diff --git a/src/lampyrid/server.py b/src/lampyrid/server.py
index 4b1682c..fad1cb4 100644
--- a/src/lampyrid/server.py
+++ b/src/lampyrid/server.py
@@ -8,7 +8,11 @@
 from fastmcp.server.auth.providers.google import GoogleProvider
 from fastmcp.utilities.logging import configure_logging
 from fastmcp.utilities.types import Image
-from key_value.aio.stores.filetree import FileTreeStore
+from key_value.aio.stores.filetree import (
+    FileTreeStore,
+    FileTreeV1CollectionSanitizationStrategy,
+    FileTreeV1KeySanitizationStrategy,
+)
 from key_value.aio.wrappers.encryption import FernetEncryptionWrapper
 from mcp.types import Icon
 
@@ -35,8 +39,20 @@ def _create_auth_provider() -> Optional[AuthProvider]:
             # Create storage directory if it doesn't exist
             settings.oauth_storage_path.mkdir(parents=True, exist_ok=True)
 
-            # Initialize file-tree storage with Fernet encryption
-            file_tree_store = FileTreeStore(data_directory=settings.oauth_storage_path)
+            # Initialize file-tree storage with Fernet encryption.
+            # Sanitize keys/collections so URL-based client_ids (e.g. Goose's CIMD
+            # client_id `https://.../client-metadata.json`) don't get interpreted as
+            # nested directory paths, which fails with FileNotFoundError. The default
+            # PassthroughStrategy leaves slashes untouched.
+            file_tree_store = FileTreeStore(
+                data_directory=settings.oauth_storage_path,
+                key_sanitization_strategy=FileTreeV1KeySanitizationStrategy(
+                    directory=settings.oauth_storage_path
+                ),
+                collection_sanitization_strategy=FileTreeV1CollectionSanitizationStrategy(
+                    directory=settings.oauth_storage_path
+                ),
+            )
             client_storage = FernetEncryptionWrapper(
                 key_value=file_tree_store,
                 fernet=Fernet(settings.oauth_storage_encryption_key),  # ty:ignore[invalid-argument-type]
diff --git a/tests/unit/test_server.py b/tests/unit/test_server.py
index 7f568ef..39d0a4b 100644
--- a/tests/unit/test_server.py
+++ b/tests/unit/test_server.py
@@ -63,10 +63,16 @@ def test_create_auth_provider_with_persistence(self):
             with (
                 patch('lampyrid.server.GoogleProvider') as mock_google_provider,
                 patch('lampyrid.server.FileTreeStore') as mock_file_tree_store,
+                patch('lampyrid.server.FileTreeV1KeySanitizationStrategy') as mock_key_strategy,
+                patch(
+                    'lampyrid.server.FileTreeV1CollectionSanitizationStrategy'
+                ) as mock_collection_strategy,
                 patch('lampyrid.server.FernetEncryptionWrapper') as mock_encryption_wrapper,
             ):
                 mock_google_provider.return_value = MagicMock()
                 mock_file_tree_store.return_value = MagicMock()
+                mock_key_strategy.return_value = MagicMock()
+                mock_collection_strategy.return_value = MagicMock()
                 mock_encryption_wrapper.return_value = MagicMock()
 
                 result = _create_auth_provider()
@@ -78,7 +84,17 @@ def test_create_auth_provider_with_persistence(self):
                 )
                 # Verify file tree store and encryption wrapper were initialized
                 mock_file_tree_store.assert_called_once_with(
-                    data_directory=mock_settings.oauth_storage_path
+                    data_directory=mock_settings.oauth_storage_path,
+                    key_sanitization_strategy=mock_key_strategy.return_value,
+                    collection_sanitization_strategy=mock_collection_strategy.return_value,
+                )
+                # Sanitization strategies must be built against the storage path so that
+                # URL-based client_ids (Goose CIMD) don't break filesystem persistence
+                mock_key_strategy.assert_called_once_with(
+                    directory=mock_settings.oauth_storage_path
+                )
+                mock_collection_strategy.assert_called_once_with(
+                    directory=mock_settings.oauth_storage_path
                 )
                 mock_encryption_wrapper.assert_called_once()
                 # Verify GoogleProvider was called with client_storage