diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 0d7dce6..f2ca05d 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -27,6 +27,9 @@ jobs:
     name: Python ${{ matrix.python-version }}
     runs-on: ubuntu-latest
     container: ubuntu:26.04
+    defaults:
+      run:
+        shell: bash
     strategy:
       fail-fast: false
       matrix:
diff --git a/src/kotonoha/karaoke_label.py b/src/kotonoha/karaoke_label.py
index 861d24d..c8e4e6e 100644
--- a/src/kotonoha/karaoke_label.py
+++ b/src/kotonoha/karaoke_label.py
@@ -13,7 +13,7 @@
 
 from __future__ import annotations
 
-from typing import Any, Protocol, cast
+from typing import Any, Protocol, TypedDict, cast
 
 from PyQt6 import QtCore
 from PyQt6.QtCore import QEasingCurve, QPropertyAnimation, QRectF, QSize, Qt
@@ -40,7 +40,13 @@
 
 # Effect strength per intensity: glow alpha (of the accent), glow radius (px), and
 # the active-word brightening (QColor.lighter percent).
-_FX = {
+class _EffectConfig(TypedDict):
+    glow_alpha: float
+    glow_radius: float
+    pop: int
+
+
+_FX: dict[str, _EffectConfig] = {
     "subtle": {"glow_alpha": 0.22, "glow_radius": 2.0, "pop": 128},
     "expressive": {"glow_alpha": 0.42, "glow_radius": 3.0, "pop": 155},
 }
diff --git a/src/kotonoha/leaf_icon.py b/src/kotonoha/leaf_icon.py
index db1d485..f73aec9 100644
--- a/src/kotonoha/leaf_icon.py
+++ b/src/kotonoha/leaf_icon.py
@@ -11,6 +11,7 @@
 
 from functools import lru_cache
 from pathlib import Path
+from typing import cast
 
 from PyQt6.QtCore import QByteArray, QPointF, QRectF, Qt
 from PyQt6.QtGui import (
@@ -71,7 +72,7 @@ def is_generated(key: str) -> bool:
 def system_is_dark() -> bool:
     """True when the system colour scheme is dark (so a light tray icon suits the
     panel). Defaults to dark when the scheme is unknown or before the app exists."""
-    app = QGuiApplication.instance()
+    app = cast(QGuiApplication | None, QGuiApplication.instance())
     hints = app.styleHints() if app is not None else None
     scheme = hints.colorScheme() if hints is not None else None
     return scheme != Qt.ColorScheme.Light
diff --git a/src/kotonoha/settings_dialog.py b/src/kotonoha/settings_dialog.py
index de24838..bdf6080 100644
--- a/src/kotonoha/settings_dialog.py
+++ b/src/kotonoha/settings_dialog.py
@@ -82,8 +82,10 @@ class _FontNameDelegate(QStyledItemDelegate):
     """Font-list delegate that previews each family name in its own font but drops
     the file-type 'T' icon QFontComboBox normally draws on the left."""
 
-    def initStyleOption(self, option: QStyleOptionViewItem, index: QModelIndex) -> None:
+    def initStyleOption(self, option: QStyleOptionViewItem | None, index: QModelIndex) -> None:
         super().initStyleOption(option, index)
+        if option is None:
+            return
         family = index.data()
         if isinstance(family, str) and family:
             option.font = QFont(family)  # render the name in its own font
@@ -277,7 +279,7 @@ def _resolve_theme(value: str) -> str:
     "auto" follows the system colour scheme (Qt 6.5+), defaulting to dark."""
     if value in ("light", "dark"):
         return value
-    app = QGuiApplication.instance()
+    app = cast(QGuiApplication | None, QGuiApplication.instance())
     hints = app.styleHints() if app is not None else None
     scheme = hints.colorScheme() if hints is not None else None
     return "light" if scheme == Qt.ColorScheme.Light else "dark"
@@ -506,10 +508,8 @@ def showEvent(self, a0: QShowEvent | None) -> None:
         # sections never resizes the window and the nav never truncates.
         self._nav.setFixedWidth(self._nav.sizeHintForColumn(0) + 30)
         self._stack.setMinimumWidth(400)
-        tallest = max(
-            (self._stack.widget(i).sizeHint().height() for i in range(self._stack.count())),
-            default=0,
-        )
+        widgets = (self._stack.widget(i) for i in range(self._stack.count()))
+        tallest = max((widget.sizeHint().height() for widget in widgets if widget is not None), default=0)
         self._stack.setMinimumHeight(tallest)
         needed = self._nav.width() + 1 + 400 + 46  # nav + divider + content + margins/spacing
         if self.minimumWidth() < needed:
diff --git a/tests/test_clock.py b/tests/test_clock.py
index 72595e6..6d1d974 100644
--- a/tests/test_clock.py
+++ b/tests/test_clock.py
@@ -101,13 +101,17 @@ def test_coarse_position_stall_keeps_flowing_while_playing():
     clock = MediaClock(monotonic=fake)
     clock.sync(media_time=10.0, playing=True)
     previous = clock.now()
+    assert previous is not None
     for _ in range(15):  # ~3s of a stalled Position report, well past the grace window
         fake.t += 0.2
         clock.sync(media_time=10.0, playing=True)
         current = clock.now()
+        assert current is not None
         assert current >= previous  # never rolls backward
         previous = current
-    assert clock.now() > 12.0  # kept interpolating forward the whole stall
+    current = clock.now()
+    assert current is not None
+    assert current > 12.0  # kept interpolating forward the whole stall
     assert clock.playing is True  # a stall while Playing is never treated as a pause
 
 
@@ -128,7 +132,9 @@ def test_backward_seek_while_paused_is_followed():
     fake.t += 0.2
     clock.sync(media_time=20.2, playing=True)  # resume
     fake.t += 0.5
-    assert 20.0 <= clock.now() <= 21.5  # plays forward from the seeked position
+    current = clock.now()
+    assert current is not None
+    assert 20.0 <= current <= 21.5  # plays forward from the seeked position
 
 
 def test_lagging_report_does_not_roll_the_sweep_back():
@@ -137,7 +143,9 @@ def test_lagging_report_does_not_roll_the_sweep_back():
     clock.sync(media_time=10.0, playing=True)
     fake.t += 0.5  # estimate ~10.5
     clock.sync(media_time=10.2, playing=True)  # advanced, but still behind estimate
-    assert clock.now() >= 10.5  # stayed forward, did not snap back to 10.2
+    current = clock.now()
+    assert current is not None
+    assert current >= 10.5  # stayed forward, did not snap back to 10.2
 
 
 def test_sync_without_media_time_is_noop():
diff --git a/tests/test_config.py b/tests/test_config.py
index 0c22285..c5b816a 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -1,3 +1,5 @@
+from typing import cast
+
 from kotonoha.config import Config, load_config, save_config
 
 
@@ -74,7 +76,7 @@ def test_effects_defaults_clamp_and_roundtrip(tmp_path):
     assert Config(fx_transition="bogus").clamped().fx_transition == "rise"
     # Fuzzy matching: on by default, coerced to bool.
     assert Config().fuzzy_match is True
-    assert Config(fuzzy_match=0).clamped().fuzzy_match is False
+    assert Config(fuzzy_match=cast(bool, 0)).clamped().fuzzy_match is False
     path = tmp_path / "c.json"
     save_config(Config(fx_animate=False, fx_glow=False, fx_word_pop=False, fx_intensity="expressive"), path)
     loaded = load_config(path)
@@ -96,7 +98,7 @@ def test_theme_and_white_panel_clamp_and_roundtrip(tmp_path):
 
 def test_frost_window_defaults_and_roundtrips(tmp_path):
     assert Config().frost_window is True
-    assert Config(frost_window=0).clamped().frost_window is False  # coerced to bool
+    assert Config(frost_window=cast(bool, 0)).clamped().frost_window is False  # coerced to bool
     path = tmp_path / "c.json"
     save_config(Config(frost_window=False), path)
     assert load_config(path).frost_window is False
diff --git a/tests/test_controller.py b/tests/test_controller.py
index 4deb630..3c29195 100644
--- a/tests/test_controller.py
+++ b/tests/test_controller.py
@@ -1,4 +1,5 @@
 import os
+from typing import cast
 
 os.environ.setdefault("QT_QPA_PLATFORM", "offscreen")
 
@@ -7,6 +8,8 @@
 
 from kotonoha.config import Config
 from kotonoha.controller import AppController
+from kotonoha.providers.mpris import MprisProvider
+from kotonoha.receiver import LyricsReceiver
 
 
 @pytest.fixture(scope="module")
@@ -38,9 +41,9 @@ async def test_start_survives_optional_receiver_bind_failure(qapp):
     # A stale instance / double-launch holding port 28745 must only disable the
     # optional Cider receiver, not take down the already-shown overlay and tray.
     controller = AppController(qapp, Config())
-    controller._receiver = _FakeReceiver()
+    controller._receiver = cast(LyricsReceiver, _FakeReceiver())
     fake_mpris = _FakeMpris()
-    controller._mpris = fake_mpris
+    controller._mpris = cast(MprisProvider, fake_mpris)
 
     await controller.start()  # must not raise
 
diff --git a/tests/test_lyrics.py b/tests/test_lyrics.py
index fef8f54..0d01136 100644
--- a/tests/test_lyrics.py
+++ b/tests/test_lyrics.py
@@ -364,7 +364,9 @@ def test_exact_title_and_artist_survive_small_duration_skew():
 def test_best_match_prefers_genuine_artist_over_missing_artist():
     track = TrackMetadata("Song", "Artist", "", 180.0)
     candidates = [Candidate("noart", "Song", "", 180.0), Candidate("art", "Song", "Artist", 180.0)]
-    assert best_match(candidates, track).candidate.song_id == "art"
+    evidence = best_match(candidates, track)
+    assert evidence is not None
+    assert evidence.candidate.song_id == "art"
 
 
 def test_query_variants_are_raw_then_base_title_primary_artist():
diff --git a/tests/test_lyrics_resolver.py b/tests/test_lyrics_resolver.py
index 5bd3454..fcade85 100644
--- a/tests/test_lyrics_resolver.py
+++ b/tests/test_lyrics_resolver.py
@@ -77,7 +77,7 @@ def resolver_with_fakes(
     network_hits = network_hits or {}
 
     def adapter(name):
-        async def fetch(_session, _track, *, fuzzy=False):
+        async def fetch(session, track, *, fuzzy=False):
             calls.append(f"network:{name}")
             return network_hits.get(name)
 
@@ -195,7 +195,7 @@ async def test_concurrent_identical_requests_share_network_work():
     started = asyncio.Event()
     release = asyncio.Event()
 
-    async def fetch(_session, _track, *, fuzzy=False):
+    async def fetch(session, track, *, fuzzy=False):
         calls.append("network:netease")
         started.set()
         await release.wait()
@@ -219,7 +219,7 @@ async def fetch(_session, _track, *, fuzzy=False):
 
 
 async def test_network_timeout_log_includes_exception_type(caplog):
-    async def timeout(_session, _track, *, fuzzy=False):
+    async def timeout(session, track, *, fuzzy=False):
         raise TimeoutError
 
     resolver = LyricsResolver(
diff --git a/tests/test_mpris_provider.py b/tests/test_mpris_provider.py
index 51f07d3..cb85792 100644
--- a/tests/test_mpris_provider.py
+++ b/tests/test_mpris_provider.py
@@ -61,6 +61,9 @@ def set_cache_enabled(self, _enabled):
     def set_prefer_best(self, _enabled):
         return None
 
+    def set_fuzzy(self, _enabled):
+        return None
+
     async def clear_cache(self):
         return None
 
diff --git a/tests/test_native_packaging.py b/tests/test_native_packaging.py
index 52c24fe..8c003cc 100644
--- a/tests/test_native_packaging.py
+++ b/tests/test_native_packaging.py
@@ -113,6 +113,13 @@ def test_cmake_is_documented_and_verified_by_ci() -> None:
     assert 'archive.namelist().count("kotonoha/libkoto-layer.so")' in package_workflow
 
 
+def test_python_workflow_uses_bash_for_bash_only_commands() -> None:
+    workflow = read_packaging_file(TEST_WORKFLOW)
+    python_job = workflow.split("  python:\n", maxsplit=1)[1].split("\n  cider:\n", maxsplit=1)[0]
+
+    assert "    defaults:\n      run:\n        shell: bash\n" in python_job
+
+
 def test_debian_control_declares_package_metadata_and_dependencies() -> None:
     control = read_packaging_file(DEBIAN_DIR / "control")
 
diff --git a/tests/test_overlay.py b/tests/test_overlay.py
index 3c96fa5..1baa460 100644
--- a/tests/test_overlay.py
+++ b/tests/test_overlay.py
@@ -8,12 +8,14 @@
 from PyQt6.QtWidgets import QApplication
 
 from kotonoha.config import Config
+from kotonoha.native import LayerShellController
 from kotonoha.overlay import LyricsOverlay
 from kotonoha.state import LyricsState
 
 
-class UnavailableController:
-    available = False
+class UnavailableController(LayerShellController):
+    def __init__(self) -> None:
+        super().__init__("", "wayland", "GNOME")
 
 
 class RecordingOverlay(LyricsOverlay):
@@ -169,7 +171,9 @@ def test_white_panel_flips_text_and_context_shadow_to_light(qapp):
     # A dark panel keeps light text with a dark halo.
     overlay.apply_config(Config(panel_style="pill"))
     assert overlay._text_colors()[0].lightness() > 160
-    assert overlay._prev_label.graphicsEffect().color().lightness() < 100
+    effect = overlay._prev_label.graphicsEffect()
+    assert isinstance(effect, QGraphicsDropShadowEffect)
+    assert effect.color().lightness() < 100
     overlay.deleteLater()
     qapp.processEvents()
 
@@ -222,6 +226,7 @@ def test_lyric_script_converts_displayed_line(qapp):
         LyricsState(), Config(lyrics_script="zh-Hant"), UnavailableController()
     )
     out = converted._convert_line(line)
+    assert out is not None
     assert out.text == "簡體字"  # display converted to Traditional
     assert out.words[0].text == "簡"  # words converted too (for the karaoke sweep)
     off = LyricsOverlay(LyricsState(), Config(lyrics_script="off"), UnavailableController())
diff --git a/tests/test_receiver.py b/tests/test_receiver.py
index 8036fb7..aad31a7 100644
--- a/tests/test_receiver.py
+++ b/tests/test_receiver.py
@@ -1,6 +1,8 @@
 import json
+from typing import cast
 
 import pytest
+from aiohttp import web
 
 pytest.importorskip("PyQt6.QtCore")
 pytest.importorskip("aiohttp")
@@ -150,7 +152,7 @@ async def send_str(self, _text):
             raise ConnectionResetError("socket closed mid-send")
 
     receiver = LyricsReceiver(LyricsState())
-    receiver._clients.add(FakeWS())
+    receiver._clients.add(cast(web.WebSocketResponse, FakeWS()))
 
     receiver.update_translation_language("ja")  # must not raise
     # The send task is retained (RUF006), not fire-and-forget.
diff --git a/tests/test_settings_dialog.py b/tests/test_settings_dialog.py
index 69433c5..7c9eda0 100644
--- a/tests/test_settings_dialog.py
+++ b/tests/test_settings_dialog.py
@@ -160,7 +160,9 @@ def test_content_sits_in_a_raised_card_and_page_switch_is_safe(qapp):
     # effect is left on the page (it can never be stuck dim/blank).
     dialog._nav.setCurrentRow(2)
     assert dialog._stack.currentIndex() == 2
-    assert dialog._stack.currentWidget().graphicsEffect() is None
+    current = dialog._stack.currentWidget()
+    assert current is not None
+    assert current.graphicsEffect() is None
     dialog.close()
 
 
@@ -189,17 +191,17 @@ def test_theme_selector_roundtrips_and_switches_palette(qapp):
 
     dark = SettingsDialog(Config(theme="dark"))
     assert dark._theme == "dark"
-    assert _PALETTES["dark"]["TEXT"] in dark.styleSheet()
+    assert cast(str, _PALETTES["dark"]["TEXT"]) in dark.styleSheet()
     assert dark.current_config().theme == "dark"
 
     light = SettingsDialog(Config(theme="light"))
     assert light._theme == "light"
-    assert _PALETTES["light"]["TEXT"] in light.styleSheet()
+    assert cast(str, _PALETTES["light"]["TEXT"]) in light.styleSheet()
     # Switching theme on Apply re-skins the dialog live.
     light._theme_combo.setCurrentIndex(light._theme_combo.findData("dark"))
     light._emit()
     assert light._theme == "dark"
-    assert _PALETTES["dark"]["TEXT"] in light.styleSheet()
+    assert cast(str, _PALETTES["dark"]["TEXT"]) in light.styleSheet()
     dark.close()
     light.close()
 
@@ -208,7 +210,11 @@ def test_connection_section_removed_but_port_preserved(qapp):
     # The WS-port control was dropped; the sidebar no longer lists Connection,
     # and current_config keeps the config's port untouched (still used by the CLI).
     dialog = SettingsDialog(Config(port=41234))
-    labels = [dialog._nav.item(i).text() for i in range(dialog._nav.count())]
+    labels = []
+    for i in range(dialog._nav.count()):
+        item = dialog._nav.item(i)
+        assert item is not None
+        labels.append(item.text())
     assert not any("onnect" in label or "连接" in label or "連接" in label or "接続" in label for label in labels)
     assert not hasattr(dialog, "_port")
     assert dialog.current_config().port == 41234  # preserved from the config
@@ -317,10 +323,11 @@ def test_icon_picker_includes_generated_leaf_styles(qapp):
     from kotonoha import leaf_icon
 
     dialog = SettingsDialog(Config(icon_name=leaf_icon.TILE))
-    keys = [
-        str(dialog._tray_icon_list.item(i).data(Qt.ItemDataRole.UserRole))
-        for i in range(dialog._tray_icon_list.count())
-    ]
+    keys = []
+    for i in range(dialog._tray_icon_list.count()):
+        item = dialog._tray_icon_list.item(i)
+        assert item is not None
+        keys.append(str(item.data(Qt.ItemDataRole.UserRole)))
     for style in leaf_icon.PICKER_STYLES:  # accent / white / black / tile are offered
         assert style in keys
     assert leaf_icon.WHITE in keys and leaf_icon.BLACK in keys  # explicit monochromes
@@ -349,10 +356,11 @@ def test_tray_and_window_icons_are_chosen_independently(qapp):
     assert dialog._picked_icon(dialog._tray_icon_list) == leaf_icon.WHITE
     assert dialog._picked_icon(dialog._window_icon_list) == leaf_icon.TILE
     # Changing one does not move the other.
-    window_keys = [
-        str(dialog._window_icon_list.item(i).data(Qt.ItemDataRole.UserRole))
-        for i in range(dialog._window_icon_list.count())
-    ]
+    window_keys = []
+    for i in range(dialog._window_icon_list.count()):
+        item = dialog._window_icon_list.item(i)
+        assert item is not None
+        window_keys.append(str(item.data(Qt.ItemDataRole.UserRole)))
     dialog._window_icon_list.setCurrentRow(window_keys.index(leaf_icon.BLACK))
     cfg = dialog.current_config()
     assert cfg.icon_name == leaf_icon.WHITE