diff --git a/test/test_composer_path_containment.py b/test/test_composer_path_containment.py
index cc5f067b..c47ca884 100644
--- a/test/test_composer_path_containment.py
+++ b/test/test_composer_path_containment.py
@@ -45,19 +45,18 @@ def plugins_dir(tmp_path, monkeypatch):
 
 @pytest.mark.parametrize("payload", TRAVERSAL)
 def test_traversal_payloads_are_refused(plugins_dir, payload):
-    with pytest.raises(C.ComposerInputError):
-        C._plugin_dir(payload)
+    assert C._plugin_dir(payload) is None
 
 
 @pytest.mark.parametrize("payload", MALFORMED)
 def test_malformed_ids_are_refused(plugins_dir, payload):
-    with pytest.raises(C.ComposerInputError):
-        C._plugin_dir(payload)
+    assert C._plugin_dir(payload) is None
 
 
 @pytest.mark.parametrize("payload", ["a", "my-plugin", "x9", "a" * 63])
 def test_valid_ids_resolve_inside_the_base(plugins_dir, payload):
     resolved = C._plugin_dir(payload)
+    assert resolved is not None, f"{payload!r} was rejected but is valid"
     assert resolved.parent == plugins_dir.resolve(), (
         f"{payload!r} resolved to {resolved}, outside {plugins_dir}")
 
@@ -71,18 +70,34 @@ def test_no_payload_can_escape_even_if_the_regex_is_loosened(plugins_dir, monkey
     """
     import re
     monkeypatch.setattr(C, "_PLUGIN_ID_RE", re.compile(r"\A[\w./\\~-]+\Z"))
+    import os
     escaped = []
+    base = os.path.realpath(str(plugins_dir))
     for payload in TRAVERSAL:
-        try:
-            resolved = C._plugin_dir(payload)
-        except C.ComposerInputError:
+        resolved = C._plugin_dir(payload)
+        if resolved is None:
             continue
-        base = plugins_dir.resolve()
-        if resolved != base and base not in resolved.parents:
-            escaped.append((payload, str(resolved)))
+        real = os.path.realpath(str(resolved))
+        if real != base and os.path.commonpath([base, real]) != base:
+            escaped.append((payload, real))
     assert not escaped, f"these escaped the base with a loosened regex: {escaped}"
 
 
+def test_a_sibling_directory_with_a_shared_prefix_is_not_inside(tmp_path, monkeypatch):
+    """commonpath, not startswith.
+
+    "/x/plugins-evil" starts with "/x/plugins" but is a different directory, so
+    a prefix test would accept it.
+    """
+    base = tmp_path / "plugins"
+    base.mkdir()
+    (tmp_path / "plugins-evil").mkdir()
+    monkeypatch.setattr(C.composer_bp, "plugins_dir", str(base), raising=False)
+    import re
+    monkeypatch.setattr(C, "_PLUGIN_ID_RE", re.compile(r"\A[\w./\\~-]+\Z"))
+    assert C._plugin_dir("../plugins-evil") is None
+
+
 def test_a_trailing_newline_is_not_a_valid_id():
     r"""Python's `$` also matches before a trailing newline, so the original
     `^...$` accepted "myplugin\n" and would have created a directory whose
diff --git a/web_interface/blueprints/composer.py b/web_interface/blueprints/composer.py
index 3eb0002c..eac307d1 100644
--- a/web_interface/blueprints/composer.py
+++ b/web_interface/blueprints/composer.py
@@ -12,7 +12,9 @@ import ast
 import io
 import json
 import logging
+import os
 import re
+from typing import Optional
 import zipfile
 from datetime import datetime
 from pathlib import Path
@@ -598,7 +600,7 @@ class ComposerInputError(ValueError):
     """
 
 
-def _plugin_dir(plugin_id: str) -> Path:
+def _plugin_dir(plugin_id: str) -> Optional[Path]:
     """Resolve a plugin directory, refusing anything outside plugins_dir.
 
     _PLUGIN_ID_RE already rejects '/', '.' and '..', so this cannot currently
@@ -609,15 +611,21 @@ def _plugin_dir(plugin_id: str) -> Path:
     recognises, which is why CodeQL reported sixteen path-injection alerts
     against code that was already safe.
 
-    Raises ComposerInputError if the id is malformed or escapes the base.
+    Returns None for a malformed id or one that escapes the base. It returns
+    rather than raises so the handlers answer with a fixed literal: routing a
+    caught exception's text into a response is what py/stack-trace-exposure
+    flags, and there is nothing here a caller needs beyond "that id is not ok".
     """
     if not _PLUGIN_ID_RE.match(plugin_id or ''):
-        raise ComposerInputError('Invalid plugin ID')
-    base = Path(composer_bp.plugins_dir).resolve()
-    candidate = (base / plugin_id).resolve()
-    if candidate != base and base not in candidate.parents:
-        raise ComposerInputError('Invalid plugin ID')
-    return candidate
+        return None
+    base = os.path.realpath(str(composer_bp.plugins_dir))
+    candidate = os.path.realpath(os.path.join(base, plugin_id))
+    # commonpath, not startswith: "/plugins-evil" starts with "/plugins" but is
+    # a different directory. This is also the form static analysis recognises
+    # as a containment check.
+    if candidate != base and os.path.commonpath([base, candidate]) != base:
+        return None
+    return Path(candidate)
 
 
 def _save_composer_state(target_dir: Path, payload: dict) -> None:
@@ -697,10 +705,9 @@ def install_locally():
     # different function -- re-check here, at the point the path is actually
     # built, so this route stays safe on its own if that call is ever
     # reordered or changed.
-    try:
-        target = _plugin_dir(plugin_id)
-    except ComposerInputError as exc:
-        return jsonify({'status': 'error', 'message': str(exc)}), 400
+    target = _plugin_dir(plugin_id)
+    if target is None:
+        return jsonify({'status': 'error', 'message': 'Invalid plugin ID'}), 400
     force = bool(data.get('_force', False))
 
     if target.exists() and not force:
@@ -754,10 +761,10 @@ def validate_id(plugin_id):
     if not _PLUGIN_ID_RE.match(plugin_id):
         return jsonify({'valid': False, 'available': False, 'reason': 'Invalid format'})
     if composer_bp.plugins_dir:
-        try:
-            taken = _plugin_dir(plugin_id).exists()
-        except ComposerInputError:
+        resolved = _plugin_dir(plugin_id)
+        if resolved is None:
             return jsonify({'valid': False, 'available': False, 'reason': 'Invalid format'})
+        taken = resolved.exists()
         if taken:
             return jsonify({'valid': True, 'available': False, 'reason': 'Already installed'})
     return jsonify({'valid': True, 'available': True})
@@ -826,10 +833,9 @@ def load_plugin(plugin_id):
     """
     if not composer_bp.plugins_dir:
         return jsonify({'status': 'error', 'message': 'Plugin directory not configured'}), 503
-    try:
-        plugin_dir = _plugin_dir(plugin_id)
-    except ComposerInputError as exc:
-        return jsonify({'status': 'error', 'message': str(exc)}), 400
+    plugin_dir = _plugin_dir(plugin_id)
+    if plugin_dir is None:
+        return jsonify({'status': 'error', 'message': 'Invalid plugin ID'}), 400
     if not plugin_dir.exists():
         return jsonify({'status': 'error', 'message': 'Plugin not found'}), 404
 
diff --git a/web_interface/static/v3/app.css b/web_interface/static/v3/app.css
index a1342955..fa73108b 100644
--- a/web_interface/static/v3/app.css
+++ b/web_interface/static/v3/app.css
@@ -421,6 +421,9 @@ a, button, input, select, textarea {
     .md\:grid-cols-4 { grid-template-columns: repeat(4, minmax(0, 1fr)); }
     .md\:flex { display: flex; }
     .md\:hidden { display: none; }
+    /* composer.html labels its toolbar buttons `hidden md:inline`, so without
+       this the label is hidden at every width and the buttons stay icon-only. */
+    .md\:inline { display: inline; }
 }
 
 @media (min-width: 1024px) {