diff --git a/aider/commands.py b/aider/commands.py
index 3881403c5c1..20145a81cc4 100644
--- a/aider/commands.py
+++ b/aider/commands.py
@@ -847,7 +847,7 @@ def cmd_add(self, args):
             abs_file_path = self.coder.abs_root_path(matched_file)
 
             if (
-                not abs_file_path.startswith(self.coder.root)
+                not Path(abs_file_path).is_relative_to(self.coder.root)
                 and not is_image_file(matched_file)
                 and self.coder.auto_commits
             ):
diff --git a/tests/basic/test_commands.py b/tests/basic/test_commands.py
index 406e928ebf2..0d6b6372a3c 100644
--- a/tests/basic/test_commands.py
+++ b/tests/basic/test_commands.py
@@ -506,6 +506,40 @@ def test_cmd_add_from_outside_git(self):
 
             self.assertEqual(len(coder.abs_fnames), 0)
 
+    def test_cmd_add_abs_path_in_sibling_dir_with_prefix(self):
+        """An absolute path inside a sibling directory whose name starts with
+        the repo root's name must be rejected.
+
+        Regression: the within-root guard used str.startswith(), so a path
+        like /tmp/x/repoXYZ/secret.env was wrongly accepted when the repo
+        root was /tmp/x/repo (no trailing path separator), leaking an
+        out-of-tree file into the chat / to the LLM.
+        See https://github.com/Aider-AI/aider/issues/178
+        """
+        with ChdirTemporaryDirectory() as tmp_dname:
+            root = Path("repo")
+            root.mkdir()
+            os.chdir(str(root))
+
+            make_repo()
+
+            io = InputOutput(pretty=False, fancy_input=False, yes=False)
+            from aider.coders import Coder
+
+            coder = Coder.create(self.GPT35, None, io)
+            commands = Commands(io, coder)
+
+            # Sibling dir whose name shares the root's prefix, holding a real file.
+            sibling = Path(tmp_dname) / "repoXYZ"
+            sibling.mkdir()
+            outside_file = sibling / "secret.env"
+            outside_file.touch()
+
+            commands.cmd_add(str(outside_file.resolve()))
+
+            self.assertNotIn(str(outside_file.resolve()), coder.abs_fnames)
+            self.assertEqual(len(coder.abs_fnames), 0)
+
     def test_cmd_add_filename_with_special_chars(self):
         with ChdirTemporaryDirectory():
             io = InputOutput(pretty=False, fancy_input=False, yes=False)