diff --git a/cista/protocol.py b/cista/protocol.py
index 86617f6..ff76abe 100644
--- a/cista/protocol.py
+++ b/cista/protocol.py
@@ -120,6 +120,9 @@ class FileEntry(msgspec.Struct, array_like=True):
     size: int
     isfile: int
 
+    def __repr__(self):
+        return self.key or "FileEntry()"
+
 
 class Update(msgspec.Struct, array_like=True):
     ...
@@ -137,6 +140,10 @@ class UpdIns(Update, tag="i"):
     items: list[FileEntry]
 
 
+class UpdateMessage(msgspec.Struct):
+    update: list[UpdKeep | UpdDel | UpdIns]
+
+
 class Space(msgspec.Struct):
     disk: int
     free: int
diff --git a/cista/watching.py b/cista/watching.py
index ab62ab6..b8d4ae8 100644
--- a/cista/watching.py
+++ b/cista/watching.py
@@ -282,13 +282,11 @@ def format_update(old, new):
 
         del_count = 0
         rest = new[nidx:]
-        while old[oidx] not in rest:
+        while oidx < len(old) and old[oidx] not in rest:
             del_count += 1
             oidx += 1
-
         if del_count:
             update.append(UpdDel(del_count))
-            oidx += 1
             continue
 
         insert_items = []
diff --git a/tests/test_watching.py b/tests/test_watching.py
new file mode 100644
index 0000000..8e65faf
--- /dev/null
+++ b/tests/test_watching.py
@@ -0,0 +1,77 @@
+import msgspec
+
+from cista.protocol import FileEntry, UpdateMessage, UpdDel, UpdIns, UpdKeep
+from cista.watching import format_update
+
+
+def decode(data: str):
+    return msgspec.json.decode(data, type=UpdateMessage).update
+
+
+# Helper function to create a list of FileEntry objects
+def f(count, start=0):
+    return [FileEntry(i, str(i), str(i), 0, 0, 0) for i in range(start, start + count)]
+
+
+def test_identical_lists():
+    old_list = f(3)
+    new_list = old_list.copy()
+    expected = [UpdKeep(3)]
+    assert decode(format_update(old_list, new_list)) == expected
+
+
+def test_completely_different_lists():
+    old_list = f(3)
+    new_list = f(3, 3)  # Different entries
+    expected = [UpdDel(3), UpdIns(new_list)]
+    assert decode(format_update(old_list, new_list)) == expected
+
+
+def test_insertions():
+    old_list = f(3)
+    new_list = old_list[:2] + f(1, 10) + old_list[2:]
+    expected = [UpdKeep(2), UpdIns(f(1, 10)), UpdKeep(1)]
+    assert decode(format_update(old_list, new_list)) == expected
+
+
+def test_deletions():
+    old_list = f(3)
+    new_list = [old_list[0], old_list[2]]
+    expected = [UpdKeep(1), UpdDel(1), UpdKeep(1)]
+    assert decode(format_update(old_list, new_list)) == expected
+
+
+def test_mixed_operations():
+    old_list = f(4)
+    new_list = [old_list[0], old_list[2], *f(1, 10)]
+    expected = [UpdKeep(1), UpdDel(1), UpdKeep(1), UpdDel(1), UpdIns(f(1, 10))]
+    assert decode(format_update(old_list, new_list)) == expected
+
+
+def test_empty_old_list():
+    old_list = []
+    new_list = f(3)
+    expected = [UpdIns(new_list)]
+    assert decode(format_update(old_list, new_list)) == expected
+
+
+def test_empty_new_list():
+    old_list = f(3)
+    new_list = []
+    expected = [UpdDel(3)]
+    assert decode(format_update(old_list, new_list)) == expected
+
+
+def test_longer_lists():
+    old_list = f(6)
+    new_list = f(1, 6) + old_list[1:3] + old_list[4:5] + f(2, 7)
+    expected = [
+        UpdDel(1),
+        UpdIns(f(1, 6)),
+        UpdKeep(2),
+        UpdDel(1),
+        UpdKeep(1),
+        UpdDel(1),
+        UpdIns(f(2, 7)),
+    ]
+    assert decode(format_update(old_list, new_list)) == expected