happy-dom
+
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { createAuthClient } from "./solid";
+import { testClientPlugin } from "./test-plugin";
+
+/**
+ * @see https://github.com/better-auth/better-auth/issues/8420
+ */
+describe("useAuthQuery - error handling", () => {
+	beforeEach(() => {
+		vi.useFakeTimers();
+	});
+
+	afterEach(() => {
+		vi.useRealTimers();
+		vi.restoreAllMocks();
+	});
+
+	it("should preserve stale data on network error (fetch throws)", async () => {
+		let shouldFail = false;
+
+		const client = createAuthClient({
+			plugins: [testClientPlugin()],
+			fetchOptions: {
+				customFetchImpl: async (_url) => {
+					if (shouldFail) {
+						throw new TypeError("Failed to fetch");
+					}
+					return new Response(
+						JSON.stringify({
+							user: { id: "1", email: "test@test.com" },
+							session: { id: "session-1" },
+						}),
+					);
+				},
+				baseURL: "http://localhost:3000",
+			},
+		});
+
+		const session = client.useSession();
+		await vi.runAllTimersAsync();
+
+		expect(session().data).toMatchObject({
+			user: { id: "1", email: "test@test.com" },
+		});
+		expect(session().error).toBeNull();
+
+		// Network failure on refetch
+		shouldFail = true;
+		await session().refetch();
+		await vi.runAllTimersAsync();
+
+		// Stale data should be preserved
+		expect(session().data).toMatchObject({
+			user: { id: "1", email: "test@test.com" },
+		});
+	});
+
+	it("should clear data on 401 unauthorized response", async () => {
+		let returnUnauthorized = false;
+
+		const client = createAuthClient({
+			plugins: [testClientPlugin()],
+			fetchOptions: {
+				customFetchImpl: async (url) => {
+					const urlStr = typeof url === "string" ? url : url.toString();
+					if (returnUnauthorized && urlStr.includes("/get-session")) {
+						return new Response(JSON.stringify({ message: "Unauthorized" }), {
+							status: 401,
+						});
+					}
+					return new Response(
+						JSON.stringify({
+							user: { id: "1", email: "test@test.com" },
+							session: { id: "session-1" },
+						}),
+					);
+				},
+				baseURL: "http://localhost:3000",
+			},
+		});
+
+		const session = client.useSession();
+		await vi.runAllTimersAsync();
+
+		expect(session().data).toMatchObject({
+			user: { id: "1", email: "test@test.com" },
+		});
+
+		// Refetch with 401
+		returnUnauthorized = true;
+		await session().refetch();
+		await vi.runAllTimersAsync();
+
+		expect(session().data).toBeNull();
+	});
+
+	it("should preserve stale data on 500 server error", async () => {
+		let returnServerError = false;
+
+		const client = createAuthClient({
+			plugins: [testClientPlugin()],
+			fetchOptions: {
+				customFetchImpl: async (url) => {
+					const urlStr = typeof url === "string" ? url : url.toString();
+					if (returnServerError && urlStr.includes("/get-session")) {
+						return new Response(
+							JSON.stringify({ message: "Internal Server Error" }),
+							{ status: 500 },
+						);
+					}
+					return new Response(
+						JSON.stringify({
+							user: { id: "1", email: "test@test.com" },
+							session: { id: "session-1" },
+						}),
+					);
+				},
+				baseURL: "http://localhost:3000",
+			},
+		});
+
+		const session = client.useSession();
+		await vi.runAllTimersAsync();
+
+		expect(session().data).toMatchObject({
+			user: { id: "1", email: "test@test.com" },
+		});
+
+		// Refetch with 500
+		returnServerError = true;
+		await session().refetch();
+		await vi.runAllTimersAsync();
+
+		// Stale data should be preserved on 500
+		expect(session().data).toMatchObject({
+			user: { id: "1", email: "test@test.com" },
+		});
+	});
+});
diff --git a/packages/better-auth/src/client/query.ts b/packages/better-auth/src/client/query.ts
index e486960895..2271cfa1c8 100644
--- a/packages/better-auth/src/client/query.ts
+++ b/packages/better-auth/src/client/query.ts
@@ -79,9 +79,12 @@ export const useAuthQuery = <T>(
 							: request.retry?.attempts;
 					const retryAttempt = request.retryAttempt || 0;
 					if (retryAttempts && retryAttempt < retryAttempts) return;
+					const isUnauthorized = context.error.status === 401;
 					value.set({
 						error: context.error,
-						data: null,
+						data: isUnauthorized
+							? null // clear session on HTTP 401
+							: value.get().data, // preserve stale data on other errors
 						isPending: false,
 						isRefetching: false,
 						refetch: value.value.refetch,
@@ -103,7 +106,7 @@ export const useAuthQuery = <T>(
 				.catch((error) => {
 					value.set({
 						error,
-						data: null,
+						data: value.get().data,
 						isPending: false,
 						isRefetching: false,
 						refetch: value.value.refetch,