diff --git a/codearchiver/cli.py b/codearchiver/cli.py
index 1affcc8..de698fd 100644
--- a/codearchiver/cli.py
+++ b/codearchiver/cli.py
@@ -143,6 +143,8 @@ def parse_args():
 	parser.add_argument('--version', action = 'version', version = f'codearchiver {codearchiver.version.__version__}')
 	parser.add_argument('-v', '--verbose', '--verbosity', dest = 'verbosity', action = 'count', default = 0, help = 'Increase output verbosity')
 	parser.add_argument('--dump-locals', dest = 'dumpLocals', action = 'store_true', default = False, help = 'Dump local variables on serious log messages (warnings or higher)')
+	# Undocumented option to write one line for each artefact filename produced by this process to FD 3.
+	parser.add_argument('--write-artefacts-fd-3', dest = 'writeArtefactsFd3', action = 'store_true', help = argparse.SUPPRESS)
 	parser.add_argument('url', help = 'Target URL')
 	args = parser.parse_args()
 	return args
@@ -190,6 +192,8 @@ def main():
 	import codearchiver.storage
 	with _dump_locals_on_exception():
 		inputUrl = codearchiver.core.InputURL(args.url)
+		if args.writeArtefactsFd3:
+			artefactsFd = os.fdopen(3, 'w')
 		storage = codearchiver.storage.DirectoryStorage(os.getcwd())
 		module = codearchiver.core.get_module_instance(inputUrl, storage = storage)
 		with tempfile.TemporaryDirectory(prefix = 'tmp.codearchiver.', dir = os.getcwd()) as td:
@@ -199,6 +203,9 @@ def main():
 				result = module.process()
 			finally:
 				os.chdir('..')
+		if args.writeArtefactsFd3:
+			for filename in storage.newFiles:
+				print(filename, file = artefactsFd)
 
 if __name__ == '__main__':
 	main()
diff --git a/codearchiver/storage.py b/codearchiver/storage.py
index 26d6e55..d94db42 100644
--- a/codearchiver/storage.py
+++ b/codearchiver/storage.py
@@ -13,6 +13,12 @@ _logger = logging.getLogger(__name__)
 
 
 class Storage(abc.ABC):
+	'''
+	Interface for storage backing the codearchiver collection
+	This serves primarily to aid deduplication by locating prior archives of the same or closely related repositories.
+	Filenames must not contain LF.
+	'''
+
 	@abc.abstractmethod
 	def put(self, filename: str, metadata: typing.Optional['codearchiver.core.Metadata'] = None):
 		'''Put a local file and (if provided) its metadata into storage. If an error occurs, a partial copy may remain in storage. If it completes, the local input file is removed.'''
@@ -24,6 +30,15 @@ class Storage(abc.ABC):
 		for _, subresult in result.submoduleResults:
 			self.put_result(subresult)
 
+	@property
+	@abc.abstractmethod
+	def newFiles(self) -> list[str]:
+		'''
+		List of all files that have been `.put()` on this instance.
+		This may include additional files for storing metadata.
+		'''
+		# The return value must be a copy of the state.
+
 	@abc.abstractmethod
 	def search_metadata(self, criteria: list[tuple[str, typing.Union[str, tuple[str]]]]) -> collections.abc.Iterator[str]:
 		'''
@@ -47,6 +62,7 @@ class DirectoryStorage(Storage):
 	def __init__(self, directory):
 		super().__init__()
 		self._directory = directory
+		self._newFiles = []
 
 	def _check_directory(self):
 		exists = os.path.exists(self._directory)
@@ -60,11 +76,14 @@ class DirectoryStorage(Storage):
 
 	def put(self, filename, metadata = None):
 		self._ensure_directory()
+		if '\n' in filename:
+			raise ValueError(fr'filenames cannot contain \n: {filename!r}')
 		#FIXME: Race condition
 		if os.path.exists((targetFilename := os.path.join(self._directory, os.path.basename(filename)))):
 			raise FileExistsError(f'{targetFilename} already exists')
 		_logger.info(f'Moving {filename} to {self._directory}')
 		shutil.move(filename, self._directory)
+		self._newFiles.append(filename)
 		if not metadata:
 			return
 		metadataFilename = os.path.join(self._directory, f'{filename}_codearchiver_metadata.txt')
@@ -72,6 +91,11 @@ class DirectoryStorage(Storage):
 		_logger.info(f'Writing metadata for {filename} to {metadataFilename}')
 		with open(metadataFilename, 'x') as fp:
 			fp.write(metadata.serialise())
+		self._newFiles.append(metadataFilename)
+
+	@property
+	def newFiles(self):
+		return self._newFiles.copy()
 
 	def search_metadata(self, criteria):
 		_logger.info(f'Searching metadata by criteria: {criteria!r}')
@@ -82,6 +106,7 @@ class DirectoryStorage(Storage):
 		files.sort()
 		for metadataFilename in files:
 			metadataFilename = metadataFilename[escapedDirPrefixLen:]
+			assert '\n' not in metadataFilename
 			_logger.info(f'Searching metadata {metadataFilename}')
 			with self.open(metadataFilename, 'r') as fp:
 				idx = codearchiver.core.Metadata.deserialise(fp, validate = False)
@@ -97,5 +122,7 @@ class DirectoryStorage(Storage):
 
 	@contextlib.contextmanager
 	def open(self, filename, mode = 'rb'):
+		if '\n' in filename:
+			raise ValueError(fr'filenames cannot contain \n: {filename!r}')
 		with open(os.path.join(self._directory, filename), mode) as fp:
 			yield fp