diff --git a/packages/better-auth/src/api/routes/update-user.test.ts b/packages/better-auth/src/api/routes/update-user.test.ts
index 97edb070f2..720895c459 100644
--- a/packages/better-auth/src/api/routes/update-user.test.ts
+++ b/packages/better-auth/src/api/routes/update-user.test.ts
@@ -366,6 +366,50 @@ describe("updateUser", async () => {
 		expect(firstSession?.user.name).toBe("updatedName");
 	});
 
+	/**
+	 * @see https://github.com/better-auth/better-auth/issues/8156
+	 */
+	it("should propagate updates when secondaryStorage auto-deserializes JSON (e.g. @vercel/kv)", async () => {
+		const store = new Map<string, unknown>();
+		const { client: authClient, signInWithTestUser: signIn } =
+			await getTestInstance({
+				secondaryStorage: {
+					set(key, value, ttl) {
+						// Simulate @vercel/kv behavior: stores string but get() returns parsed object
+						store.set(key, JSON.parse(value));
+					},
+					get(key) {
+						return store.get(key) ?? null;
+					},
+					delete(key) {
+						store.delete(key);
+					},
+				},
+			});
+
+		const { headers: headers1 } = await signIn();
+
+		// Verify the store actually holds a parsed object with string dates
+		const session1 = await authClient.getSession({
+			fetchOptions: { headers: headers1, throw: true },
+		});
+		const cached = store.get(session1!.session.token) as
+			| { session?: { expiresAt?: unknown } }
+			| undefined;
+		expect(typeof cached?.session?.expiresAt).toBe("string");
+
+		// This was the crash in #8156: TypeError: expiresAt.getTime is not a function
+		await authClient.updateUser({
+			name: "updatedViaKV",
+			fetchOptions: { headers: headers1, throw: true },
+		});
+
+		const refreshed = await authClient.getSession({
+			fetchOptions: { headers: headers1, throw: true },
+		});
+		expect(refreshed?.user.name).toBe("updatedViaKV");
+	});
+
 	it("should not write to secondary storage multiple times for the same session token during updateUser", async () => {
 		const store = new Map<string, string>();
 		const writeLog: { key: string; timestamp: number }[] = [];
diff --git a/packages/better-auth/src/db/internal-adapter.test.ts b/packages/better-auth/src/db/internal-adapter.test.ts
index be9b148ba8..6cb587a233 100644
--- a/packages/better-auth/src/db/internal-adapter.test.ts
+++ b/packages/better-auth/src/db/internal-adapter.test.ts
@@ -602,6 +602,68 @@ describe("internal adapter test", async () => {
 		expect(finalTTL).toBeGreaterThanOrEqual(7198); // Allow for test execution time
 	});
 
+	/**
+	 * @see https://github.com/better-auth/better-auth/issues/8156
+	 */
+	it("should handle secondaryStorage that auto-deserializes JSON (e.g. @vercel/kv)", async () => {
+		const objectStore = new Map<string, unknown>();
+
+		const testOpts = {
+			database: new DatabaseSync(":memory:"),
+			secondaryStorage: {
+				set(key: string, value: string, _ttl?: number) {
+					// Simulate @vercel/kv: stores string but get() returns parsed object
+					objectStore.set(key, JSON.parse(value));
+				},
+				get(key: string) {
+					return objectStore.get(key) ?? null;
+				},
+				delete(key: string) {
+					objectStore.delete(key);
+				},
+			},
+		} satisfies BetterAuthOptions;
+
+		(await getMigrations(testOpts)).runMigrations();
+		const testCtx = await init(testOpts);
+		const testAdapter = testCtx.internalAdapter;
+
+		const user = await testAdapter.createUser({
+			name: "KV User",
+			email: `kv-test-${Date.now()}@example.com`,
+		});
+		const session = await testAdapter.createSession(user.id);
+
+		// Verify store holds parsed objects with string dates (not Date instances)
+		const cachedBefore = objectStore.get(session.token) as
+			| { session?: { expiresAt?: unknown } }
+			| undefined;
+		expect(typeof cachedBefore?.session?.expiresAt).toBe("string");
+
+		// This was the crash in #8156: TypeError: expiresAt.getTime is not a function
+		// updateUser calls refreshUserSessions which reads from secondary storage
+		await expect(
+			testAdapter.updateUser(user.id, { name: "Updated Name" }),
+		).resolves.toBeDefined();
+
+		const afterUpdate = objectStore.get(session.token) as
+			| { user?: { name?: string } }
+			| undefined;
+		expect(afterUpdate?.user?.name).toBe("Updated Name");
+
+		// Also verify updateUserByEmail works
+		await expect(
+			testAdapter.updateUserByEmail(user.email, {
+				name: "Updated By Email",
+			}),
+		).resolves.toBeDefined();
+
+		const afterEmailUpdate = objectStore.get(session.token) as
+			| { user?: { name?: string } }
+			| undefined;
+		expect(afterEmailUpdate?.user?.name).toBe("Updated By Email");
+	});
+
 	it("should create on secondary storage", async () => {
 		// Create session
 		const now = Date.now();