diff --git a/core/tagger.py b/core/tagger.py
new file mode 100644
index 0000000..02df60d
--- /dev/null
+++ b/core/tagger.py
@@ -0,0 +1,109 @@
+"""Scrittura tag ID3 su file MP3 via mutagen, con cover art da URL."""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+from typing import Optional
+
+import requests
+from mutagen.easyid3 import EasyID3
+from mutagen.id3 import ID3, APIC
+from mutagen.mp3 import MP3
+
+
+SAFE_FILENAME_RE = re.compile(r'[<>:"/\\|?*\x00-\x1f]')
+
+
+def sanitize_filename_stem(stem: str, max_len: int = 180) -> str:
+    """Rimuove caratteri non validi per un filename e limita lunghezza."""
+    s = SAFE_FILENAME_RE.sub("_", stem or "").strip()
+    s = re.sub(r"\s+", " ", s)
+    if len(s) > max_len:
+        s = s[:max_len].rstrip()
+    return s or "untitled"
+
+
+def build_filename_stem(artist: str, title: str) -> str:
+    """Compone 'Artista - Titolo' sanitizzato."""
+    artist = (artist or "").strip()
+    title = (title or "").strip()
+    if artist and title:
+        return sanitize_filename_stem(f"{artist} - {title}")
+    return sanitize_filename_stem(title or artist or "untitled")
+
+
+def _download_cover(url: str) -> Optional[bytes]:
+    """Scarica bytes cover, ritorna None su qualsiasi errore."""
+    try:
+        resp = requests.get(url, timeout=15)
+        if resp.status_code == 200:
+            return resp.content
+    except Exception:
+        pass
+    return None
+
+
+def _cover_mime(url: str, content: bytes) -> str:
+    """Deduci MIME dell'immagine da URL o magic bytes."""
+    u = url.lower()
+    if u.endswith(".png"):
+        return "image/png"
+    if u.endswith((".jpg", ".jpeg")):
+        return "image/jpeg"
+    # magic bytes
+    if content[:8] == b"\x89PNG\r\n\x1a\n":
+        return "image/png"
+    if content[:3] == b"\xff\xd8\xff":
+        return "image/jpeg"
+    return "image/jpeg"  # default
+
+
+def write_tags(filepath: str, metadata: dict) -> None:
+    """Scrive ID3 tag su file MP3 esistente.
+
+    metadata: dict con chiavi opzionali:
+        title, artist, album, date (YYYY o YYYY-MM-DD), genre,
+        tracknumber, cover_url
+
+    Errori non fatali (file mancante, tag non scrivibili): silenziosi
+    per non bloccare il download principale.
+    """
+    p = Path(filepath)
+    if not p.exists():
+        return
+
+    # 1. Tag testuali via EasyID3
+    try:
+        try:
+            audio = EasyID3(str(p))
+        except Exception:
+            # Se il file non ha header ID3, aggiungine uno vuoto
+            mp3 = MP3(str(p))
+            if mp3.tags is None:
+                mp3.add_tags()
+                mp3.save()
+            audio = EasyID3(str(p))
+        for key in ("title", "artist", "album", "date", "genre", "tracknumber"):
+            val = metadata.get(key)
+            if val:
+                audio[key] = str(val)
+        audio.save(str(p))
+    except Exception:
+        return  # ID3 broken, non provo la cover
+
+    # 2. Cover art via ID3 APIC
+    cover_url = (metadata.get("cover_url") or "").strip()
+    if not cover_url:
+        return
+    cover_bytes = _download_cover(cover_url)
+    if not cover_bytes:
+        return
+    try:
+        mime = _cover_mime(cover_url, cover_bytes)
+        tags = ID3(str(p))
+        tags.delall("APIC")
+        tags.add(APIC(encoding=3, mime=mime, type=3, desc="Cover", data=cover_bytes))
+        tags.save(str(p))
+    except Exception:
+        pass
diff --git a/tests/test_tagger.py b/tests/test_tagger.py
new file mode 100644
index 0000000..514892b
--- /dev/null
+++ b/tests/test_tagger.py
@@ -0,0 +1,57 @@
+"""Test per core.tagger — filename sanitization + write_tags safety net."""
+
+from __future__ import annotations
+
+from core import tagger
+
+
+class TestSanitizeFilenameStem:
+    def test_removes_bad_chars(self):
+        # Tutti i char vietati su Windows/macOS: < > : " / \ | ? *
+        got = tagger.sanitize_filename_stem('bad<name>:"/\\|?*test')
+        # Nessuno dei char proibiti sopravvive
+        for ch in '<>:"/\\|?*':
+            assert ch not in got
+        # E ho ottenuto qualcosa di non vuoto
+        assert got
+
+    def test_collapses_whitespace(self):
+        assert tagger.sanitize_filename_stem("a    b    c") == "a b c"
+
+    def test_truncates_long_stem(self):
+        s = "x" * 500
+        got = tagger.sanitize_filename_stem(s, max_len=180)
+        assert len(got) <= 180
+
+    def test_empty_returns_untitled(self):
+        assert tagger.sanitize_filename_stem("") == "untitled"
+        assert tagger.sanitize_filename_stem("   ") == "untitled"
+
+
+class TestBuildFilenameStem:
+    def test_format_artist_dash_title(self):
+        assert tagger.build_filename_stem("Kapuchon", "Hot Sauce") == "Kapuchon - Hot Sauce"
+
+    def test_multiple_artists_ok(self):
+        got = tagger.build_filename_stem("Kapuchon, Miss Monique & GLZ", "Hot Sauce (Extended)")
+        assert got == "Kapuchon, Miss Monique & GLZ - Hot Sauce (Extended)"
+
+    def test_missing_artist_uses_title_only(self):
+        assert tagger.build_filename_stem("", "Only Title") == "Only Title"
+
+    def test_missing_title_uses_artist_only(self):
+        assert tagger.build_filename_stem("Only Artist", "") == "Only Artist"
+
+    def test_sanitizes_bad_chars(self):
+        got = tagger.build_filename_stem("A/B", "C:D")
+        # Nessuno slash o colon residuo
+        assert "/" not in got
+        assert ":" not in got
+
+
+class TestWriteTagsMissingFile:
+    def test_missing_file_is_noop(self):
+        # Non deve sollevare alcuna eccezione se il file non esiste
+        tagger.write_tags("/nonexistent/path/to/file.mp3", {
+            "title": "x", "artist": "y", "cover_url": "http://example.com/x.jpg",
+        })