diff --git a/backend/apps/swarm/closure.py b/backend/apps/swarm/closure.py
index b841ca00..0293ff3b 100644
--- a/backend/apps/swarm/closure.py
+++ b/backend/apps/swarm/closure.py
@@ -315,6 +315,13 @@ def p_read_payload(sandbox: str, ref: EntityRef) -> dict:
         return json.load(f)
 
 
+def bundle_key(full: str, base: str) -> str:
+    """A staged file's key inside the bundle: slash-separated on every OS. On Windows relpath gave
+    `workspace\\app\\...`, the app importer looked for `workspace/`, skipped every file, and saved a
+    hollow app that then said its files were missing (the Tisusa import, 2026-09-04)."""
+    return os.path.relpath(full, base).replace(os.sep, "/")
+
+
 def p_read_files(sandbox: str, ref: EntityRef) -> dict[str, bytes]:
     base = p_safe_join(sandbox, os.path.join(ref.path, "files"))
     out: dict[str, bytes] = {}
@@ -324,7 +331,7 @@ def p_read_files(sandbox: str, ref: EntityRef) -> dict[str, bytes]:
         for fn in fnames:
             full = os.path.join(root, fn)
             with open(full, "rb") as f:
-                out[os.path.relpath(full, base)] = f.read()
+                out[bundle_key(full, base)] = f.read()
     return out
 
 
diff --git a/backend/apps/swarm/entities/apps.py b/backend/apps/swarm/entities/apps.py
index 409a871f..828cfd0c 100644
--- a/backend/apps/swarm/entities/apps.py
+++ b/backend/apps/swarm/entities/apps.py
@@ -93,6 +93,10 @@ class AppExportable:
             with open(dest, "wb") as f:
                 f.write(data)
             wrote_workspace = True
+        if files and not wrote_workspace:
+            # The bundle shipped files but none under workspace/: a hollow app would import "successfully" and
+            # then say its files are missing. Fail here, where the rollback still knows what to undo.
+            raise ValueError("the app's files are not where the bundle format puts them; nothing was imported")
         if wrote_workspace:
             p_localize_env(folder)
 
diff --git a/backend/apps/swarm/entities/skills.py b/backend/apps/swarm/entities/skills.py
index 8530d638..0c604c35 100644
--- a/backend/apps/swarm/entities/skills.py
+++ b/backend/apps/swarm/entities/skills.py
@@ -111,7 +111,8 @@ def read_supporting_files(skill_dir: str) -> dict[str, bytes]:
         p_dirs[:] = [d for d in p_dirs if d not in WALK_SKIP_DIRS]
         for n in names:
             full = os.path.join(root, n)
-            rel = os.path.relpath(full, skill_dir)
+            # Slash keys on every OS, like the app exporter; a Windows export used to ship `scripts\\x.py`.
+            rel = os.path.relpath(full, skill_dir).replace(os.sep, "/")
             if rel == "SKILL.md" or n.startswith("."):
                 continue
             try:
diff --git a/backend/tests/test_swarm_windows_paths.py b/backend/tests/test_swarm_windows_paths.py
new file mode 100644
index 00000000..75a86725
--- /dev/null
+++ b/backend/tests/test_swarm_windows_paths.py
@@ -0,0 +1,38 @@
+"""A Windows user imported the Tisusa .swarm (507 workspace files) and got an app card that said
+"This app's files are missing" (2026-09-04). The staging reader keyed files with the OS separator,
+the app importer looked for `workspace/`, skipped every file, and saved a hollow app. Bundle keys
+are slash-separated everywhere now, and a bundle that ships files but none under workspace/ fails
+loudly instead of importing a shell."""
+
+import os
+
+import pytest
+
+from backend.apps.swarm import closure
+from backend.apps.swarm.entities.apps import AppExportable
+
+
+def test_bundle_keys_are_slashes_even_when_the_os_answers_with_backslashes(monkeypatch):
+    real_relpath = os.path.relpath
+    # The Windows shape: relpath answers with backslashes and the separator is a backslash.
+    monkeypatch.setattr(os.path, "relpath", lambda full, start: real_relpath(full, start).replace("/", "\\"))
+    monkeypatch.setattr(os, "sep", "\\")
+    assert closure.bundle_key("/sb/entities/e1/files/workspace/app/index.html", "/sb/entities/e1/files") == "workspace/app/index.html"
+
+
+def test_the_staging_reader_uses_the_slash_keys(tmp_path):
+    base = tmp_path / "entities" / "e1" / "files" / "workspace" / "app"
+    base.mkdir(parents=True)
+    (base / "index.html").write_bytes(b"<h1>x</h1>")
+    ref = type("Ref", (), {"path": "entities/e1"})()
+    assert list(closure.p_read_files(str(tmp_path), ref)) == ["workspace/app/index.html"]
+    src = open(closure.__file__).read()
+    assert "out[bundle_key(full, base)]" in src
+
+
+def test_a_bundle_whose_files_miss_the_workspace_prefix_fails_loudly(tmp_path, monkeypatch):
+    monkeypatch.setattr("backend.apps.swarm.entities.apps.OUTPUTS_WORKSPACE_DIR", str(tmp_path / "ws"))
+    monkeypatch.setattr("backend.apps.swarm.entities.apps.OUTPUTS_DIR", str(tmp_path / "out"))
+    with pytest.raises(ValueError, match="nothing was imported"):
+        AppExportable.import_({"name": "Hollow"}, {"workspace\\\\app\\\\index.html": b"<h1>x</h1>"}, {})
+    assert not (tmp_path / "out").exists() or not list((tmp_path / "out").glob("*.json"))