diff --git a/tests/tools/test_skills_list_modified_diff.py b/tests/tools/test_skills_list_modified_diff.py
new file mode 100644
index 00000000000..972b0e103b9
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+++ b/tests/tools/test_skills_list_modified_diff.py
@@ -0,0 +1,132 @@
+"""Tests for discovering and diffing user-modified bundled skills.
+
+`hermes update` keeps (does not overwrite) bundled skills the user edited
+locally, but historically only printed a *count* — there was no way to find
+which skills, or see what changed. These tests cover the two helpers that close
+that gap, exercising the real sync pipeline (no mocks of the comparison logic):
+
+* ``list_user_modified_bundled_skills()`` — the discovery half of the exact
+  test the sync loop uses to decide what to skip.
+* ``diff_bundled_skill()`` — a unified diff of the user copy vs the stock copy.
+
+Revert already exists (``reset_bundled_skill``); the last test confirms it
+clears the modified state so the two stay consistent.
+"""
+
+from contextlib import ExitStack
+from unittest.mock import patch
+
+from tools.skills_sync import (
+    sync_skills,
+    reset_bundled_skill,
+    list_user_modified_bundled_skills,
+    diff_bundled_skill,
+)
+
+
+def _make_bundled(tmp_path):
+    """A fake bundled skills tree with one skill: category/foo."""
+    bundled = tmp_path / "bundled_skills"
+    foo = bundled / "category" / "foo"
+    foo.mkdir(parents=True)
+    (foo / "SKILL.md").write_text("---\nname: foo\n---\n# Foo Skill\n")
+    (foo / "helper.py").write_text("print('stock')\n")
+    return bundled
+
+
+def _patches(bundled, skills_dir, manifest_file):
+    stack = ExitStack()
+    stack.enter_context(
+        patch("tools.skills_sync._get_bundled_dir", return_value=bundled)
+    )
+    stack.enter_context(
+        patch(
+            "tools.skills_sync._get_optional_dir",
+            return_value=bundled.parent / "optional-skills",
+        )
+    )
+    stack.enter_context(patch("tools.skills_sync.SKILLS_DIR", skills_dir))
+    stack.enter_context(patch("tools.skills_sync.MANIFEST_FILE", manifest_file))
+    return stack
+
+
+def _env(tmp_path):
+    bundled = _make_bundled(tmp_path)
+    skills_dir = tmp_path / "user_skills"
+    manifest_file = skills_dir / ".bundled_manifest"
+    return bundled, skills_dir, manifest_file
+
+
+def test_pristine_skill_is_not_listed_as_modified(tmp_path):
+    bundled, skills_dir, manifest_file = _env(tmp_path)
+    with _patches(bundled, skills_dir, manifest_file):
+        sync_skills(quiet=True)
+        assert list_user_modified_bundled_skills() == []
+
+
+def test_edited_skill_is_listed_as_modified(tmp_path):
+    bundled, skills_dir, manifest_file = _env(tmp_path)
+    with _patches(bundled, skills_dir, manifest_file):
+        sync_skills(quiet=True)
+        (skills_dir / "category" / "foo" / "helper.py").write_text("print('mine')\n")
+
+        modified = list_user_modified_bundled_skills()
+        names = [m["name"] for m in modified]
+        assert names == ["foo"]
+        entry = modified[0]
+        assert entry["dest"] == skills_dir / "category" / "foo"
+        assert entry["bundled_src"] == bundled / "category" / "foo"
+
+
+def test_diff_reports_no_changes_when_pristine(tmp_path):
+    bundled, skills_dir, manifest_file = _env(tmp_path)
+    with _patches(bundled, skills_dir, manifest_file):
+        sync_skills(quiet=True)
+        result = diff_bundled_skill("foo")
+        assert result["ok"] is True
+        assert result["modified"] is False
+        assert result["diffs"] == []
+
+
+def test_diff_shows_modified_and_added_files(tmp_path):
+    bundled, skills_dir, manifest_file = _env(tmp_path)
+    with _patches(bundled, skills_dir, manifest_file):
+        sync_skills(quiet=True)
+        user_foo = skills_dir / "category" / "foo"
+        (user_foo / "helper.py").write_text("print('mine')\n")
+        (user_foo / "extra.txt").write_text("local note\n")
+
+        result = diff_bundled_skill("foo")
+        assert result["ok"] is True
+        assert result["modified"] is True
+
+        by_path = {d["path"]: d for d in result["diffs"]}
+        assert by_path["helper.py"]["status"] == "modified"
+        # The unified diff shows the user's line replacing the stock line.
+        assert "print('mine')" in by_path["helper.py"]["diff"]
+        assert "print('stock')" in by_path["helper.py"]["diff"]
+        # A file only in the user copy is reported as added.
+        assert by_path["extra.txt"]["status"] == "added"
+
+
+def test_diff_unknown_skill_is_not_ok(tmp_path):
+    bundled, skills_dir, manifest_file = _env(tmp_path)
+    with _patches(bundled, skills_dir, manifest_file):
+        sync_skills(quiet=True)
+        result = diff_bundled_skill("does-not-exist")
+        assert result["ok"] is False
+        assert result["found"] is False
+
+
+def test_reset_clears_modified_state(tmp_path):
+    """Revert (existing) and discovery (new) must agree: after reset, not modified."""
+    bundled, skills_dir, manifest_file = _env(tmp_path)
+    with _patches(bundled, skills_dir, manifest_file):
+        sync_skills(quiet=True)
+        (skills_dir / "category" / "foo" / "helper.py").write_text("print('mine')\n")
+        assert [m["name"] for m in list_user_modified_bundled_skills()] == ["foo"]
+
+        # Restore from the stock source, then it must no longer be flagged.
+        result = reset_bundled_skill("foo", restore=True)
+        assert result["ok"] is True
+        assert list_user_modified_bundled_skills() == []