diff --git a/tests/test_lyrics.py b/tests/test_lyrics.py
new file mode 100644
index 0000000..e1357af
--- /dev/null
+++ b/tests/test_lyrics.py
@@ -0,0 +1,247 @@
+"""Lyrics service (§6.7) + OpenSubsonic structured-lyrics serializer — DB-free.
+
+Service: get-or-fetch caching, not_found TTL, force refetch, graceful miss.
+Serializer: LRC → timed lines, plain fallback, empty payload when absent.
+"""
+
+import datetime as dt
+import uuid
+
+import pytest
+from app.api.rest.serializers import _parse_lrc, structured_lyrics
+from app.application.lyrics_service import LyricsService
+from app.domain.entities import Artist, Track
+from app.domain.entities.album import Album
+from app.domain.entities.lyrics import Lyrics, LyricsResult
+from app.domain.errors import NotFoundError
+
+
+def _now() -> dt.datetime:
+    return dt.datetime.now(dt.UTC)
+
+
+def _lyrics(*, synced=None, plain=None, source=None, status="found", age_days=0) -> Lyrics:
+    return Lyrics(
+        track_id=uuid.uuid4(),
+        synced=synced,
+        plain=plain,
+        source=source,
+        status=status,
+        fetched_at=_now() - dt.timedelta(days=age_days),
+    )
+
+
+def _track() -> Track:
+    now = _now()
+    return Track(
+        id=uuid.uuid4(),
+        title="Song",
+        artist_id=uuid.uuid4(),
+        album_id=None,
+        storage_uri="tracks/x.mp3",
+        file_format="mp3",
+        file_size=1,
+        source="upload",
+        source_id="x",
+        duration_seconds=200,
+        genre=None,
+        year=None,
+        track_number=None,
+        metadata_status="pending",
+        metadata_error=None,
+        enriched_at=None,
+        availability="local",
+        created_at=now,
+        updated_at=now,
+    )
+
+
+class FakeLyricsRepo:
+    def __init__(self, cached: Lyrics | None) -> None:
+        self._cached = cached
+        self.upserts: list[dict[str, object]] = []
+
+    async def get(self, track_id: uuid.UUID) -> Lyrics | None:
+        return self._cached
+
+    async def upsert(self, *, track_id, synced, plain, source, status) -> Lyrics:
+        self.upserts.append({"status": status, "source": source})
+        return Lyrics(
+            track_id=track_id,
+            synced=synced,
+            plain=plain,
+            source=source,
+            status=status,
+            fetched_at=_now(),
+        )
+
+
+class FakeTrackRepo:
+    def __init__(self, track: Track | None) -> None:
+        self._track = track
+
+    async def get_by_id(self, track_id: uuid.UUID) -> Track | None:
+        return self._track
+
+
+class FakeArtistRepo:
+    def __init__(self, artist: Artist | None = None) -> None:
+        self._artist = artist
+
+    async def get_by_id(self, artist_id: uuid.UUID) -> Artist | None:
+        return self._artist
+
+
+class FakeAlbumRepo:
+    async def get_by_id(self, album_id: uuid.UUID) -> Album | None:
+        return None
+
+
+class FakeProvider:
+    def __init__(self, result: LyricsResult | None) -> None:
+        self._result = result
+        self.calls = 0
+
+    async def fetch(self, *, artist, title, album, duration_seconds) -> LyricsResult | None:
+        self.calls += 1
+        return self._result
+
+
+def _service(*, cached, track, provider) -> tuple[LyricsService, FakeLyricsRepo, FakeProvider]:
+    repo = FakeLyricsRepo(cached)
+    prov = FakeProvider(provider)
+    svc = LyricsService(
+        lyrics=repo,
+        tracks=FakeTrackRepo(track),
+        artists=FakeArtistRepo(),
+        albums=FakeAlbumRepo(),
+        provider=prov,
+    )
+    return svc, repo, prov
+
+
+# -- service ------------------------------------------------------------------
+
+
+async def test_fresh_found_is_served_from_cache_without_provider() -> None:
+    cached = _lyrics(plain="hi", status="found")
+    svc, _repo, prov = _service(cached=cached, track=_track(), provider=None)
+    out = await svc.get_lyrics(cached.track_id)
+    assert out is cached
+    assert prov.calls == 0
+
+
+async def test_miss_fetches_and_caches_found() -> None:
+    track = _track()
+    result = LyricsResult(synced="[00:01.00]hi", plain="hi", source="lrclib")
+    svc, repo, prov = _service(cached=None, track=track, provider=result)
+    out = await svc.get_lyrics(track.id)
+    assert prov.calls == 1
+    assert out.status == "found"
+    assert repo.upserts[-1]["status"] == "found"
+
+
+async def test_provider_miss_caches_not_found() -> None:
+    track = _track()
+    svc, repo, prov = _service(cached=None, track=track, provider=None)
+    out = await svc.get_lyrics(track.id)
+    assert prov.calls == 1
+    assert out.status == "not_found"
+    assert repo.upserts[-1]["status"] == "not_found"
+
+
+async def test_stale_not_found_is_refetched() -> None:
+    # A not_found older than the 7-day TTL is retried against the provider.
+    stale = _lyrics(status="not_found", age_days=8)
+    result = LyricsResult(synced=None, plain="found now", source="lrclib")
+    svc, _repo, prov = _service(cached=stale, track=_track(), provider=result)
+    out = await svc.get_lyrics(stale.track_id)
+    assert prov.calls == 1
+    assert out.status == "found"
+
+
+async def test_fresh_not_found_is_not_refetched() -> None:
+    fresh = _lyrics(status="not_found", age_days=1)
+    svc, _repo, prov = _service(cached=fresh, track=_track(), provider=None)
+    out = await svc.get_lyrics(fresh.track_id)
+    assert out is fresh
+    assert prov.calls == 0
+
+
+async def test_force_bypasses_a_fresh_cache() -> None:
+    cached = _lyrics(plain="old", status="found")
+    result = LyricsResult(synced=None, plain="new", source="lrclib")
+    svc, _repo, prov = _service(cached=cached, track=_track(), provider=result)
+    out = await svc.get_lyrics(cached.track_id, force=True)
+    assert prov.calls == 1
+    assert out.plain == "new"
+
+
+async def test_unknown_track_raises() -> None:
+    svc, _repo, _prov = _service(cached=None, track=None, provider=None)
+    with pytest.raises(NotFoundError):
+        await svc.get_lyrics(uuid.uuid4())
+
+
+# -- serializer ---------------------------------------------------------------
+
+
+def test_parse_lrc_times_and_order() -> None:
+    lines = _parse_lrc("[00:12.34]second\n[00:01.00]first\n[bad]meta\n\n")
+    # Metadata/blank lines dropped; output ordered by time; centiseconds → ms.
+    assert lines == [
+        {"start": 1000, "value": "first"},
+        {"start": 12340, "value": "second"},
+    ]
+
+
+def test_parse_lrc_repeated_stamps_on_one_line() -> None:
+    lines = _parse_lrc("[00:01.00][00:05.00]chorus")
+    assert lines == [
+        {"start": 1000, "value": "chorus"},
+        {"start": 5000, "value": "chorus"},
+    ]
+
+
+def test_structured_lyrics_prefers_synced() -> None:
+    out = structured_lyrics(
+        _lyrics(synced="[00:01.00]hi", plain="hi", status="found"),
+        display_artist="A",
+        display_title="T",
+    )
+    entry = out["lyricsList"]["structuredLyrics"][0]
+    assert entry["synced"] is True
+    assert entry["displayArtist"] == "A"
+    assert entry["line"] == [{"start": 1000, "value": "hi"}]
+
+
+def test_structured_lyrics_falls_back_to_plain() -> None:
+    out = structured_lyrics(
+        _lyrics(synced=None, plain="line one\nline two", status="found"),
+        display_artist="A",
+        display_title="T",
+    )
+    entry = out["lyricsList"]["structuredLyrics"][0]
+    assert entry["synced"] is False
+    assert entry["line"] == [{"value": "line one"}, {"value": "line two"}]
+
+
+def test_structured_lyrics_empty_when_absent() -> None:
+    out = structured_lyrics(
+        _lyrics(synced=None, plain=None, status="not_found"),
+        display_artist="A",
+        display_title="T",
+    )
+    assert out == {"lyricsList": {}}
+
+
+def test_structured_lyrics_unparseable_synced_falls_back_to_plain() -> None:
+    # synced present but no valid timecodes → treat as plain, synced=False.
+    out = structured_lyrics(
+        _lyrics(synced="no timecodes here", plain="plain text", status="found"),
+        display_artist="A",
+        display_title="T",
+    )
+    entry = out["lyricsList"]["structuredLyrics"][0]
+    assert entry["synced"] is False
+    assert entry["line"] == [{"value": "plain text"}]
diff --git a/tests/test_recommendation_service.py b/tests/test_recommendation_service.py
new file mode 100644
index 0000000..f8990e7
--- /dev/null
+++ b/tests/test_recommendation_service.py
@@ -0,0 +1,248 @@
+"""RecommendationService (§6.5) — DB-free, in-memory fakes.
+
+Covers the two paths that matter: the ML recommender when available (reason
+``ml``, order-preserving batched hydration) and the metadata fallback when it
+declines (similar/radio still work — the graceful-degradation invariant).
+"""
+
+import datetime as dt
+import uuid
+
+import pytest
+from app.application.recommendation_service import (
+    REASON_FROM_LIKES,
+    REASON_ML,
+    REASON_SIMILAR,
+    RecommendationService,
+)
+from app.domain.entities import Artist, Track
+from app.domain.errors import NotFoundError
+from app.infrastructure.ml.recommender import NullRecommender
+
+
+def _now() -> dt.datetime:
+    return dt.datetime.now(dt.UTC)
+
+
+def _track(*, genre: str | None = "rock", artist_id: uuid.UUID | None = None) -> Track:
+    now = _now()
+    return Track(
+        id=uuid.uuid4(),
+        title="T",
+        artist_id=artist_id or uuid.uuid4(),
+        album_id=None,
+        storage_uri="tracks/x.mp3",
+        file_format="mp3",
+        file_size=1,
+        source="upload",
+        source_id="x",
+        duration_seconds=None,
+        genre=genre,
+        year=None,
+        track_number=None,
+        metadata_status="pending",
+        metadata_error=None,
+        enriched_at=None,
+        availability="local",
+        created_at=now,
+        updated_at=now,
+    )
+
+
+def _artist() -> Artist:
+    now = _now()
+    return Artist(
+        id=uuid.uuid4(), name="A", source=None, source_id=None, created_at=now, updated_at=now
+    )
+
+
+class FakeTrackRepo:
+    def __init__(self, tracks: list[Track]) -> None:
+        self._by_id = {t.id: t for t in tracks}
+        self.similar: list[Track] = []
+        self.sample: list[Track] = []
+        self.similar_calls: list[dict[str, object]] = []
+
+    async def get_by_id(self, track_id: uuid.UUID) -> Track | None:
+        return self._by_id.get(track_id)
+
+    async def get_many(self, ids: list[uuid.UUID]) -> list[Track]:
+        # Deliberately unordered (mirrors a real ``WHERE id IN`` query) so the
+        # service is responsible for restoring request order.
+        return [self._by_id[i] for i in reversed(ids) if i in self._by_id]
+
+    async def list_similar(self, *, genre, artist_id, exclude_ids, limit) -> list[Track]:
+        self.similar_calls.append({"exclude_ids": list(exclude_ids), "limit": limit})
+        return [t for t in self.similar if t.id not in exclude_ids][:limit]
+
+    async def sample_playable(self, *, exclude_ids, limit) -> list[Track]:
+        return [t for t in self.sample if t.id not in exclude_ids][:limit]
+
+
+class FakeArtistRepo:
+    def __init__(self, artists: list[Artist]) -> None:
+        self._by_id = {a.id: a for a in artists}
+        self.similar: list[Artist] = []
+
+    async def get_by_id(self, artist_id: uuid.UUID) -> Artist | None:
+        return self._by_id.get(artist_id)
+
+    async def get_many(self, ids: list[uuid.UUID]) -> list[Artist]:
+        return [self._by_id[i] for i in reversed(ids) if i in self._by_id]
+
+    async def list_similar(self, *, artist_id, limit) -> list[Artist]:
+        return self.similar[:limit]
+
+
+class FakeLikeRepo:
+    def __init__(self, liked: list[Track]) -> None:
+        self.liked = liked
+
+    async def list_liked_tracks(self, *, user_id, limit, offset) -> list[Track]:
+        return self.liked[offset : offset + limit]
+
+
+class StubRecommender:
+    """An 'available' ML recommender returning fixed ids for the ML path."""
+
+    def __init__(self, ids: list[uuid.UUID] | None) -> None:
+        self._ids = ids
+
+    def is_available(self) -> bool:
+        return True
+
+    async def similar_track_ids(self, track_id, *, limit, exclude_ids):
+        return self._ids
+
+    async def similar_artist_ids(self, artist_id, *, limit):
+        return self._ids
+
+    async def radio_track_ids(self, *, seed_track_id, exploration, limit, exclude_ids):
+        return self._ids
+
+
+def _service(tracks, artists, likes, recommender) -> RecommendationService:
+    return RecommendationService(
+        recommender=recommender, tracks=tracks, artists=artists, likes=likes
+    )
+
+
+# -- similar ------------------------------------------------------------------
+
+
+async def test_similar_tracks_unknown_seed_raises() -> None:
+    svc = _service(FakeTrackRepo([]), FakeArtistRepo([]), FakeLikeRepo([]), NullRecommender())
+    with pytest.raises(NotFoundError):
+        await svc.similar_tracks(uuid.uuid4(), limit=5)
+
+
+async def test_similar_tracks_falls_back_to_metadata() -> None:
+    seed = _track()
+    neighbours = [_track(), _track()]
+    repo = FakeTrackRepo([seed, *neighbours])
+    repo.similar = neighbours
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([]), NullRecommender())
+
+    reason, found = await svc.similar_tracks(seed.id, limit=5)
+    assert reason == REASON_SIMILAR
+    assert [t.id for t in found] == [n.id for n in neighbours]
+    # The seed itself is always excluded from its own neighbours.
+    assert repo.similar_calls[-1]["exclude_ids"] == [seed.id]
+
+
+async def test_similar_tracks_uses_ml_and_preserves_order() -> None:
+    seed = _track()
+    a, b, c = _track(), _track(), _track()
+    repo = FakeTrackRepo([seed, a, b, c])
+    # ML returns a specific order; hydration must preserve it despite get_many
+    # returning rows unordered, and skip ids that no longer exist.
+    ml_ids = [c.id, uuid.uuid4(), a.id, b.id]
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([]), StubRecommender(ml_ids))
+
+    reason, found = await svc.similar_tracks(seed.id, limit=10)
+    assert reason == REASON_ML
+    assert [t.id for t in found] == [c.id, a.id, b.id]
+
+
+async def test_similar_artists_falls_back_to_metadata() -> None:
+    seed = _artist()
+    neighbours = [_artist(), _artist()]
+    repo = FakeArtistRepo([seed])
+    repo.similar = neighbours
+    svc = _service(FakeTrackRepo([]), repo, FakeLikeRepo([]), NullRecommender())
+
+    reason, found = await svc.similar_artists(seed.id, limit=5)
+    assert reason == REASON_SIMILAR
+    assert [a.id for a in found] == [n.id for n in neighbours]
+
+
+# -- radio --------------------------------------------------------------------
+
+
+async def test_radio_from_likes_seeds_and_fills(monkeypatch) -> None:
+    liked = _track()
+    similar = [_track(), _track()]
+    explore = [_track(), _track(), _track()]
+    repo = FakeTrackRepo([liked, *similar, *explore])
+    repo.similar = similar
+    repo.sample = explore
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([liked]), NullRecommender())
+    # Deterministic seed choice + no shuffling for a stable assertion.
+    monkeypatch.setattr("app.application.recommendation_service.random.choice", lambda s: s[0])
+    monkeypatch.setattr("app.application.recommendation_service.random.shuffle", lambda s: None)
+
+    reason, picks = await svc.radio(
+        user_id=uuid.uuid4(),
+        seed_track_id=None,
+        from_likes=True,
+        exploration=0.5,
+        limit=4,
+        exclude_ids=[],
+    )
+    assert reason == "metadata"
+    ids = [p.track.id for p in picks]
+    # 4 picks, all distinct, no seed repeated, mix of similar + discover.
+    assert len(ids) == 4
+    assert len(set(ids)) == 4
+    reasons = {p.reason for p in picks}
+    assert REASON_FROM_LIKES in reasons  # similar picks carry the seed's reason
+
+
+async def test_radio_respects_exclude_ids(monkeypatch) -> None:
+    similar = [_track(), _track()]
+    explore = [_track(), _track()]
+    repo = FakeTrackRepo([*similar, *explore])
+    repo.similar = similar
+    repo.sample = explore
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([]), NullRecommender())
+    monkeypatch.setattr("app.application.recommendation_service.random.shuffle", lambda s: None)
+
+    excluded = similar[0].id
+    _, picks = await svc.radio(
+        user_id=uuid.uuid4(),
+        seed_track_id=None,
+        from_likes=False,
+        exploration=1.0,
+        limit=4,
+        exclude_ids=[excluded],
+    )
+    assert excluded not in {p.track.id for p in picks}
+
+
+async def test_radio_uses_ml_when_available(monkeypatch) -> None:
+    a, b = _track(), _track()
+    repo = FakeTrackRepo([a, b])
+    svc = _service(repo, FakeArtistRepo([]), FakeLikeRepo([]), StubRecommender([b.id, a.id]))
+    monkeypatch.setattr("app.application.recommendation_service.random.shuffle", lambda s: None)
+
+    reason, picks = await svc.radio(
+        user_id=uuid.uuid4(),
+        seed_track_id=a.id,
+        from_likes=False,
+        exploration=0.3,
+        limit=5,
+        exclude_ids=[],
+    )
+    assert reason == REASON_ML
+    assert [p.track.id for p in picks] == [b.id, a.id]
+    assert all(p.reason == REASON_ML for p in picks)
diff --git a/tests/test_transcode_service.py b/tests/test_transcode_service.py
new file mode 100644
index 0000000..da4a458
--- /dev/null
+++ b/tests/test_transcode_service.py
@@ -0,0 +1,157 @@
+"""Transcode cache-path helpers + request-side service (§6.6) — DB-free.
+
+Covers the pure path math, the segment-name traversal guard, cache
+hit/miss lookups, and the "not yet downloaded" / unknown-track 404 paths.
+"""
+
+import datetime as dt
+import uuid
+from pathlib import Path
+
+import pytest
+from app.application.transcode_service import (
+    QUALITY_BITRATE,
+    TranscodeService,
+    bitrate_for_quality,
+    hls_playlist_path,
+    hls_segment_path,
+    opus_path,
+    remove_track_cache,
+    track_cache_dir,
+)
+from app.domain.entities import Track
+from app.domain.errors import NotFoundError
+
+
+def _track(*, storage_uri: str | None = "tracks/aa/song.mp3") -> Track:
+    now = dt.datetime.now(dt.UTC)
+    return Track(
+        id=uuid.uuid4(),
+        title="Song",
+        artist_id=uuid.uuid4(),
+        album_id=None,
+        storage_uri=storage_uri,
+        file_format="mp3",
+        file_size=1,
+        source="upload",
+        source_id="x",
+        duration_seconds=None,
+        genre=None,
+        year=None,
+        track_number=None,
+        metadata_status="pending",
+        metadata_error=None,
+        enriched_at=None,
+        availability="local",
+        created_at=now,
+        updated_at=now,
+    )
+
+
+class FakeTrackRepo:
+    def __init__(self, track: Track | None) -> None:
+        self._track = track
+
+    async def get_by_id(self, track_id: uuid.UUID) -> Track | None:
+        return self._track
+
+
+# -- pure helpers -------------------------------------------------------------
+
+
+def test_bitrate_for_quality_known_and_unknown() -> None:
+    assert bitrate_for_quality("high") == QUALITY_BITRATE["high"]
+    assert bitrate_for_quality("medium") == 96
+    assert bitrate_for_quality("low") == 64
+    # "original" and anything unrecognised mean "serve the master, no transcode".
+    assert bitrate_for_quality("original") is None
+    assert bitrate_for_quality("nonsense") is None
+
+
+def test_path_helpers_are_under_the_track_dir(tmp_path: Path) -> None:
+    tid = uuid.uuid4()
+    cache = track_cache_dir(tmp_path, tid)
+    assert cache == tmp_path / str(tid)
+    assert opus_path(tmp_path, tid, 128) == cache / "opus_128.opus"
+    assert hls_playlist_path(tmp_path, tid) == cache / "hls" / "playlist.m3u8"
+
+
+@pytest.mark.parametrize(
+    "name",
+    ["seg_000.ts", "seg_1234.ts"],
+)
+def test_hls_segment_path_accepts_valid_names(tmp_path: Path, name: str) -> None:
+    tid = uuid.uuid4()
+    resolved = hls_segment_path(tmp_path, tid, name)
+    assert resolved == track_cache_dir(tmp_path, tid) / "hls" / name
+
+
+@pytest.mark.parametrize(
+    "name",
+    ["../../etc/passwd", "seg_.ts", "seg_00.tsx", "playlist.m3u8", "seg_00.ts", "..", ""],
+)
+def test_hls_segment_path_rejects_traversal_and_junk(tmp_path: Path, name: str) -> None:
+    assert hls_segment_path(tmp_path, uuid.uuid4(), name) is None
+
+
+def test_remove_track_cache_is_best_effort(tmp_path: Path) -> None:
+    tid = uuid.uuid4()
+    d = track_cache_dir(tmp_path, tid)
+    (d / "hls").mkdir(parents=True)
+    (d / "opus_128.opus").write_bytes(b"x")
+    remove_track_cache(tmp_path, tid)
+    assert not d.exists()
+    # Idempotent: deleting an already-absent cache doesn't raise.
+    remove_track_cache(tmp_path, tid)
+
+
+# -- request-side service -----------------------------------------------------
+
+
+async def test_resolve_quality_file_hit_and_miss(tmp_path: Path) -> None:
+    track = _track()
+    svc = TranscodeService(tracks=FakeTrackRepo(track), cache_root=tmp_path)
+
+    # Miss: nothing on disk yet.
+    assert await svc.resolve_quality_file(track.id, "high") is None
+    # "original" never has a rendition.
+    assert await svc.resolve_quality_file(track.id, "original") is None
+
+    # Hit: once the rendition exists it's returned.
+    path = opus_path(tmp_path, track.id, 128)
+    path.parent.mkdir(parents=True)
+    path.write_bytes(b"opus")
+    assert await svc.resolve_quality_file(track.id, "high") == path
+
+
+async def test_hls_playlist_requires_downloaded_track(tmp_path: Path) -> None:
+    tid = uuid.uuid4()
+    # Unknown track → 404.
+    with pytest.raises(NotFoundError):
+        await TranscodeService(tracks=FakeTrackRepo(None), cache_root=tmp_path).hls_playlist(tid)
+
+    # Known but not downloaded (no storage_uri) → 404.
+    pending = _track(storage_uri=None)
+    with pytest.raises(NotFoundError):
+        await TranscodeService(tracks=FakeTrackRepo(pending), cache_root=tmp_path).hls_playlist(
+            pending.id
+        )
+
+
+async def test_hls_playlist_miss_then_hit(tmp_path: Path) -> None:
+    track = _track()
+    svc = TranscodeService(tracks=FakeTrackRepo(track), cache_root=tmp_path)
+    assert await svc.hls_playlist(track.id) is None
+
+    playlist = hls_playlist_path(tmp_path, track.id)
+    playlist.parent.mkdir(parents=True)
+    playlist.write_text("#EXTM3U")
+    assert await svc.hls_playlist(track.id) == playlist
+
+
+def test_hls_segment_returns_none_for_missing_file(tmp_path: Path) -> None:
+    svc = TranscodeService(tracks=FakeTrackRepo(_track()), cache_root=tmp_path)
+    # Valid name but no file on disk.
+    assert svc.hls_segment(uuid.uuid4(), "seg_000.ts") is None
+    # Junk name is refused outright.
+    assert svc.hls_segment(uuid.uuid4(), "../secret") is None