diff --git a/pysus/api/client.py b/pysus/api/client.py
index bb54f2aa..3fa80c19 100644
--- a/pysus/api/client.py
+++ b/pysus/api/client.py
@@ -761,7 +761,11 @@ async def query(
 
         if client:
             prefix = f"public/data/{client.lower()}/"
-            files = [f for f in files if str(f.path).startswith(prefix)]
+            files = [
+                f
+                for f in files
+                if str(f.path).replace("\\", "/").startswith(prefix)
+            ]
 
         if not as_dataframe:
             return files
diff --git a/pysus/tests/api/test_client.py b/pysus/tests/api/test_client.py
index 78a24122..b36b9d0f 100644
--- a/pysus/tests/api/test_client.py
+++ b/pysus/tests/api/test_client.py
@@ -513,6 +513,7 @@ async def test_query_no_dataset_iterates_all(self, test_db_path):
 
     @pytest.mark.asyncio
     async def test_query_with_client_filter(self, test_db_path):
+        from pathlib import PureWindowsPath
         from unittest.mock import AsyncMock, MagicMock
 
         from pysus.api.ducklake.client import DuckLake
@@ -529,7 +530,12 @@ async def test_query_with_client_filter(self, test_db_path):
         mock_file2 = MagicMock()
         mock_file2.path = "public/data/dadosgov/otherfile"
 
-        ds.query = AsyncMock(return_value=[mock_file1, mock_file2])
+        # DuckLake may return WindowsPath instances on Windows. The client
+        # filter must compare normalized POSIX paths so FTP files are retained.
+        mock_file3 = MagicMock()
+        mock_file3.path = PureWindowsPath("public/data/ftp/windows-file")
+
+        ds.query = AsyncMock(return_value=[mock_file1, mock_file2, mock_file3])
         mock_ducklake.datasets = AsyncMock(return_value=[ds])
 
         client._ducklake = mock_ducklake
@@ -537,7 +543,7 @@ async def test_query_with_client_filter(self, test_db_path):
 
         result = await client.query(dataset="sinan", client=FTP)
 
-        assert result == [mock_file1]
+        assert result == [mock_file1, mock_file3]
 
         await client.__aexit__(None, None, None)