diff --git a/Sources/RxAuthSwift/OAuthManager.swift b/Sources/RxAuthSwift/OAuthManager.swift
index 4fcc6ae..840f8c2 100644
--- a/Sources/RxAuthSwift/OAuthManager.swift
+++ b/Sources/RxAuthSwift/OAuthManager.swift
@@ -47,9 +47,13 @@ public final class OAuthManager: Sendable {
         } else if tokenStorage.getRefreshToken() != nil {
             do {
                 try await refreshTokenIfNeeded()
+                startTokenRefreshTimer()
                 logger.info("Refreshed token from existing session")
             } catch {
                 logger.warning("Token refresh failed: \(error.localizedDescription)")
+                // Ensure stale tokens are fully cleared so that subsequent
+                // calls to checkExistingAuth() don't retry a failed refresh.
+                try? tokenStorage.clearAll()
                 authState = .unauthenticated
             }
         } else {
@@ -86,6 +90,10 @@ public final class OAuthManager: Sendable {
             try tokenStorage.clearAll()
         } catch {
             logger.error("Failed to clear token storage: \(error.localizedDescription)")
+            // Attempt to clear individual tokens even if clearAll() fails,
+            // so that a partial failure doesn't leave stale tokens behind.
+            try? tokenStorage.deleteAccessToken()
+            try? tokenStorage.deleteRefreshToken()
         }
 
         currentUser = nil
diff --git a/Tests/RxAuthSwiftTests/OAuthManagerTokenCleanupTests.swift b/Tests/RxAuthSwiftTests/OAuthManagerTokenCleanupTests.swift
new file mode 100644
index 0000000..17d7668
--- /dev/null
+++ b/Tests/RxAuthSwiftTests/OAuthManagerTokenCleanupTests.swift
@@ -0,0 +1,318 @@
+import Foundation
+import Testing
+@testable import RxAuthSwift
+
+// MARK: - Mock Token Storage (clearAll fails but individual deletes work)
+
+private final class ClearAllFailingTokenStorage: TokenStorageProtocol, @unchecked Sendable {
+    private let lock = NSLock()
+    private var accessToken: String?
+    private var refreshToken: String?
+    private var expiresAt: Date?
+
+    func saveAccessToken(_ token: String) throws {
+        lock.lock()
+        defer { lock.unlock() }
+        accessToken = token
+    }
+
+    func getAccessToken() -> String? {
+        lock.lock()
+        defer { lock.unlock() }
+        return accessToken
+    }
+
+    func deleteAccessToken() throws {
+        lock.lock()
+        defer { lock.unlock() }
+        accessToken = nil
+    }
+
+    func saveRefreshToken(_ token: String) throws {
+        lock.lock()
+        defer { lock.unlock() }
+        refreshToken = token
+    }
+
+    func getRefreshToken() -> String? {
+        lock.lock()
+        defer { lock.unlock() }
+        return refreshToken
+    }
+
+    func deleteRefreshToken() throws {
+        lock.lock()
+        defer { lock.unlock() }
+        refreshToken = nil
+    }
+
+    func saveExpiresAt(_ date: Date) throws {
+        lock.lock()
+        defer { lock.unlock() }
+        expiresAt = date
+    }
+
+    func getExpiresAt() -> Date? {
+        lock.lock()
+        defer { lock.unlock() }
+        return expiresAt
+    }
+
+    func isTokenExpired() -> Bool {
+        lock.lock()
+        defer { lock.unlock() }
+        guard let expiresAt else { return true }
+        return expiresAt.timeIntervalSinceNow < 600
+    }
+
+    /// Always throws to simulate a storage failure during clearAll().
+    func clearAll() throws {
+        throw ClearAllError.simulatedFailure
+    }
+
+    enum ClearAllError: Error {
+        case simulatedFailure
+    }
+}
+
+// MARK: - Mock URL Protocol
+
+private final class MockURLProtocol: URLProtocol, @unchecked Sendable {
+    nonisolated(unsafe) static var requestHandler: (@Sendable (URLRequest) -> (HTTPURLResponse, Data))?
+
+    override class func canInit(with request: URLRequest) -> Bool { true }
+    override class func canonicalRequest(for request: URLRequest) -> URLRequest { request }
+
+    override func startLoading() {
+        guard let handler = Self.requestHandler else {
+            client?.urlProtocolDidFinishLoading(self)
+            return
+        }
+        let (response, data) = handler(request)
+        client?.urlProtocol(self, didReceive: response, cacheStoragePolicy: .notAllowed)
+        client?.urlProtocol(self, didLoad: data)
+        client?.urlProtocolDidFinishLoading(self)
+    }
+
+    override func stopLoading() {}
+}
+
+// MARK: - Thread-safe Counter
+
+private final class LockedCounter: @unchecked Sendable {
+    private let lock = NSLock()
+    private var _value = 0
+
+    var value: Int {
+        lock.lock()
+        defer { lock.unlock() }
+        return _value
+    }
+
+    func increment() {
+        lock.lock()
+        defer { lock.unlock() }
+        _value += 1
+    }
+}
+
+// MARK: - Tests
+
+@Suite("OAuthManager Token Cleanup", .serialized)
+struct OAuthManagerTokenCleanupTests {
+    private func makeConfig() -> RxAuthConfiguration {
+        RxAuthConfiguration(
+            issuer: "https://auth.example.com",
+            clientID: "test-client",
+            redirectURI: "testapp://callback"
+        )
+    }
+
+    // MARK: - logout() Tests
+
+    @Test @MainActor func logoutClearsAllTokens() async throws {
+        let storage = InMemoryTokenStorage()
+        try storage.saveAccessToken("access-123")
+        try storage.saveRefreshToken("refresh-456")
+        try storage.saveExpiresAt(Date().addingTimeInterval(3600))
+
+        let manager = OAuthManager(
+            configuration: makeConfig(),
+            tokenStorage: storage
+        )
+
+        await manager.logout()
+
+        #expect(storage.getAccessToken() == nil)
+        #expect(storage.getRefreshToken() == nil)
+        #expect(storage.getExpiresAt() == nil)
+        #expect(manager.authState == .unauthenticated)
+        #expect(manager.currentUser == nil)
+    }
+
+    @Test @MainActor func logoutClearsTokensWhenClearAllFails() async throws {
+        let storage = ClearAllFailingTokenStorage()
+        try storage.saveAccessToken("access-123")
+        try storage.saveRefreshToken("refresh-456")
+        try storage.saveExpiresAt(Date().addingTimeInterval(3600))
+
+        let manager = OAuthManager(
+            configuration: makeConfig(),
+            tokenStorage: storage
+        )
+
+        await manager.logout()
+
+        // Even though clearAll() throws, individual deletes should remove tokens
+        #expect(storage.getAccessToken() == nil)
+        #expect(storage.getRefreshToken() == nil)
+        #expect(manager.authState == .unauthenticated)
+        #expect(manager.currentUser == nil)
+    }
+
+    // MARK: - checkExistingAuth() Tests
+
+    @Test @MainActor func checkExistingAuthUnauthenticatedWithNoTokens() async {
+        let storage = InMemoryTokenStorage()
+        let manager = OAuthManager(
+            configuration: makeConfig(),
+            tokenStorage: storage
+        )
+
+        await manager.checkExistingAuth()
+
+        #expect(manager.authState == .unauthenticated)
+    }
+
+    @Test @MainActor func checkExistingAuthClearsStaleTokensAfterRefreshFailure() async throws {
+        // Set up mock handler before registering protocol to avoid race conditions
+        MockURLProtocol.requestHandler = { request in
+            let response = HTTPURLResponse(
+                url: request.url!,
+                statusCode: 400,
+                httpVersion: nil,
+                headerFields: nil
+            )!
+            return (response, Data())
+        }
+
+        URLProtocol.registerClass(MockURLProtocol.self)
+        defer { URLProtocol.unregisterClass(MockURLProtocol.self) }
+
+        let storage = InMemoryTokenStorage()
+        // Set up stale tokens: expired access token + refresh token
+        try storage.saveAccessToken("expired-access")
+        try storage.saveRefreshToken("stale-refresh")
+        try storage.saveExpiresAt(Date().addingTimeInterval(-3600)) // expired 1 hour ago
+
+        let manager = OAuthManager(
+            configuration: makeConfig(),
+            tokenStorage: storage
+        )
+
+        // First call finds stale refresh token, tries refresh → 400 → clears tokens
+        await manager.checkExistingAuth()
+
+        #expect(manager.authState == .unauthenticated)
+        // Tokens must be cleared so the next checkExistingAuth() won't retry
+        #expect(storage.getAccessToken() == nil)
+        #expect(storage.getRefreshToken() == nil)
+    }
+
+    @Test @MainActor func checkExistingAuthDoesNotRetryAfterTokensCleared() async throws {
+        let counter = LockedCounter()
+        MockURLProtocol.requestHandler = { request in
+            counter.increment()
+            let response = HTTPURLResponse(
+                url: request.url!,
+                statusCode: 400,
+                httpVersion: nil,
+                headerFields: nil
+            )!
+            return (response, Data())
+        }
+
+        URLProtocol.registerClass(MockURLProtocol.self)
+        defer { URLProtocol.unregisterClass(MockURLProtocol.self) }
+
+        let storage = InMemoryTokenStorage()
+        try storage.saveAccessToken("expired-access")
+        try storage.saveRefreshToken("stale-refresh")
+        try storage.saveExpiresAt(Date().addingTimeInterval(-3600))
+
+        let manager = OAuthManager(
+            configuration: makeConfig(),
+            tokenStorage: storage
+        )
+
+        // First call: finds stale refresh token, tries refresh, fails, clears tokens
+        await manager.checkExistingAuth()
+        #expect(manager.authState == .unauthenticated)
+
+        let firstCallCount = counter.value
+
+        // Second call: no tokens should remain, so no refresh attempt
+        await manager.checkExistingAuth()
+        #expect(manager.authState == .unauthenticated)
+        #expect(counter.value == firstCallCount, "Should not make additional refresh requests after tokens are cleared")
+    }
+
+    @Test @MainActor func fullCycleRefreshFailureThenRecheck() async throws {
+        // Simulate the full bug scenario:
+        // 1. User has tokens
+        // 2. Refresh fails with 400
+        // 3. Tokens should be fully cleared
+        // 4. Subsequent checkExistingAuth() should NOT attempt to refresh
+
+        MockURLProtocol.requestHandler = { request in
+            let response = HTTPURLResponse(
+                url: request.url!,
+                statusCode: 400,
+                httpVersion: nil,
+                headerFields: nil
+            )!
+            return (response, Data())
+        }
+
+        URLProtocol.registerClass(MockURLProtocol.self)
+        defer { URLProtocol.unregisterClass(MockURLProtocol.self) }
+
+        let storage = InMemoryTokenStorage()
+        try storage.saveAccessToken("old-access")
+        try storage.saveRefreshToken("old-refresh")
+        try storage.saveExpiresAt(Date().addingTimeInterval(-100)) // expired
+
+        let manager = OAuthManager(
+            configuration: makeConfig(),
+            tokenStorage: storage
+        )
+
+        // Step 1: Check existing auth → refresh fails → signs out
+        await manager.checkExistingAuth()
+        #expect(manager.authState == .unauthenticated)
+        #expect(storage.getAccessToken() == nil)
+        #expect(storage.getRefreshToken() == nil)
+
+        // Step 2: Simulate user signing in again by saving new tokens
+        try storage.saveAccessToken("new-access")
+        try storage.saveRefreshToken("new-refresh")
+        try storage.saveExpiresAt(Date().addingTimeInterval(3600)) // valid
+
+        // Step 3: Now update the mock to return valid responses for the new session
+        MockURLProtocol.requestHandler = { request in
+            let userInfoJSON = #"{"id":"user-1","name":"Test User","email":"test@example.com"}"#
+            let response = HTTPURLResponse(
+                url: request.url!,
+                statusCode: 200,
+                httpVersion: nil,
+                headerFields: ["Content-Type": "application/json"]
+            )!
+            return (response, userInfoJSON.data(using: .utf8)!)
+        }
+
+        // Step 4: Check existing auth with new valid tokens
+        await manager.checkExistingAuth()
+        #expect(manager.authState == .authenticated)
+        #expect(manager.currentUser?.id == "user-1")
+    }
+}