diff --git a/tests/agent/test_prompt_builder.py b/tests/agent/test_prompt_builder.py
index a2d8ec56d7e35..858c880ec8fb6 100644
--- a/tests/agent/test_prompt_builder.py
+++ b/tests/agent/test_prompt_builder.py
@@ -928,6 +928,43 @@ class TestFindHermesMd:
         (repo / ".git").mkdir()
         assert _find_hermes_md(repo) is None
 
+    def test_no_git_root_checks_cwd_only(self, tmp_path):
+        """Outside a git repo, only cwd is checked — parents are NOT walked.
+
+        Walking parents with no git root to stop the loop would climb all
+        the way to / and pick up a .hermes.md planted in /tmp, /home, or /
+        on a shared system — a cross-user prompt-injection vector.
+        """
+        from unittest.mock import patch
+
+        parent = tmp_path / "parent"
+        parent.mkdir()
+        (parent / ".hermes.md").write_text("planted by another user")
+        cwd = parent / "work"
+        cwd.mkdir()
+        # No git root anywhere up the tree.
+        with patch("agent.prompt_builder._find_git_root", return_value=None):
+            assert _find_hermes_md(cwd) is None
+
+    def test_no_git_root_finds_in_cwd(self, tmp_path):
+        """Outside a git repo, a .hermes.md in cwd itself is still found."""
+        from unittest.mock import patch
+
+        (tmp_path / ".hermes.md").write_text("local rules")
+        with patch("agent.prompt_builder._find_git_root", return_value=None):
+            assert _find_hermes_md(tmp_path) == tmp_path / ".hermes.md"
+
+    def test_walks_parents_inside_git_repo(self, tmp_path):
+        """Inside a git repo, parent walk up to the git root still works."""
+        from unittest.mock import patch
+
+        (tmp_path / ".hermes.md").write_text("repo root rules")
+        sub = tmp_path / "a" / "b"
+        sub.mkdir(parents=True)
+        # Simulate cwd being inside a repo rooted at tmp_path.
+        with patch("agent.prompt_builder._find_git_root", return_value=tmp_path):
+            assert _find_hermes_md(sub) == tmp_path / ".hermes.md"
+
 
 class TestFindGitRoot:
     def test_finds_git_dir(self, tmp_path):