diff --git a/Dockerfile b/Dockerfile
index d1e8de0..31d3722 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -18,7 +18,7 @@ WORKDIR /app
 #     front-end (examples/web_app.py) could not give them.
 COPY requirements.txt ./
 RUN apt-get update && apt-get install -y --no-install-recommends git \
-    && pip install --no-cache-dir "tak @ git+https://github.com/Stephenson-Software/tak@1bac386777d4b2468faeda00b31e960af8d57164" \
+    && pip install --no-cache-dir "tak @ git+https://github.com/Stephenson-Software/tak@9b539d8760e2c02cc262a7dcff6d0c541572ab71" \
     && apt-get purge -y git && apt-get autoremove -y && rm -rf /var/lib/apt/lists/*
 
 COPY src/ ./src/
diff --git a/requirements.txt b/requirements.txt
index d69fb7c..4d2a53a 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -7,5 +7,5 @@
 # tak is the text-adventure kit extracted from this game
 # (https://github.com/Stephenson-Software/tak), which FishE now runs on.
 # Installing it needs git on the PATH.
-tak @ git+https://github.com/Stephenson-Software/tak@1bac386777d4b2468faeda00b31e960af8d57164
+tak @ git+https://github.com/Stephenson-Software/tak@9b539d8760e2c02cc262a7dcff6d0c541572ab71
 jsonschema>=4.0,<5.0
diff --git a/src/saveFileManager.py b/src/saveFileManager.py
index 88f70b5..909175c 100644
--- a/src/saveFileManager.py
+++ b/src/saveFileManager.py
@@ -43,28 +43,36 @@ def __init__(self, data_directory="data"):
         )
 
     def migrate_old_save_files(self):
-        """Migrate old save files (data/*.json) to slot_1 if they exist"""
+        """Move a pre-slot save (data/*.json) into a slot, if there is one.
+
+        The save goes into slot_1 when slot_1 is free, otherwise into the next
+        free slot: an existing slot is never written over. (Before cloud saves
+        that could only happen with a hand-copied file; now a save set brought
+        in from another device can hold both layouts, and the one already in
+        a slot must survive.) With every slot taken, nothing is moved and the
+        old files stay where they are."""
         old_player = os.path.join(self.data_directory, "player.json")
-        old_stats = os.path.join(self.data_directory, "stats.json")
-        old_time = os.path.join(self.data_directory, "timeService.json")
+        old_files = ["player.json", "stats.json", "timeService.json"]
 
         # Check if old save files exist
         if not os.path.exists(old_player):
             return False
 
-        # Create slot_1 directory
-        slot_1_path = os.path.join(self.data_directory, "slot_1")
-        if not os.path.exists(slot_1_path):
-            os.makedirs(slot_1_path, exist_ok=True)
+        # The kit's rule: the lowest slot whose directory holds no file.
+        slot_number = self.get_next_available_slot()
+        if slot_number is None:
+            return False
+        slot_path = os.path.join(self.data_directory, "slot_%d" % slot_number)
+        os.makedirs(slot_path, exist_ok=True)
 
-        # Move files to slot_1
+        # Move files into the slot. The slot held no file, so nothing in it is
+        # replaced; a name that is somehow there already is left alone.
         try:
-            if os.path.exists(old_player):
-                shutil.move(old_player, os.path.join(slot_1_path, "player.json"))
-            if os.path.exists(old_stats):
-                shutil.move(old_stats, os.path.join(slot_1_path, "stats.json"))
-            if os.path.exists(old_time):
-                shutil.move(old_time, os.path.join(slot_1_path, "timeService.json"))
+            for name in old_files:
+                source = os.path.join(self.data_directory, name)
+                target = os.path.join(slot_path, name)
+                if os.path.exists(source) and not os.path.exists(target):
+                    shutil.move(source, target)
             # The migration rewrote the save directory's layout; flush it so a
             # browser-storage player doesn't re-migrate on every page load.
             syncBrowserSaves()
diff --git a/tests/test_saveFileManager.py b/tests/test_saveFileManager.py
index 50f1bfa..e6a9e6c 100644
--- a/tests/test_saveFileManager.py
+++ b/tests/test_saveFileManager.py
@@ -714,7 +714,8 @@ def test_migrate_old_save_files_partial():
 
 
 def test_migrate_old_save_files_slot1_exists():
-    """Test migration when slot_1 already exists"""
+    """A pre-slot save never replaces a save already in slot_1: it goes into
+    the next free slot, and slot_1 is left exactly as it was."""
     temp_dir = tempfile.mkdtemp()
     try:
         manager = SaveFileManager(temp_dir)
@@ -728,15 +729,66 @@ def test_migrate_old_save_files_slot1_exists():
         # Create old format files
         with open(os.path.join(temp_dir, "player.json"), "w") as f:
             json.dump({"money": 100}, f)
+        with open(os.path.join(temp_dir, "stats.json"), "w") as f:
+            json.dump({"totalFishCaught": 3}, f)
 
-        # Migration should still succeed (will overwrite)
         result = manager.migrate_old_save_files()
         assert result is True
 
-        # Check that old file was moved and overwrote existing
         with open(os.path.join(slot_1_path, "player.json"), "r") as f:
-            data = json.load(f)
-            assert data["money"] == 100  # Should be the migrated value
+            assert json.load(f)["money"] == 999  # untouched
+        assert not os.path.exists(os.path.join(slot_1_path, "stats.json"))
+        slot_2_path = os.path.join(temp_dir, "slot_2")
+        with open(os.path.join(slot_2_path, "player.json"), "r") as f:
+            assert json.load(f)["money"] == 100
+        with open(os.path.join(slot_2_path, "stats.json"), "r") as f:
+            assert json.load(f)["totalFishCaught"] == 3
+        assert not os.path.exists(os.path.join(temp_dir, "player.json"))
+        assert not os.path.exists(os.path.join(temp_dir, "stats.json"))
+    finally:
+        shutil.rmtree(temp_dir)
+
+
+def test_migrate_old_save_files_skips_a_slot_holding_any_file():
+    """A slot whose player.json is gone but whose other files survive is not
+    free either (the kit's rule): the old save goes past it."""
+    temp_dir = tempfile.mkdtemp()
+    try:
+        manager = SaveFileManager(temp_dir)
+        os.makedirs(os.path.join(temp_dir, "slot_1"))
+        with open(os.path.join(temp_dir, "slot_1", "stats.json"), "w") as f:
+            f.write('{"totalFishCaught": 7}')
+        with open(os.path.join(temp_dir, "player.json"), "w") as f:
+            json.dump({"money": 5}, f)
+
+        assert manager.migrate_old_save_files() is True
+
+        assert not os.path.exists(os.path.join(temp_dir, "slot_1", "player.json"))
+        with open(os.path.join(temp_dir, "slot_1", "stats.json")) as f:
+            assert f.read() == '{"totalFishCaught": 7}'
+        assert os.path.exists(os.path.join(temp_dir, "slot_2", "player.json"))
+    finally:
+        shutil.rmtree(temp_dir)
+
+
+def test_migrate_old_save_files_every_slot_taken_moves_nothing():
+    temp_dir = tempfile.mkdtemp()
+    try:
+        manager = SaveFileManager(temp_dir)
+        for n in range(1, 100):
+            os.makedirs(os.path.join(temp_dir, "slot_%d" % n))
+            with open(os.path.join(temp_dir, "slot_%d" % n, "player.json"), "w") as f:
+                json.dump({"money": n}, f)
+        with open(os.path.join(temp_dir, "player.json"), "w") as f:
+            json.dump({"money": 100}, f)
+
+        with patch("src.saveFileManager.syncBrowserSaves") as sync:
+            assert manager.migrate_old_save_files() is False
+        sync.assert_not_called()
+
+        assert os.path.exists(os.path.join(temp_dir, "player.json"))
+        with open(os.path.join(temp_dir, "slot_1", "player.json")) as f:
+            assert json.load(f)["money"] == 1
     finally:
         shutil.rmtree(temp_dir)
 
diff --git a/tests/web/test_serve.py b/tests/web/test_serve.py
index 025919f..852faab 100644
--- a/tests/web/test_serve.py
+++ b/tests/web/test_serve.py
@@ -50,6 +50,8 @@ def test_the_page_keeps_the_saves_database_name():
         html = page.read()
     assert 'idbName: "fishe-saves"' in html
     assert 'saveDirEnv: "FISHE_SAVE_DIR"' in html
+    # Cloud saves (RFC 0016), only on arcade and only once a player turns them on.
+    assert "cloudSaves: true," in html
     assert 'entry: "web/pyodide_main.py"' in html
 
 
diff --git a/web/index.html b/web/index.html
index ffa3523..e989cc5 100644
--- a/web/index.html
+++ b/web/index.html
@@ -22,7 +22,7 @@
 <h2>FishE <span class="tagline">— fish a seaside village and build a fortune of $10,000</span></h2>
 <div id="status" class="status">Starting&hellip;</div>
 <div id="app"></div>
-<p class="controls">Tip: click an option or press its number key (1-9). Enter or Space continues. Your saved games live in this browser.</p>
+<p class="controls">Tip: click an option or press its number key (1-9). Enter or Space continues. Your saved games live in this browser; at fishe.play.danielstephenson.dev you can also back them up to your account once signed in.</p>
 <footer class="credit">More by Daniel Stephenson &rarr; <a href="https://danielstephenson.dev">danielstephenson.dev</a></footer>
 <script src="/tak/client.js"></script>
 <script src="/tak/boot.js"></script>
@@ -33,6 +33,11 @@ <h2>FishE <span class="tagline">— fish a seaside village and build a fortune o
   bundleUrl: "/web/game.zip",
   entry: "web/pyodide_main.py",
   logPrefix: "[fishe]",
+  // Cloud saves on arcade (RFC 0016): a signed-in player can back these
+  // saves up and get them on their other devices. Off until they turn it on,
+  // and only on fishe.play.danielstephenson.dev: the old alias host never
+  // loads cloud.js (tak's onArcadeHost), so its saves stay in its browser.
+  cloudSaves: true,
   messages: { download: "Downloading the village…", start: "Casting off…" },
 });
 </script>