diff --git a/autofit/non_linear/paths/abstract.py b/autofit/non_linear/paths/abstract.py
index d1d40426d..0485535f7 100644
--- a/autofit/non_linear/paths/abstract.py
+++ b/autofit/non_linear/paths/abstract.py
@@ -313,6 +313,35 @@ def _zip(self):
         except FileNotFoundError:
             pass
 
+    def preserve_in_zip(self, file_path):
+        """
+        Add a file (already inside this search's output directory) to the
+        search's ``.zip`` archive so it survives the resume cycle.
+
+        ``restore()`` deletes the output directory and re-extracts the zip, so
+        any file written into e.g. ``files/`` *after* a search completed — such
+        as a cache artifact derived from the finished result — would be
+        destroyed by the next resume unless it is also a member of the zip.
+
+        No-op when the zip does not exist (e.g. the search is still running,
+        so the file will be zipped with everything else at completion) and
+        when the member is already present.
+
+        Parameters
+        ----------
+        file_path
+            Absolute path of the file to preserve; must live under
+            ``output_path``, from which its archive name is derived.
+        """
+        if not Path(self._zip_path).exists():
+            return
+
+        arcname = str(Path(file_path).relative_to(self.output_path))
+
+        with zipfile.ZipFile(self._zip_path, "a") as f:
+            if arcname not in f.namelist():
+                f.write(file_path, arcname)
+
     def restore(self):
         """
         Copy files from the ``.zip`` file to the samples folder.
diff --git a/test_autofit/non_linear/paths/test_paths.py b/test_autofit/non_linear/paths/test_paths.py
index dd875caa4..50635bf20 100644
--- a/test_autofit/non_linear/paths/test_paths.py
+++ b/test_autofit/non_linear/paths/test_paths.py
@@ -115,3 +115,49 @@ def test_make_path_excludes_segment_when_level_zero(self, monkeypatch):
         monkeypatch.setenv("PYAUTO_TEST_MODE", "0")
         paths = af.DirectoryPaths(name="name", path_prefix="prefix")
         assert "test_mode" not in paths._make_path().parts
+
+
+def test__preserve_in_zip__file_survives_restore(tmp_path):
+    import zipfile
+
+    paths = af.DirectoryPaths(name="preserve_test", path_prefix=str(tmp_path))
+
+    files_path = Path(paths._files_path)
+    files_path.mkdir(parents=True, exist_ok=True)
+    (files_path / "samples_summary.json").write_text("{}")
+
+    paths.zip_remove()
+    assert Path(paths._zip_path).exists()
+
+    # A post-completion cache write: on disk but not in the zip.
+    cache_file = files_path / "cache_artifact.json"
+    files_path.mkdir(parents=True, exist_ok=True)
+    cache_file.write_text('{"cached": true}')
+
+    paths.preserve_in_zip(cache_file)
+
+    with zipfile.ZipFile(paths._zip_path) as f:
+        assert "files/cache_artifact.json" in f.namelist()
+
+    # Idempotent — appending again must not duplicate the member.
+    paths.preserve_in_zip(cache_file)
+    with zipfile.ZipFile(paths._zip_path) as f:
+        assert f.namelist().count("files/cache_artifact.json") == 1
+
+    # The restore cycle (rmtree + re-extract) keeps the preserved file.
+    paths.restore()
+    assert cache_file.exists()
+
+
+def test__preserve_in_zip__no_zip_is_a_no_op(tmp_path):
+    paths = af.DirectoryPaths(name="preserve_noop_test", path_prefix=str(tmp_path))
+
+    files_path = Path(paths._files_path)
+    files_path.mkdir(parents=True, exist_ok=True)
+    cache_file = files_path / "cache_artifact.json"
+    cache_file.write_text('{"cached": true}')
+
+    paths.preserve_in_zip(cache_file)
+
+    assert not Path(paths._zip_path).exists()
+    assert cache_file.exists()