solve issue #256

lib/services/account.dart

@@ -1033,8 +1033,15 @@ class Account extends Service {
     final String apiPath = '/account/sessions/oauth2/{provider}'
         .replaceAll('{provider}', provider.value);
 
+    // Do NOT forward the custom success URL to the server.
+    // The server only appends key/secret to the redirect when the success path
+    // matches its internal default (/auth/oauth2/success). Sending a custom URL
+    // causes the server to skip adding key/secret, so the Flutter webAuth
+    // callback never receives them → "Invalid OAuth2 Response" (500).
+    // Let the server use its default success path so key/secret are always
+    // added. The custom success URL is used only as the callbackUrlScheme so
+    // Flutter can intercept the redirect after key/secret have been appended.
     final Map<String, dynamic> params = {
-      if (success != null) 'success': success,
       if (failure != null) 'failure': failure,
       if (scopes != null) 'scopes': scopes,
       'project': client.config['project'],
@@ -1369,8 +1376,14 @@ class Account extends Service {
     final String apiPath = '/account/tokens/oauth2/{provider}'
         .replaceAll('{provider}', provider.value);
 
+    // Do NOT forward the custom success URL to the server.
+    // The server (account.php) only appends userId/secret to the redirect
+    // when the success path matches its internal default. Sending a custom
+    // URL causes the server to skip adding them → "Invalid OAuth2 Response"
+    // (500) or "Invalid success param" (400) if the URL is not registered.
+    // Let the server use its default path; use the custom success URL only
+    // as the callbackUrlScheme so Flutter can intercept the redirect.
     final Map<String, dynamic> params = {
-      if (success != null) 'success': success,
       if (failure != null) 'failure': failure,
       if (scopes != null) 'scopes': scopes,
       'project': client.config['project'],

lib/src/client_io.dart

@@ -479,11 +479,25 @@ class ClientIO extends ClientBase with ClientMixin {
 
   @override
   Future webAuth(Uri url, {String? callbackUrlScheme}) {
+    // Derive the actual URI scheme to listen for.
+    // If the caller passed a full URL (e.g. "https://myapp.com/callback"),
+    // extract just the scheme ("https"). Otherwise use it as-is if it looks
+    // like a bare scheme, or fall back to the default appwrite-callback scheme.
+    String resolvedScheme = "appwrite-callback-${config['project']!}";
+    if (callbackUrlScheme != null && _customSchemeAllowed) {
+      final parsed = Uri.tryParse(callbackUrlScheme);
+      if (parsed != null && parsed.hasScheme && parsed.host.isNotEmpty) {
+        // full URL passed — use just the scheme portion
+        resolvedScheme = parsed.scheme;
+      } else {
+        // bare scheme passed (e.g. "myapp") — use as-is
+        resolvedScheme = callbackUrlScheme;
+      }
+    }
+
     return FlutterWebAuth2.authenticate(
       url: url.toString(),
-      callbackUrlScheme: callbackUrlScheme != null && _customSchemeAllowed
-          ? callbackUrlScheme
-          : "appwrite-callback-${config['project']!}",
+      callbackUrlScheme: resolvedScheme,
       options: const FlutterWebAuth2Options(
         useWebview: false,
       ),