Implement server-mediated OAuth polling flow

Replace deep link-based OAuth with server-mediated polling:

Client changes:
- AuthClient now uses PollForOAuthCompletionAsync instead of ExchangeCodeAsync
- OAuthManager no longer needs deep link handling
- Client polls CheckOAuthStatus every 2s until success/failure/timeout

Server changes:
- OAuthService now stores pending/completed OAuth sessions by state
- OAuthServiceImpl handles callback internally, stores result for polling
- AuthServiceImpl implements checkOAuthStatus gRPC method
- New OAuthHttpHandler provides HTTP endpoint for OAuth callback
- Callback shows "Login successful" or error page to user

Proto changes:
- Add CheckOAuthStatus RPC and messages
- Add OAuthStatus enum (PENDING, SUCCESS, FAILED, EXPIRED)
- Remove ExchangeCode RPC (no longer needed)

This approach works in Unity Editor and all platforms without URL scheme
registration. Client just opens browser and polls for completion.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/main/csharp/net/eagle0/clients/unity/eagle0/Assets/Auth/AuthClient.cs

@@ -11,18 +11,15 @@ using GrpcAuthClient = Net.Eagle0.Eagle.Api.Auth.Auth.AuthClient;
 namespace Auth {
     /// <summary>
     /// gRPC client for the Auth service.
-    /// Handles OAuth URL generation, code exchange, and token refresh.
+    /// Handles OAuth URL generation, status polling, and token refresh.
     /// </summary>
     public class AuthClient : IDisposable {
-        private const string RedirectUri = "eagle0://auth/callback";
+        private const int PollIntervalMs = 2000;   // Poll every 2 seconds
+        private const int PollTimeoutMs = 300000;  // 5 minute timeout
 
         private readonly GrpcAuthClient _client;
         private readonly GrpcChannel _channel;
 
-        // State parameter for CSRF protection (stored between GetOAuthUrl and ExchangeCode)
-        private string _pendingState;
-        private OAuthProvider _pendingProvider;
-
         /// <summary>
         /// Create auth client for the given server URL.
         /// </summary>
@@ -43,51 +40,56 @@ namespace Auth {
 
         /// <summary>
         /// Get OAuth URL to open in system browser.
+        /// Returns both the URL and the state token for polling.
         /// </summary>
         /// <param name="provider">Discord or Google</param>
-        /// <returns>URL to open in browser</returns>
-        public async Task<string> GetOAuthUrlAsync(OAuthProvider provider) {
-            var request = new GetOAuthUrlRequest { Provider = provider, RedirectUri = RedirectUri };
+        /// <returns>Tuple of (URL to open in browser, state token for polling)</returns>
+        public async Task<(string authUrl, string state)> GetOAuthUrlAsync(OAuthProvider provider) {
+            var request = new GetOAuthUrlRequest { Provider = provider };
 
             var response = await _client.GetOAuthUrlAsync(request);
 
-            // Store state for validation when code comes back
-            _pendingState = response.State;
-            _pendingProvider = provider;
-
             Debug.Log($"[AuthClient] Got OAuth URL for {provider}, state={response.State}");
-            return response.AuthUrl;
+            return (response.AuthUrl, response.State);
         }
 
         /// <summary>
-        /// Exchange authorization code for JWT tokens.
-        /// Call this after receiving the OAuth callback.
+        /// Poll for OAuth completion. Blocks until success, failure, or timeout.
+        /// The server handles the OAuth callback and token exchange.
         /// </summary>
-        /// <param name="code">Authorization code from callback</param>
-        /// <param name="state">State parameter from callback (for CSRF validation)</param>
-        /// <returns>Exchange response with tokens and user info</returns>
-        public async Task<ExchangeCodeResponse> ExchangeCodeAsync(string code, string state) {
-            // Validate state matches what we sent
-            if (state != _pendingState) {
-                throw new InvalidOperationException(
-                        $"State mismatch: expected {_pendingState}, got {state}. Possible CSRF attack.");
+        /// <param name="state">State token from GetOAuthUrlAsync</param>
+        /// <returns>Response with tokens and user info on success</returns>
+        public async Task<CheckOAuthStatusResponse> PollForOAuthCompletionAsync(string state) {
+            var startTime = DateTime.UtcNow;
+
+            while (true) {
+                var request = new CheckOAuthStatusRequest { State = state };
+                var response = await _client.CheckOAuthStatusAsync(request);
+
+                switch (response.Status) {
+                    case OAuthStatus.Success:
+                        Debug.Log(
+                                $"[AuthClient] OAuth successful, user={response.User?.DisplayName}, isNew={response.IsNewUser}");
+                        return response;
+
+                    case OAuthStatus.Failed:
+                        throw new Exception($"OAuth failed: {response.ErrorMessage}");
+
+                    case OAuthStatus.Expired:
+                        throw new Exception("OAuth session expired. Please try again.");
+
+                    case OAuthStatus.Pending:
+                        // Check timeout
+                        if ((DateTime.UtcNow - startTime).TotalMilliseconds > PollTimeoutMs) {
+                            throw new TimeoutException("OAuth login timed out. Please try again.");
+                        }
+                        // Wait before polling again
+                        await Task.Delay(PollIntervalMs);
+                        break;
+
+                    default: throw new Exception($"Unknown OAuth status: {response.Status}");
+                }
             }
-
-            var request = new ExchangeCodeRequest {
-                Provider = _pendingProvider,
-                AuthorizationCode = code,
-                State = state,
-                RedirectUri = RedirectUri
-            };
-
-            var response = await _client.ExchangeCodeAsync(request);
-
-            // Clear pending state
-            _pendingState = null;
-
-            Debug.Log(
-                    $"[AuthClient] Code exchange successful, user={response.User?.DisplayName}, isNew={response.IsNewUser}");
-            return response;
         }
 
         /// <summary>

src/main/csharp/net/eagle0/clients/unity/eagle0/Assets/Auth/OAuthManager.cs

@@ -1,14 +1,13 @@
 using System;
-using System.Collections.Generic;
 using System.Threading.Tasks;
 using Net.Eagle0.Eagle.Api.Auth;
 using UnityEngine;
 
 namespace Auth {
     /// <summary>
-    /// Manages OAuth authentication flow including:
-    /// - Opening system browser for OAuth consent
-    /// - Handling deep link callbacks (eagle0://auth/callback)
+    /// Manages OAuth authentication flow using server-mediated polling:
+    /// - Opens system browser for OAuth consent
+    /// - Polls server for OAuth completion (no deep links needed)
     /// - Token storage and refresh
     /// </summary>
     public class OAuthManager : MonoBehaviour {
@@ -16,7 +15,6 @@ namespace Auth {
 
         private AuthClient _authClient;
         private string _currentServerUrl;
-        private TaskCompletionSource<ExchangeCodeResponse> _pendingAuth;
 
         // Events for UI updates
         public event Action<UserInfo> OnLoginSuccess;
@@ -54,18 +52,6 @@ namespace Auth {
             Debug.Log($"[OAuthManager] Configured for server: {serverUrl}");
         }
 
-        private void OnEnable() {
-            // Register for deep link callbacks
-            Application.deepLinkActivated += OnDeepLinkActivated;
-
-            // Check if app was launched via deep link
-            if (!string.IsNullOrEmpty(Application.absoluteURL)) {
-                OnDeepLinkActivated(Application.absoluteURL);
-            }
-        }
-
-        private void OnDisable() { Application.deepLinkActivated -= OnDeepLinkActivated; }
-
         private void OnDestroy() {
             _authClient?.Dispose();
             if (Instance == this) { Instance = null; }
@@ -80,23 +66,20 @@ namespace Auth {
 
         /// <summary>
         /// Start OAuth login with specified provider.
-        /// Opens system browser for user consent.
+        /// Opens system browser for user consent, then polls for completion.
         /// </summary>
-        public async Task<ExchangeCodeResponse> LoginAsync(OAuthProvider provider) {
+        public async Task<CheckOAuthStatusResponse> LoginAsync(OAuthProvider provider) {
             EnsureConfigured();
             try {
-                // Get OAuth URL from server
-                var authUrl = await _authClient.GetOAuthUrlAsync(provider);
-
-                // Create completion source for callback
-                _pendingAuth = new TaskCompletionSource<ExchangeCodeResponse>();
+                // Get OAuth URL and state from server
+                var (authUrl, state) = await _authClient.GetOAuthUrlAsync(provider);
 
                 // Open system browser
                 Debug.Log($"[OAuthManager] Opening browser: {authUrl}");
                 Application.OpenURL(authUrl);
 
-                // Wait for deep link callback
-                var response = await _pendingAuth.Task;
+                // Poll for OAuth completion (server handles the callback)
+                var response = await _authClient.PollForOAuthCompletionAsync(state);
 
                 // Store tokens
                 TokenStorage.StoreTokens(
@@ -118,44 +101,6 @@ namespace Auth {
                 Debug.LogError($"[OAuthManager] Login failed: {ex.Message}");
                 OnLoginFailed?.Invoke(ex.Message);
                 throw;
-            } finally { _pendingAuth = null; }
-        }
-
-        /// <summary>
-        /// Handle deep link callback from OAuth provider.
-        /// URL format: eagle0://auth/callback?code=xxx&state=yyy
-        /// </summary>
-        private async void OnDeepLinkActivated(string url) {
-            Debug.Log($"[OAuthManager] Deep link received: {url}");
-
-            if (!url.StartsWith("eagle0://auth/callback")) { return; }
-
-            try {
-                // Parse query parameters
-                var queryParams = ParseQueryString(url);
-
-                if (queryParams.TryGetValue("error", out var error)) {
-                    var errorDesc =
-                            queryParams.GetValueOrDefault("error_description", "Unknown error");
-                    throw new Exception($"OAuth error: {error} - {errorDesc}");
-                }
-
-                if (!queryParams.TryGetValue("code", out var code)) {
-                    throw new Exception("No authorization code in callback");
-                }
-
-                if (!queryParams.TryGetValue("state", out var state)) {
-                    throw new Exception("No state parameter in callback");
-                }
-
-                // Exchange code for tokens
-                var response = await _authClient.ExchangeCodeAsync(code, state);
-
-                // Complete pending auth task
-                _pendingAuth?.TrySetResult(response);
-            } catch (Exception ex) {
-                Debug.LogError($"[OAuthManager] Callback handling failed: {ex.Message}");
-                _pendingAuth?.TrySetException(ex);
             }
         }
 
@@ -234,27 +179,5 @@ namespace Auth {
             }
             OnLogout?.Invoke();
         }
-
-        /// <summary>
-        /// Parse query string from URL.
-        /// </summary>
-        private static Dictionary<string, string> ParseQueryString(string url) {
-            var result = new Dictionary<string, string>();
-
-            var queryStart = url.IndexOf('?');
-            if (queryStart < 0) return result;
-
-            var query = url.Substring(queryStart + 1);
-            var pairs = query.Split('&');
-
-            foreach (var pair in pairs) {
-                var parts = pair.Split('=');
-                if (parts.Length == 2) {
-                    result[Uri.UnescapeDataString(parts[0])] = Uri.UnescapeDataString(parts[1]);
-                }
-            }
-
-            return result;
-        }
     }
 }

src/main/protobuf/net/eagle0/eagle/api/auth.proto

@@ -13,11 +13,13 @@ option objc_class_prefix = "E0A";
 
 // Authentication service for OAuth login
 service Auth {
-    // Get OAuth URL to open in system browser
+    // Get OAuth URL to open in system browser.
+    // The server handles the OAuth callback - client polls CheckOAuthStatus.
     rpc GetOAuthUrl(GetOAuthUrlRequest) returns (GetOAuthUrlResponse) {}
 
-    // Exchange authorization code for JWT tokens
-    rpc ExchangeCode(ExchangeCodeRequest) returns (ExchangeCodeResponse) {}
+    // Poll for OAuth completion. Call this after opening the URL from GetOAuthUrl.
+    // The server receives the OAuth callback and exchanges the code for tokens.
+    rpc CheckOAuthStatus(CheckOAuthStatusRequest) returns (CheckOAuthStatusResponse) {}
 
     // Set or update display name (requires valid JWT)
     rpc SetDisplayName(SetDisplayNameRequest) returns (SetDisplayNameResponse) {}
@@ -42,28 +44,36 @@ enum OAuthProvider {
 // Request to initiate OAuth flow
 message GetOAuthUrlRequest {
     OAuthProvider provider = 1;
-    string redirect_uri = 2;  // e.g., "eagle0://auth/callback"
 }
 
 message GetOAuthUrlResponse {
     string auth_url = 1;  // Full OAuth URL to open in system browser
-    string state = 2;     // State parameter for CSRF protection
+    string state = 2;     // State parameter - use this to poll CheckOAuthStatus
 }
 
-// Exchange authorization code for JWT
-message ExchangeCodeRequest {
-    OAuthProvider provider = 1;
-    string authorization_code = 2;
-    string state = 3;         // Must match state from GetOAuthUrlResponse
-    string redirect_uri = 4;  // Must match exactly what was used in GetOAuthUrlRequest
+// Poll for OAuth completion status
+message CheckOAuthStatusRequest {
+    string state = 1;  // State from GetOAuthUrlResponse
 }
 
-message ExchangeCodeResponse {
-    string access_token = 1;   // JWT for API access
-    string refresh_token = 2;  // Long-lived token for getting new access tokens
-    int64 expires_at = 3;      // Unix timestamp when access_token expires
-    UserInfo user = 4;
-    bool is_new_user = 5;      // True if user needs to set display name
+message CheckOAuthStatusResponse {
+    OAuthStatus status = 1;
+    // Only set when status is OAUTH_STATUS_SUCCESS:
+    string access_token = 2;
+    string refresh_token = 3;
+    int64 expires_at = 4;
+    UserInfo user = 5;
+    bool is_new_user = 6;
+    // Only set when status is OAUTH_STATUS_FAILED:
+    string error_message = 7;
+}
+
+enum OAuthStatus {
+    OAUTH_STATUS_UNSPECIFIED = 0;
+    OAUTH_STATUS_PENDING = 1;   // User hasn't completed OAuth yet
+    OAUTH_STATUS_SUCCESS = 2;   // OAuth complete, tokens available
+    OAUTH_STATUS_FAILED = 3;    // OAuth failed (user denied, etc.)
+    OAUTH_STATUS_EXPIRED = 4;   // State token expired (typically 10 minutes)
 }
 
 // User information returned from auth endpoints

src/main/scala/net/eagle0/eagle/BUILD.bazel

@@ -21,6 +21,7 @@ scala_binary(
         "//src/main/scala/net/eagle0/eagle/service:custom_battle_manager",
         "//src/main/scala/net/eagle0/eagle/service:exception_interceptor",
         "//src/main/scala/net/eagle0/eagle/service:games_manager",
+        "//src/main/scala/net/eagle0/eagle/service:oauth_http_handler",
         "//src/main/scala/net/eagle0/eagle/service:server_setup_helpers",
         "//src/main/scala/net/eagle0/eagle/service/persistence:async_s3_persister",
         "//src/main/scala/net/eagle0/eagle/service/persistence:local_file_persister",

src/main/scala/net/eagle0/eagle/Main.scala

@@ -8,7 +8,7 @@ import io.grpc.{Server, ServerBuilder}
 import net.eagle0.common.{FunctionalRandom, SeededRandom, SimpleTimedLogger}
 import net.eagle0.eagle.api.auth.auth.AuthGrpc
 import net.eagle0.eagle.api.eagle.EagleGrpc
-import net.eagle0.eagle.auth.{JwtService, JwtServiceImpl, OAuthServiceImpl, UserServiceImpl}
+import net.eagle0.eagle.auth.{JwtService, JwtServiceImpl, OAuthService, OAuthServiceImpl, UserServiceImpl}
 import net.eagle0.eagle.service.*
 import net.eagle0.eagle.service.persistence.{AsyncS3Persister, LocalFilePersister, SaveDirectory}
 
@@ -25,6 +25,12 @@ object Main {
 
   private val gptModelNameKey = Symbol("gptModelName")
 
+  private val serverBaseUrlKey     = Symbol("serverBaseUrl")
+  private val defaultServerBaseUrl = "https://prod.eagle0.net"
+
+  private val oauthHttpPortKey     = Symbol("oauthHttpPort")
+  private val defaultOauthHttpPort = "8080"
+
   def parsedOptions(args: Array[String]): Map[Symbol, String] = {
     def nextOption(
         map: Map[Symbol, String],
@@ -43,6 +49,16 @@ object Main {
             map ++ Map(gptModelNameKey -> value),
             tail
           )
+        case "--server-base-url" +: value +: tail                  =>
+          nextOption(
+            map ++ Map(serverBaseUrlKey -> value),
+            tail
+          )
+        case "--oauth-http-port" +: value +: tail                  =>
+          nextOption(
+            map ++ Map(oauthHttpPortKey -> value),
+            tail
+          )
         case option +: _                                           =>
           throw new IllegalArgumentException("Unknown option " + option)
 
@@ -77,11 +93,24 @@ object Main {
     val customBattleManager = newCustomBattleManager(shardokInterfaceAddress)
     customBattleManager.begin()
 
+    val serverBaseUrl =
+      options.getOrElse(serverBaseUrlKey, defaultServerBaseUrl)
+
+    // Create OAuth service (shared between gRPC and HTTP)
+    val oauthService = new OAuthServiceImpl(serverBaseUrl)
+
+    // Start OAuth HTTP server for callbacks
+    val oauthHttpPort    =
+      options.getOrElse(oauthHttpPortKey, defaultOauthHttpPort).toInt
+    val oauthHttpHandler = new OAuthHttpHandler(oauthService, oauthHttpPort)
+    oauthHttpHandler.start()
+
     val server = buildServer(
       gamesManager,
       customBattleManager,
       options.getOrElse(eagleGrpcPortKey, defaultEagleGrpcPort).toInt,
-      SeededRandom(random.nextLong())
+      SeededRandom(random.nextLong()),
+      oauthService
     ).start
     SimpleTimedLogger.printLogger.logLine(
       s"Eagle server is listening on port ${server.getPort}"
@@ -94,7 +123,8 @@ object Main {
       gamesManager: GamesManager,
       customBattleManager: CustomBattleManager,
       grpcPort: Int,
-      functionalRandom: FunctionalRandom
+      functionalRandom: FunctionalRandom,
+      oauthService: OAuthService
   ): Server = {
     val serverBuilder = ServerBuilder.forPort(grpcPort)
 
@@ -102,7 +132,6 @@ object Main {
     val jwtService: Option[JwtService] = JwtServiceImpl.fromEnvironment()
     val authPersister                  = LocalFilePersister(SaveDirectory.saveDirectory)
     val userService                    = new UserServiceImpl(authPersister)
-    val oauthService                   = new OAuthServiceImpl()
 
     // Add Eagle game service
     serverBuilder.addService(

src/main/scala/net/eagle0/eagle/auth/BUILD.bazel

@@ -28,6 +28,9 @@ scala_library(
         "//src/main/scala/net/eagle0/eagle:__subpackages__",
         "//src/test/scala/net/eagle0/eagle:__subpackages__",
     ],
+    exports = [
+        "//src/main/protobuf/net/eagle0/eagle/api:auth_scala_grpc",
+    ],
     deps = [
         ":oauth_config",
         "//src/main/protobuf/net/eagle0/eagle/api:auth_scala_grpc",

src/main/scala/net/eagle0/eagle/auth/OAuthService.scala

@@ -24,22 +24,30 @@ case class ProviderUserInfo(
     username: String
 )
 
+/** Result of OAuth flow */
+sealed trait OAuthResult
+case class OAuthSuccess(providerInfo: ProviderUserInfo, provider: OAuthProvider) extends OAuthResult
+case class OAuthFailure(error: String)                                           extends OAuthResult
+case object OAuthPending                                                         extends OAuthResult
+case object OAuthExpired                                                         extends OAuthResult
+
 /** Service for OAuth code exchange */
 trait OAuthService {
 
-  /** Generate OAuth authorization URL */
-  def getAuthUrl(provider: OAuthProvider, redirectUri: String): (String, String) // (url, state)
+  /** Generate OAuth authorization URL. Server handles callback internally. */
+  def getAuthUrl(provider: OAuthProvider): (String, String) // (url, state)
 
-  /** Exchange authorization code for user info */
-  def exchangeCode(
-      provider: OAuthProvider,
-      code: String,
-      state: String,
-      redirectUri: String
-  ): Either[String, ProviderUserInfo]
+  /** Check status of OAuth flow for given state */
+  def checkStatus(state: String): OAuthResult
+
+  /** Handle OAuth callback - exchange code for user info and store result */
+  def handleCallback(code: String, state: String): Either[String, ProviderUserInfo]
+
+  /** Get the server's OAuth callback URL */
+  def callbackUrl: String
 }
 
-class OAuthServiceImpl extends OAuthService {
+class OAuthServiceImpl(serverBaseUrl: String) extends OAuthService {
 
   private implicit val formats: DefaultFormats.type = DefaultFormats
   private val json                                  = new Json(formats)
@@ -49,25 +57,29 @@ class OAuthServiceImpl extends OAuthService {
     .connectTimeout(Duration.ofSeconds(10))
     .build()
 
-  // State parameter storage with timestamp (for CSRF protection)
-  private val stateStore = TrieMap[String, Long]()
+  // Pending OAuth sessions: state -> (provider, timestamp)
+  private val pendingOAuth = TrieMap[String, (OAuthProvider, Long)]()
+
+  // Completed OAuth results: state -> result
+  private val completedOAuth = TrieMap[String, OAuthResult]()
 
   // State expiration (10 minutes)
   private val stateExpirationMs = 600000L
 
-  override def getAuthUrl(provider: OAuthProvider, redirectUri: String): (String, String) = {
+  override def callbackUrl: String = s"$serverBaseUrl/oauth/callback"
+
+  override def getAuthUrl(provider: OAuthProvider): (String, String) = {
     val config = getConfig(provider)
 
     val state = UUID.randomUUID().toString
-    stateStore.put(state, System.currentTimeMillis())
+    pendingOAuth.put(state, (provider, System.currentTimeMillis()))
 
     // Clean old states
-    val cutoff = System.currentTimeMillis() - stateExpirationMs
-    stateStore.filterInPlace((_, timestamp) => timestamp > cutoff)
+    cleanupExpiredStates()
 
     val params = Map(
       "client_id"     -> config.clientId,
-      "redirect_uri"  -> redirectUri,
+      "redirect_uri"  -> callbackUrl,
       "response_type" -> "code",
       "scope"         -> config.scopes.mkString(" "),
       "state"         -> state
@@ -83,30 +95,67 @@ class OAuthServiceImpl extends OAuthService {
     (url, state)
   }
 
-  override def exchangeCode(
-      provider: OAuthProvider,
-      code: String,
-      state: String,
-      redirectUri: String
-  ): Either[String, ProviderUserInfo] = {
-    // Verify state
-    stateStore.remove(state) match {
-      case Some(timestamp) if System.currentTimeMillis() - timestamp < stateExpirationMs =>
-      // Valid state
-      case _                                                                             =>
-        return Left("Invalid or expired state parameter")
+  override def checkStatus(state: String): OAuthResult =
+    // First check completed results
+    completedOAuth.get(state) match {
+      case Some(result) => result
+      case None         =>
+        // Check if still pending
+        pendingOAuth.get(state) match {
+          case Some((_, timestamp)) if System.currentTimeMillis() - timestamp < stateExpirationMs =>
+            OAuthPending
+          case Some(_)                                                                            =>
+            // Expired
+            pendingOAuth.remove(state)
+            OAuthExpired
+          case None                                                                               =>
+            // Unknown state - might be expired and cleaned up
+            OAuthExpired
+        }
     }
 
-    val config = getConfig(provider)
+  override def handleCallback(code: String, state: String): Either[String, ProviderUserInfo] =
+    // Get pending OAuth info
+    pendingOAuth.remove(state) match {
+      case Some((provider, timestamp)) if System.currentTimeMillis() - timestamp < stateExpirationMs =>
+        val config = getConfig(provider)
 
-    // Exchange code for access token
-    val tokenResult = exchangeCodeForToken(config, code, redirectUri)
-    tokenResult match {
-      case Left(error)        => Left(error)
-      case Right(accessToken) =>
-        // Fetch user info
-        fetchUserInfo(provider, config, accessToken)
+        // Exchange code for access token
+        val tokenResult = exchangeCodeForToken(config, code)
+        tokenResult match {
+          case Left(error)        =>
+            completedOAuth.put(state, OAuthFailure(error))
+            Left(error)
+          case Right(accessToken) =>
+            // Fetch user info
+            fetchUserInfo(provider, config, accessToken) match {
+              case Left(error)     =>
+                completedOAuth.put(state, OAuthFailure(error))
+                Left(error)
+              case Right(userInfo) =>
+                completedOAuth.put(state, OAuthSuccess(userInfo, provider))
+                Right(userInfo)
+            }
+        }
+
+      case Some(_) =>
+        completedOAuth.put(state, OAuthExpired)
+        Left("OAuth session expired")
+
+      case None =>
+        Left("Invalid or expired state parameter")
     }
+
+  private def cleanupExpiredStates(): Unit = {
+    val cutoff = System.currentTimeMillis() - stateExpirationMs
+    pendingOAuth.filterInPlace((_, v) => v._2 > cutoff)
+    completedOAuth.filterInPlace((state, _) =>
+      pendingOAuth.contains(state) ||
+        completedOAuth.get(state).exists {
+          case OAuthSuccess(_, _) => true // Keep successes for a bit
+          case _                  => false
+        }
+    )
   }
 
   private def getConfig(provider: OAuthProvider): OAuthProviderConfig =
@@ -118,8 +167,7 @@ class OAuthServiceImpl extends OAuthService {
 
   private def exchangeCodeForToken(
       config: OAuthProviderConfig,
-      code: String,
-      redirectUri: String
+      code: String
   ): Either[String, String] =
     Try {
       val formData = Map(
@@ -127,7 +175,7 @@ class OAuthServiceImpl extends OAuthService {
         "client_secret" -> config.clientSecret,
         "grant_type"    -> "authorization_code",
         "code"          -> code,
-        "redirect_uri"  -> redirectUri
+        "redirect_uri"  -> callbackUrl
       )
 
       val formBody = formData.map {

src/main/scala/net/eagle0/eagle/service/AuthServiceImpl.scala

@@ -6,7 +6,15 @@ import scala.concurrent.Future
 
 import net.eagle0.eagle.api.auth.auth.*
 import net.eagle0.eagle.api.auth.auth.AuthGrpc.Auth
-import net.eagle0.eagle.auth.{JwtService, OAuthService, ProviderUserInfo, UserService}
+import net.eagle0.eagle.auth.{
+  JwtService,
+  OAuthExpired,
+  OAuthFailure,
+  OAuthPending,
+  OAuthService,
+  OAuthSuccess,
+  UserService
+}
 import net.eagle0.eagle.internal.user.user.User
 
 /** gRPC service implementation for OAuth authentication */
@@ -18,58 +26,62 @@ class AuthServiceImpl(
 
   override def getOAuthUrl(request: GetOAuthUrlRequest): Future[GetOAuthUrlResponse] =
     Future {
-      val (url, state) = oauthService.getAuthUrl(request.provider, request.redirectUri)
+      val (url, state) = oauthService.getAuthUrl(request.provider)
       GetOAuthUrlResponse(authUrl = url, state = state)
     }
 
-  override def exchangeCode(request: ExchangeCodeRequest): Future[ExchangeCodeResponse] =
-    Future {
-      oauthService.exchangeCode(
-        provider = request.provider,
-        code = request.authorizationCode,
-        state = request.state,
-        redirectUri = request.redirectUri
-      ) match {
-        case Left(error) =>
-          throw new io.grpc.StatusRuntimeException(
-            io.grpc.Status.UNAUTHENTICATED.withDescription(error)
-          )
+  override def checkOAuthStatus(
+      request: CheckOAuthStatusRequest
+  ): Future[CheckOAuthStatusResponse] = Future {
+    oauthService.checkStatus(request.state) match {
+      case OAuthPending =>
+        CheckOAuthStatusResponse(status = OAuthStatus.OAUTH_STATUS_PENDING)
 
-        case Right(providerInfo) =>
-          // Find or create user
-          val providerName = request.provider match {
-            case OAuthProvider.OAUTH_PROVIDER_DISCORD => "discord"
-            case OAuthProvider.OAUTH_PROVIDER_GOOGLE  => "google"
-            case _                                    => "unknown"
-          }
+      case OAuthExpired =>
+        CheckOAuthStatusResponse(status = OAuthStatus.OAUTH_STATUS_EXPIRED)
 
-          val user = userService.findOrCreateUser(
-            provider = providerName,
-            providerUserId = providerInfo.id,
-            email = providerInfo.email,
-            avatarUrl = providerInfo.avatarUrl
-          )
+      case OAuthFailure(error) =>
+        CheckOAuthStatusResponse(
+          status = OAuthStatus.OAUTH_STATUS_FAILED,
+          errorMessage = error
+        )
 
-          val isNewUser = user.displayName.isEmpty
+      case OAuthSuccess(providerInfo, provider) =>
+        // Find or create user
+        val providerName = provider match {
+          case OAuthProvider.OAUTH_PROVIDER_DISCORD => "discord"
+          case OAuthProvider.OAUTH_PROVIDER_GOOGLE  => "google"
+          case _                                    => "unknown"
+        }
 
-          // Create JWT tokens
-          val accessToken = jwtService.createAccessToken(
-            userId = user.userId,
-            displayName = user.displayName,
-            isAdmin = user.isAdmin
-          )
+        val user = userService.findOrCreateUser(
+          provider = providerName,
+          providerUserId = providerInfo.id,
+          email = providerInfo.email,
+          avatarUrl = providerInfo.avatarUrl
+        )
 
-          val (refreshToken, refreshInfo) = jwtService.createRefreshToken(user.userId)
+        val isNewUser = user.displayName.isEmpty
 
-          ExchangeCodeResponse(
-            accessToken = accessToken,
-            refreshToken = refreshToken,
-            expiresAt = refreshInfo.expiresAt.toEpochMilli / 1000,
-            user = Some(toUserInfo(user, request.provider)),
-            isNewUser = isNewUser
-          )
-      }
+        // Create JWT tokens
+        val accessToken = jwtService.createAccessToken(
+          userId = user.userId,
+          displayName = user.displayName,
+          isAdmin = user.isAdmin
+        )
+
+        val (refreshToken, refreshInfo) = jwtService.createRefreshToken(user.userId)
+
+        CheckOAuthStatusResponse(
+          status = OAuthStatus.OAUTH_STATUS_SUCCESS,
+          accessToken = accessToken,
+          refreshToken = refreshToken,
+          expiresAt = refreshInfo.expiresAt.toEpochMilli / 1000,
+          user = Some(toUserInfo(user, provider)),
+          isNewUser = isNewUser
+        )
     }
+  }
 
   override def setDisplayName(
       request: SetDisplayNameRequest

src/main/scala/net/eagle0/eagle/service/BUILD.bazel

@@ -18,6 +18,18 @@ scala_library(
     ],
 )
 
+scala_library(
+    name = "oauth_http_handler",
+    srcs = ["OAuthHttpHandler.scala"],
+    visibility = [
+        "//src/main/scala/net/eagle0/eagle:__pkg__",
+    ],
+    deps = [
+        "//src/main/scala/net/eagle0/common:simple_timed_logger",
+        "//src/main/scala/net/eagle0/eagle/auth:oauth_service",
+    ],
+)
+
 scala_library(
     name = "service",
     srcs = [

src/main/scala/net/eagle0/eagle/service/OAuthHttpHandler.scala

@@ -0,0 +1,162 @@
+// Copyright 2025 Dan Crosby
+package net.eagle0.eagle.service
+
+import java.io.{OutputStream, PrintWriter, StringWriter}
+import java.net.{InetSocketAddress, URLDecoder}
+import java.nio.charset.StandardCharsets
+
+import com.sun.net.httpserver.{HttpExchange, HttpHandler, HttpServer}
+import net.eagle0.common.SimpleTimedLogger
+import net.eagle0.eagle.auth.OAuthService
+
+/** Simple HTTP server for OAuth callback handling */
+class OAuthHttpHandler(oauthService: OAuthService, httpPort: Int) {
+
+  private var server: HttpServer = _
+
+  def start(): Unit = {
+    server = HttpServer.create(new InetSocketAddress(httpPort), 0)
+    server.createContext("/oauth/callback", new CallbackHandler)
+    server.setExecutor(null) // Use default executor
+    server.start()
+    SimpleTimedLogger.printLogger.logLine(
+      s"OAuth HTTP server listening on port $httpPort"
+    )
+  }
+
+  def stop(): Unit =
+    if server != null then {
+      server.stop(0)
+    }
+
+  private class CallbackHandler extends HttpHandler {
+    override def handle(exchange: HttpExchange): Unit =
+      try {
+        val query  = exchange.getRequestURI.getQuery
+        val params = parseQuery(query)
+
+        // Check for OAuth error
+        params.get("error") match {
+          case Some(error) =>
+            val errorDesc = params.getOrElse("error_description", "Unknown error")
+            val state     = params.getOrElse("state", "unknown")
+            SimpleTimedLogger.printLogger.logLine(
+              s"OAuth error for state $state: $error - $errorDesc"
+            )
+            sendResponse(
+              exchange,
+              400,
+              s"""
+                 |<!DOCTYPE html>
+                 |<html>
+                 |<head><title>Login Failed</title></head>
+                 |<body style="font-family: sans-serif; text-align: center; padding: 50px;">
+                 |<h1>Login Failed</h1>
+                 |<p>$error: $errorDesc</p>
+                 |<p>You can close this tab and try again.</p>
+                 |</body>
+                 |</html>
+                 |""".stripMargin
+            )
+            return
+
+          case None => // Continue
+        }
+
+        // Get code and state
+        val code = params.get("code") match {
+          case Some(c) => c
+          case None    =>
+            sendResponse(exchange, 400, "Missing authorization code")
+            return
+        }
+
+        val state = params.get("state") match {
+          case Some(s) => s
+          case None    =>
+            sendResponse(exchange, 400, "Missing state parameter")
+            return
+        }
+
+        // Handle the callback
+        oauthService.handleCallback(code, state) match {
+          case Left(error) =>
+            SimpleTimedLogger.printLogger.logLine(s"OAuth callback failed: $error")
+            sendResponse(
+              exchange,
+              400,
+              s"""
+                 |<!DOCTYPE html>
+                 |<html>
+                 |<head><title>Login Failed</title></head>
+                 |<body style="font-family: sans-serif; text-align: center; padding: 50px;">
+                 |<h1>Login Failed</h1>
+                 |<p>$error</p>
+                 |<p>You can close this tab and try again.</p>
+                 |</body>
+                 |</html>
+                 |""".stripMargin
+            )
+
+          case Right(userInfo) =>
+            SimpleTimedLogger.printLogger.logLine(
+              s"OAuth callback successful for user ${userInfo.username}"
+            )
+            sendResponse(
+              exchange,
+              200,
+              s"""
+                 |<!DOCTYPE html>
+                 |<html>
+                 |<head><title>Login Successful</title></head>
+                 |<body style="font-family: sans-serif; text-align: center; padding: 50px;">
+                 |<h1>Login Successful!</h1>
+                 |<p>Welcome, ${userInfo.username}!</p>
+                 |<p>You can close this tab and return to the game.</p>
+                 |</body>
+                 |</html>
+                 |""".stripMargin
+            )
+        }
+      } catch {
+        case e: Exception =>
+          val sw = new StringWriter()
+          e.printStackTrace(new PrintWriter(sw))
+          SimpleTimedLogger.printLogger.logLine(
+            s"OAuth callback error: ${e.getMessage}\n${sw.toString}"
+          )
+          sendResponse(exchange, 500, s"Internal server error: ${e.getMessage}")
+      }
+
+    private def parseQuery(query: String): Map[String, String] = {
+      if query == null || query.isEmpty then return Map.empty
+
+      query
+        .split("&")
+        .flatMap { pair =>
+          pair.split("=", 2) match {
+            case Array(key, value) =>
+              Some(
+                URLDecoder.decode(key, StandardCharsets.UTF_8) ->
+                  URLDecoder.decode(value, StandardCharsets.UTF_8)
+              )
+            case _                 => None
+          }
+        }
+        .toMap
+    }
+
+    private def sendResponse(
+        exchange: HttpExchange,
+        code: Int,
+        body: String
+    ): Unit = {
+      val bytes            = body.getBytes(StandardCharsets.UTF_8)
+      exchange.getResponseHeaders.add("Content-Type", "text/html; charset=utf-8")
+      exchange.sendResponseHeaders(code, bytes.length)
+      val os: OutputStream = exchange.getResponseBody
+      os.write(bytes)
+      os.close()
+    }
+  }
+}