diff --git a/plugin-repos/starlark-apps/manager.py b/plugin-repos/starlark-apps/manager.py
index 5a810079..e078ec55 100644
--- a/plugin-repos/starlark-apps/manager.py
+++ b/plugin-repos/starlark-apps/manager.py
@@ -10,6 +10,7 @@ API Version: 1.0.0
 import json
 import os
 import re
+import stat
 import time
 import fcntl
 from pathlib import Path
@@ -457,12 +458,77 @@ class StarlarkAppsPlugin(BasePlugin):
             apps_dir = project_root / "starlark-apps"
         except Exception:
             # Fallback to current working directory
-            apps_dir = Path.cwd() / "starlark-apps"
+            project_root = Path.cwd()
+            apps_dir = project_root / "starlark-apps"
 
         # Create directory if it doesn't exist
         apps_dir.mkdir(parents=True, exist_ok=True)
+        self._hand_apps_dir_to_checkout_owner(apps_dir, project_root)
         return apps_dir
 
+    def _hand_apps_dir_to_checkout_owner(self, apps_dir: Path, project_root: Path) -> None:
+        """Give the apps directory to whoever owns the checkout.
+
+        This directory is not in the repository, so it is created lazily by
+        whichever process reaches it first -- and the two that do run as
+        different users. The display service is `User=root`
+        (systemd/ledmatrix.service) and instantiates this plugin at startup,
+        which is where `_get_apps_directory` is called from. The web interface
+        is `User=<login user>` (systemd/ledmatrix-web.service) and is what
+        actually installs apps.
+
+        On a fresh install the display service usually wins that race -- the
+        documented first step is to install pixlet and reboot -- so the
+        directory lands root-owned, and every subsequent install from the web
+        UI fails on PermissionError. The user sees only "Failed to install
+        from repository", with nothing pointing at ownership.
+
+        The web user cannot repair this; it lacks permission to chown. Root
+        can, so root does it here, on every startup. That also heals installs
+        already broken by this, without the user having to find the chown.
+        """
+        geteuid = getattr(os, "geteuid", None)
+        chown = getattr(os, "chown", None)
+        if geteuid is None or chown is None or geteuid() != 0:
+            # Not root, or not a platform with POSIX ownership. If the
+            # directory is wrong we cannot fix it, and must not pretend to.
+            return
+        try:
+            owner = project_root.stat()
+        except OSError:
+            return
+        if owner.st_uid == 0:
+            # The checkout genuinely belongs to root, so root owning the apps
+            # directory is correct and there is nobody to hand it to.
+            return
+
+        # Deepest first, with the directory itself last. Handing over the
+        # container before its contents would briefly let a local user rename
+        # entries underneath a repair that is still running.
+        descendants = sorted(apps_dir.rglob("*"),
+                             key=lambda p: len(p.parts), reverse=True)
+        for path in (*descendants, apps_dir):
+            try:
+                st = os.lstat(path)
+            except OSError:
+                continue
+            if stat.S_ISLNK(st.st_mode):
+                # Never hand over a link's target. Anyone able to write in
+                # this directory could otherwise point a symlink at a
+                # root-owned file and have this give it away -- the whole
+                # point of the loop is that it runs as root.
+                continue
+            if st.st_uid == owner.st_uid and st.st_gid == owner.st_gid:
+                continue
+            try:
+                chown(path, owner.st_uid, owner.st_gid, follow_symlinks=False)
+            except OSError as e:
+                self.logger.warning(
+                    "Could not hand %s to uid %s: %s -- installs from the web "
+                    "interface will fail until this is chowned manually",
+                    path, owner.st_uid, e,
+                )
+
     def _sanitize_app_id(self, app_id: str) -> str:
         """
         Sanitize app_id into a safe slug for use in file paths.
@@ -1011,6 +1077,13 @@ class StarlarkAppsPlugin(BasePlugin):
             self.logger.info(f"Installed Starlark app: {app_id} (sanitized: {safe_app_id})")
             return True
 
+        except PermissionError:
+            # Deliberately not folded into the False below. A False here is
+            # reported as a generic install failure, which is how the
+            # directory-ownership bug stayed invisible: the caller could not
+            # tell "this app is broken" from "this process cannot write here".
+            # The routes turn this into a message that names the fix.
+            raise
         except Exception as e:
             self.logger.error(f"Error installing app {app_id}: {e}")
             return False
diff --git a/test/test_starlark_apps_dir_ownership.py b/test/test_starlark_apps_dir_ownership.py
new file mode 100644
index 00000000..8d64e5a5
--- /dev/null
+++ b/test/test_starlark_apps_dir_ownership.py
@@ -0,0 +1,366 @@
+"""starlark-apps: the root display service must not lock the web UI out.
+
+Reported by a user after a fresh install: installing an app from the Starlark
+tab failed with "install failed: Failed to install from repository", and so did
+uploading a .star file and installing from a GitHub directory. The cause was
+ownership, which the error named nowhere -- they found it only by reading the
+service logs, and fixed it with
+
+    sudo chown -R ledpi:ledpi /home/ledpi/LEDMatrix/starlark-apps
+
+The starlark-apps directory is not in the repository, so it is created lazily
+by whichever process reaches it first. Those processes run as different users:
+systemd/ledmatrix.service is `User=root` and constructs this plugin at startup
+(which is what calls _get_apps_directory), while systemd/ledmatrix-web.service
+runs as the login user and is what actually installs apps. The documented
+first step is to install pixlet and reboot, so on a fresh machine the display
+service usually wins the race and the directory lands root-owned.
+
+The web user cannot repair that -- chown needs root. So root hands the
+directory over itself, every startup, which also heals machines already broken
+by this.
+"""
+
+import importlib.util
+import os
+import sys
+import types
+from pathlib import Path
+from unittest.mock import MagicMock
+
+import pytest
+
+PLUGIN_DIR = Path(__file__).resolve().parent.parent / "plugin-repos" / "starlark-apps"
+
+
+@pytest.fixture(scope="module")
+def manager_module():
+    if not PLUGIN_DIR.exists():
+        pytest.skip("starlark-apps plugin is not checked out")
+    sys.path.insert(0, str(PLUGIN_DIR))
+    injected_fcntl = "fcntl" not in sys.modules
+    if injected_fcntl:
+        stub = types.ModuleType("fcntl")
+        stub.LOCK_EX, stub.LOCK_UN = 2, 8
+        stub.flock = lambda *a, **kw: None
+        sys.modules["fcntl"] = stub
+    try:
+        spec = importlib.util.spec_from_file_location(
+            "starlark_manager_ownership", PLUGIN_DIR / "manager.py")
+        module = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(module)
+        return module
+    except ImportError as e:
+        # Only a genuinely absent dependency is a skip. A syntax error or a
+        # NameError in the plugin is a regression these tests exist to catch,
+        # and swallowing it here would turn a red suite green.
+        if any(dep in str(e) for dep in ("PIL", "Pillow", "pixlet", "frame_extractor")):
+            pytest.skip(f"starlark-apps optional dependency missing: {e}")
+        raise
+    finally:
+        sys.path.remove(str(PLUGIN_DIR))
+        if injected_fcntl:
+            sys.modules.pop("fcntl", None)
+
+
+def _plugin(manager_module):
+    """A manager with __init__ bypassed -- only ownership paths are tested."""
+    cls = manager_module.StarlarkAppsPlugin
+    inst = cls.__new__(cls)
+    inst.logger = MagicMock()
+    return inst
+
+
+class _Stat:
+    """A real stat_result with only the ownership fields overridden.
+
+    Everything else is delegated to the genuine result. A stub carrying just
+    st_uid/st_gid passed locally but broke in CI, because pathlib itself reads
+    st_mode while walking the tree on some Python versions -- and the fields
+    it needs are an implementation detail, not something this test should be
+    asserting about.
+    """
+
+    def __init__(self, real, uid, gid):
+        self._real = real
+        self.st_uid = uid
+        self.st_gid = gid
+
+    def __getattr__(self, name):
+        return getattr(self._real, name)
+
+
+@pytest.fixture
+def owned(monkeypatch):
+    """Let a test declare a fake uid/gid for specific paths.
+
+    Real ownership cannot be faked without root, and these tests must run as
+    an ordinary user in CI.
+    """
+    fake = {}
+    real_stat = Path.stat
+    real_lstat = os.lstat
+
+    def patched_stat(self, *args, **kwargs):
+        st = real_stat(self, *args, **kwargs)
+        key = str(self)
+        return _Stat(st, *fake[key]) if key in fake else st
+
+    def patched_lstat(path, *args, **kwargs):
+        st = real_lstat(path, *args, **kwargs)
+        key = str(path)
+        return _Stat(st, *fake[key]) if key in fake else st
+
+    # Both, because the code reads the checkout owner through Path.stat and
+    # each entry it repairs through os.lstat -- lstat so a symlink reports
+    # itself rather than its target.
+    monkeypatch.setattr(Path, "stat", patched_stat)
+    monkeypatch.setattr(os, "lstat", patched_lstat)
+    return fake
+
+
+@pytest.fixture
+def as_root(monkeypatch):
+    """Run the handover as root, recording chowns instead of performing them."""
+    calls = []
+    monkeypatch.setattr("os.geteuid", lambda: 0)
+    monkeypatch.setattr(
+        "os.chown",
+        lambda p, uid, gid, **kw: calls.append((str(p), uid, gid)))
+    return calls
+
+
+def _tree(tmp_path):
+    """A project root with an apps directory holding one installed app."""
+    apps = tmp_path / "starlark-apps"
+    (apps / "analogclock").mkdir(parents=True)
+    (apps / "analogclock" / "analog_clock.star").write_text("# app")
+    (apps / "manifest.json").write_text("{}")
+    return apps
+
+
+class TestRootHandsTheDirectoryOver:
+    def test_root_created_directory_is_given_to_the_checkout_owner(
+            self, manager_module, tmp_path, owned, as_root):
+        apps = _tree(tmp_path)
+        owned[str(tmp_path)] = (1000, 1000)      # checkout belongs to the login user
+        owned[str(apps)] = (0, 0)                # but root got there first
+
+        _plugin(manager_module)._hand_apps_dir_to_checkout_owner(apps, tmp_path)
+
+        assert (str(apps), 1000, 1000) in as_root
+
+    def test_contents_are_repaired_not_just_the_directory(
+            self, manager_module, tmp_path, owned, as_root):
+        """An install broken by this leaves root-owned files inside it too."""
+        apps = _tree(tmp_path)
+        owned[str(tmp_path)] = (1000, 1000)
+        for p in (apps, apps / "analogclock",
+                  apps / "analogclock" / "analog_clock.star",
+                  apps / "manifest.json"):
+            owned[str(p)] = (0, 0)
+
+        _plugin(manager_module)._hand_apps_dir_to_checkout_owner(apps, tmp_path)
+
+        chowned = {c[0] for c in as_root}
+        assert str(apps / "analogclock" / "analog_clock.star") in chowned
+        assert str(apps / "manifest.json") in chowned
+
+    def test_nothing_is_touched_when_ownership_is_already_right(
+            self, manager_module, tmp_path, owned, as_root):
+        apps = _tree(tmp_path)
+        owned[str(tmp_path)] = (1000, 1000)
+        for p in (apps, apps / "analogclock",
+                  apps / "analogclock" / "analog_clock.star",
+                  apps / "manifest.json"):
+            owned[str(p)] = (1000, 1000)
+
+        _plugin(manager_module)._hand_apps_dir_to_checkout_owner(apps, tmp_path)
+
+        assert as_root == []
+
+
+class TestItDoesNotOverreach:
+    def test_a_non_root_process_changes_nothing(
+            self, manager_module, tmp_path, owned, monkeypatch):
+        """The web service also calls this. It has no right to chown anything."""
+        apps = _tree(tmp_path)
+        owned[str(tmp_path)] = (1000, 1000)
+        owned[str(apps)] = (0, 0)
+        calls = []
+        monkeypatch.setattr("os.geteuid", lambda: 1000)
+        monkeypatch.setattr("os.chown", lambda p, u, g, **kw: calls.append(p))
+
+        _plugin(manager_module)._hand_apps_dir_to_checkout_owner(apps, tmp_path)
+
+        assert calls == []
+
+    def test_a_genuinely_root_owned_checkout_is_left_alone(
+            self, manager_module, tmp_path, owned, as_root):
+        """Installed as root on purpose: there is nobody to hand it to."""
+        apps = _tree(tmp_path)
+        owned[str(tmp_path)] = (0, 0)
+        owned[str(apps)] = (0, 0)
+
+        _plugin(manager_module)._hand_apps_dir_to_checkout_owner(apps, tmp_path)
+
+        assert as_root == []
+
+    def test_a_failed_chown_warns_and_does_not_raise(
+            self, manager_module, tmp_path, owned, monkeypatch):
+        """Startup must not die because one file could not be handed over."""
+        apps = _tree(tmp_path)
+        owned[str(tmp_path)] = (1000, 1000)
+        owned[str(apps)] = (0, 0)
+        monkeypatch.setattr("os.geteuid", lambda: 0)
+
+        def boom(*_a, **_kw):
+            raise OSError("read-only file system")
+
+        monkeypatch.setattr("os.chown", boom)
+        plugin = _plugin(manager_module)
+
+        plugin._hand_apps_dir_to_checkout_owner(apps, tmp_path)   # must not raise
+
+        assert plugin.logger.warning.called
+
+
+class TestTheDirectoryGetterUsesIt:
+    def test_get_apps_directory_performs_the_handover(
+            self, manager_module, monkeypatch):
+        """Pins the wiring: creating the directory without handing it over is
+        exactly the bug.
+
+        This calls the real getter, which resolves to the checkout's own
+        starlark-apps directory, so it removes the directory again when the
+        test was what created it.
+        """
+        plugin = _plugin(manager_module)
+        seen = []
+        monkeypatch.setattr(
+            type(plugin), "_hand_apps_dir_to_checkout_owner",
+            lambda self, apps, root: seen.append((apps, root)))
+        expected = PLUGIN_DIR.parent.parent / "starlark-apps"
+        pre_existing = expected.exists()
+
+        try:
+            result = plugin._get_apps_directory()
+
+            assert result == expected and result.exists()
+            assert seen and seen[0] == (expected, expected.parent)
+        finally:
+            if not pre_existing and expected.exists():
+                try:
+                    expected.rmdir()
+                except OSError:
+                    pass
+
+
+class TestTheErrorNamesTheCause:
+    """The reporter saw only "Failed to install from repository".
+
+    That message names no path and no cause, so the only way to the answer was
+    reading the service logs. The handover above should stop the failure
+    happening at all; this makes the failure legible if it ever does.
+    """
+
+    @pytest.fixture(scope="class")
+    def hint(self):
+        try:
+            from web_interface.blueprints.api_v3.starlark import _ownership_hint
+        except ImportError as e:
+            # Same rule as above: absent Flask is a skip, a broken module is not.
+            if "flask" in str(e).lower():
+                pytest.skip(f"Flask is not installed here: {e}")
+            raise
+        return _ownership_hint
+
+    def test_a_permission_error_explains_itself(self, hint):
+        message = hint(PermissionError(13, "Permission denied"))
+        assert message
+        assert "chown" in message
+        assert "starlark-apps" in message
+
+    def test_it_points_at_the_automatic_repair_first(self, hint):
+        """Restarting the display service is the fix that needs no root user
+        to understand it -- the manual chown is the fallback."""
+        message = hint(PermissionError(13, "Permission denied"))
+        assert "systemctl restart ledmatrix" in message
+        assert message.index("systemctl") < message.index("chown")
+
+    def test_other_failures_are_not_mislabelled(self, hint):
+        """A network error must not be reported as an ownership problem."""
+        for err in (ValueError("bad json"), OSError(28, "No space left on device"),
+                    TimeoutError("github timed out")):
+            assert hint(err) is None
+
+
+class TestItWillNotBeTrickedIntoGivingAwayAFile:
+    """Root chowning a tree is a privilege-escalation primitive if it follows
+    links: anyone who can write in the directory could point one at a
+    root-owned file and have this hand it over."""
+
+    def test_a_symlink_is_never_followed(self, manager_module, tmp_path, owned, as_root):
+        apps = _tree(tmp_path)
+        target = tmp_path / "precious"
+        target.write_text("root-owned secret")
+        (apps / "evil").symlink_to(target)
+        owned[str(tmp_path)] = (1000, 1000)
+        owned[str(apps)] = (0, 0)
+        # The link must look like it NEEDS handing over, or it would be
+        # skipped for already having the right owner and this test would pass
+        # without ever exercising the symlink check.
+        owned[str(apps / "evil")] = (0, 0)
+        owned[str(target)] = (0, 0)
+
+        _plugin(manager_module)._hand_apps_dir_to_checkout_owner(apps, tmp_path)
+
+        chowned = {c[0] for c in as_root}
+        assert str(target) not in chowned
+        assert str(apps / "evil") not in chowned
+
+    def test_the_directory_is_handed_over_last(self, manager_module, tmp_path, owned, as_root):
+        """Its contents must be settled before the container changes hands."""
+        apps = _tree(tmp_path)
+        owned[str(tmp_path)] = (1000, 1000)
+        for p in (apps, apps / "analogclock",
+                  apps / "analogclock" / "analog_clock.star",
+                  apps / "manifest.json"):
+            owned[str(p)] = (0, 0)
+
+        _plugin(manager_module)._hand_apps_dir_to_checkout_owner(apps, tmp_path)
+
+        order = [c[0] for c in as_root]
+        assert order[-1] == str(apps)
+
+
+class TestAPermissionFailureReachesTheCaller:
+    def test_install_app_does_not_swallow_permission_errors(
+            self, manager_module, tmp_path, monkeypatch):
+        """A False here reads as "this app is broken" and routes to a generic
+        message -- which is how the ownership bug stayed invisible."""
+        plugin = _plugin(manager_module)
+        plugin.apps_dir = tmp_path
+        plugin.apps = {}
+
+        def denied(self, *a, **kw):
+            raise PermissionError(13, "Permission denied")
+
+        monkeypatch.setattr(Path, "mkdir", denied)
+
+        with pytest.raises(PermissionError):
+            plugin.install_app("analogclock", str(tmp_path / "x.star"), {})
+
+    def test_other_install_failures_still_return_false(
+            self, manager_module, tmp_path, monkeypatch):
+        """Only permission errors are promoted; the bool contract is intact."""
+        plugin = _plugin(manager_module)
+        plugin.apps_dir = tmp_path
+        plugin.apps = {}
+
+        def broken(self, *a, **kw):
+            raise OSError(28, "No space left on device")
+
+        monkeypatch.setattr(Path, "mkdir", broken)
+
+        assert plugin.install_app("analogclock", str(tmp_path / "x.star"), {}) is False
diff --git a/web_interface/blueprints/api_v3/starlark.py b/web_interface/blueprints/api_v3/starlark.py
index 980672ab..856de4a4 100644
--- a/web_interface/blueprints/api_v3/starlark.py
+++ b/web_interface/blueprints/api_v3/starlark.py
@@ -26,6 +26,36 @@ import web_interface.blueprints.api_v3 as _pkg
 # package is the only patch point that covers every caller.
 
 
+def _ownership_hint(err: BaseException):
+    """An actionable message when the apps directory is not writable.
+
+    The display service runs as root and the web interface as the login user
+    (see systemd/ledmatrix.service and systemd/ledmatrix-web.service). The
+    starlark-apps directory is not in the repository, so whichever service
+    reaches it first creates it -- and when that is the display service, the
+    web user cannot write into it and every install fails.
+
+    The plugin now hands the directory back on startup, so this should not be
+    reachable. It is kept because the failure is otherwise invisible: the
+    generic message names no path and no cause, and the one user who hit it
+    had to read the service logs to find it. If the handover is ever prevented
+    -- an exotic mount, a directory root-owned for another reason -- this says
+    what to do instead of costing somebody an evening.
+
+    Returns None when `err` is not a permission problem.
+    """
+    if not isinstance(err, PermissionError):
+        return None
+    return (
+        f"Cannot write to {_STARLARK_APPS_DIR}. It is owned by another user "
+        f"-- usually because the display service, which runs as root, created "
+        f"it before the web interface did. Restarting the display service "
+        f"(sudo systemctl restart ledmatrix) repairs the ownership "
+        f"automatically. To fix it by hand: "
+        f"sudo chown -R $USER:$USER {_STARLARK_APPS_DIR}"
+    )
+
+
 @api_v3.route('/starlark/status', methods=['GET'])
 def get_starlark_status():
     """Get Starlark plugin status and Pixlet availability."""
@@ -278,7 +308,8 @@ def upload_starlark_app():
         # without it, though, and describe_exception redacts credentials and
         # truncates -- the same trade-off every other handler here makes.
         logger.exception("[Starlark] File error uploading starlark app: %s", err)
-        return jsonify({'status': 'error', 'message': 'File error during upload',
+        return jsonify({'status': 'error',
+                        'message': _ownership_hint(err) or 'File error during upload',
                         'details': describe_exception(err)}), 500
     except ImportError as err:
         logger.exception("[Starlark] Module load error uploading starlark app: %s", err)
@@ -702,6 +733,16 @@ def install_from_tronbyte_repository():
 
     except Exception as e:
         logger.exception("[Starlark] install_from_tronbyte_repository failed")
+        hint = _ownership_hint(e)
+        if hint:
+            # `details` is kept deliberately. CodeQL flags it as information
+            # exposure, but this package's rule is "if it returns 5xx, it says
+            # why" -- enforced by test_no_api_v3_handler_discards_its_exception,
+            # whose PRE_EXISTING allowance may shrink and never grow. The
+            # Starlark routes are exactly the ones that policy was written for:
+            # they answered 500 with no detail for three releases. It stays.
+            return jsonify({'status': 'error', 'message': hint,
+                            'details': describe_exception(e)}), 500
         return jsonify({'status': 'error', 'message': 'Failed to install from repository', 'details': describe_exception(e)}), 500
 @api_v3.route('/starlark/repository/categories', methods=['GET'])
 def get_tronbyte_categories():