fix: enable yt-dlp EJS solver so YouTube formats resolve

YouTube gates media formats behind signature / "n" JS challenges that
yt-dlp's bundled solver can't reliably crack. Without the EJS
remote-component solver script yt-dlp warns "Signature/n challenge
solving failed: Some formats may be missing" and downloads break — even
with valid cookies, so it masquerades as a cookie/auth problem.

Pass --remote-components ejs:github by default (env/CLI overridable) on
all four yt-dlp call sites. Needs a recent yt-dlp + deno present; set
YTDLP_REMOTE_COMPONENTS / --remote-components '' to disable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -102,6 +102,7 @@ export LIDARR_API_KEY="your-lidarr-api-key"
 | `--workers N` | Parallel metadata fetches during `--repair` (default 4). |
 | `--cookies FILE` | yt-dlp `cookies.txt` for authenticated YouTube (avoids bot-check / rate limits). |
 | `--cookies-from-browser BROWSER` | Load YouTube cookies from a local browser (e.g. `firefox`). |
+| `--remote-components SPEC` | yt-dlp EJS challenge-solver source (default `ejs:github`; needs `deno`). Fixes "signature/n challenge solving failed". `''` to disable. |
 | `--retag-from-path` | Offline: re-tag artist/title from folder + filename (see below). |
 | `-x`, `--exclude NAME` | Folder under `--root` to skip during `--repair`/`--retag-from-path` (repeatable). |
 | `--debug` | Verbose output. |
@@ -163,6 +164,13 @@ throttling. Cookies also apply to normal fetches/downloads. The same can be set
 container via `$YTDLP_COOKIES` / `$YTDLP_COOKIES_FROM_BROWSER`. If you do get flagged, **stop** —
 retrying extends it; wait ~30-60 min (429) or longer for a bot-check.
 
+**JS challenge solving (not a cookie issue).** Separately from auth, YouTube gates media formats
+behind signature / "n" JS challenges. If yt-dlp warns *"Signature solving failed"* / *"n challenge
+solving failed: Some formats may be missing"*, downloads break even with valid cookies. musicfetch
+passes `--remote-components ejs:github` by default so yt-dlp fetches the [EJS](https://github.com/yt-dlp/yt-dlp/wiki/EJS)
+solver and runs it via a local JS runtime — install **[`deno`](https://deno.land)** and use a recent
+yt-dlp. Override with `--remote-components` / `$YTDLP_REMOTE_COMPONENTS` (empty disables).
+
 #### Getting YouTube cookies
 
 > ⚠️ Use a **throwaway / secondary Google account**, not your main one — bulk automated

musicfetch

@@ -61,6 +61,14 @@ COOKIES_FROM_BROWSER = os.environ.get("YTDLP_COOKIES_FROM_BROWSER", "")
 # Tunable via env or --sleep; default 1s mirrors yt-dlp's own `-t sleep` advice.
 SLEEP_REQUESTS = os.environ.get("YTDLP_SLEEP", "1")
 
+# yt-dlp JS-challenge solver. YouTube now gates formats behind signature / "n"
+# challenges that yt-dlp's bundled solver can't reliably crack; without the EJS
+# remote-component solver script it warns "Signature/n challenge solving failed:
+# Some formats may be missing" and downloads break (looks like a cookie problem,
+# isn't). Enabling ejs:github makes yt-dlp fetch the solver and run it via a local
+# JS runtime (deno). Needs a recent yt-dlp + deno present. Set empty to disable.
+REMOTE_COMPONENTS = os.environ.get("YTDLP_REMOTE_COMPONENTS", "ejs:github")
+
 
 def _cookie_args() -> list:
     """yt-dlp cookie flags (file wins over browser); empty when neither is set."""
@@ -86,6 +94,13 @@ def _sleep_args(download: bool = False) -> list:
         args += ["--sleep-interval", str(secs), "--max-sleep-interval", str(secs * 2)]
     return args
 
+
+def _ejs_args() -> list:
+    """yt-dlp EJS challenge-solver flags; empty when REMOTE_COMPONENTS is unset."""
+    if REMOTE_COMPONENTS:
+        return ["--remote-components", REMOTE_COMPONENTS]
+    return []
+
 # Quality choices for --quality.
 QUALITY_CHOICES = ["best", "320", "m4a", "opus", "flac"]
 
@@ -391,7 +406,7 @@ def _ytmusic_search(query: str, limit: int) -> list[Hit]:
 def _ytdlp_search(query: str, limit: int) -> list[Hit]:
     try:
         result = subprocess.run(
-            ["yt-dlp", *_cookie_args(), *_sleep_args(), "--flat-playlist", "-J", f"ytsearch{limit}:{query}"],
+            ["yt-dlp", *_cookie_args(), *_ejs_args(), *_sleep_args(), "--flat-playlist", "-J", f"ytsearch{limit}:{query}"],
             capture_output=True, text=True, check=True,
         )
         data = json.loads(result.stdout)
@@ -663,6 +678,7 @@ def yt_download(url_or_query: str, target_folder: Optional[str], quality: str, d
                 hit: Optional[Hit] = None, outtmpl: Optional[str] = None):
     cmd = ["yt-dlp",
            *_cookie_args(),
+           *_ejs_args(),
            *_sleep_args(download=True),
            *_quality_args(quality),
            "--embed-metadata",
@@ -851,7 +867,7 @@ def probe_url(url: str) -> tuple[str, str, list[Hit]]:
             if hits:
                 return "playlist", title, hits
     try:
-        result = subprocess.run(["yt-dlp", *_cookie_args(), *_sleep_args(), "--flat-playlist", "-J", url],
+        result = subprocess.run(["yt-dlp", *_cookie_args(), *_ejs_args(), *_sleep_args(), "--flat-playlist", "-J", url],
                                 capture_output=True, text=True, check=True)
         data = json.loads(result.stdout)
     except (subprocess.CalledProcessError, json.JSONDecodeError) as e:
@@ -890,7 +906,7 @@ def download_single(url: str, root: str, quality: str, dry_run: bool) -> dict:
 
 
 def run_yt_dlp_get_metadata(url: str, extra_args=None) -> Optional[dict]:
-    cmd = ["yt-dlp", *_cookie_args(), *_sleep_args(), "-j", "--no-playlist", *(extra_args or []), url]
+    cmd = ["yt-dlp", *_cookie_args(), *_ejs_args(), *_sleep_args(), "-j", "--no-playlist", *(extra_args or []), url]
     try:
         result = subprocess.run(cmd, capture_output=True, text=True, check=True)
         return json.loads(result.stdout)
@@ -1338,6 +1354,10 @@ def parse_args():
     p.add_argument("--cookies-from-browser", metavar="BROWSER",
                    help="Load YouTube cookies from a local browser, e.g. firefox or "
                         "chrome. Overrides $YTDLP_COOKIES_FROM_BROWSER.")
+    p.add_argument("--remote-components", metavar="SPEC",
+                   help="yt-dlp EJS challenge-solver source (default ejs:github; "
+                        "needs deno). Fixes 'signature/n challenge solving failed'. "
+                        "Pass '' to disable. Overrides $YTDLP_REMOTE_COMPONENTS.")
     p.add_argument("--retag-from-path", action="store_true",
                    help="Offline: re-tag artist/title from folder + filename "
                         "(fixes tags damaged by a prior --repair).")
@@ -1369,7 +1389,7 @@ def _dispatch_chosen(chosen: Hit, hits: list[Hit], root: str, quality: str,
 
 
 def main():
-    global DEBUG, COOKIES_FILE, COOKIES_FROM_BROWSER, SLEEP_REQUESTS
+    global DEBUG, COOKIES_FILE, COOKIES_FROM_BROWSER, SLEEP_REQUESTS, REMOTE_COMPONENTS
     args = parse_args()
     DEBUG = args.debug
     if args.sleep is not None:
@@ -1378,6 +1398,8 @@ def main():
         COOKIES_FILE = args.cookies
     if args.cookies_from_browser:
         COOKIES_FROM_BROWSER = args.cookies_from_browser
+    if args.remote_components is not None:  # '' disables; None keeps env default
+        REMOTE_COMPONENTS = args.remote_components
     query = " ".join(args.query).strip()
 
     if args.retag_from_path:

tests/test_repair.py

@@ -401,6 +401,28 @@ def test_metadata_fetch_passes_cookies(monkeypatch):
     assert "/cookies.txt" in captured["cmd"]
 
 
+def test_ejs_args_default(monkeypatch):
+    monkeypatch.setattr(mf, "REMOTE_COMPONENTS", "ejs:github")
+    assert mf._ejs_args() == ["--remote-components", "ejs:github"]
+
+
+def test_ejs_args_disabled(monkeypatch):
+    monkeypatch.setattr(mf, "REMOTE_COMPONENTS", "")
+    assert mf._ejs_args() == []
+
+
+def test_metadata_fetch_passes_ejs(monkeypatch):
+    captured = {}
+
+    class _R:
+        stdout = '{"title": "x"}'
+    monkeypatch.setattr(mf, "REMOTE_COMPONENTS", "ejs:github")
+    monkeypatch.setattr(mf.subprocess, "run", lambda cmd, **k: captured.update(cmd=cmd) or _R())
+    mf.run_yt_dlp_get_metadata("http://u")
+    assert "--remote-components" in captured["cmd"]
+    assert "ejs:github" in captured["cmd"]
+
+
 def test_metadata_fetch_logs_stderr(monkeypatch, capsys):
     def boom(cmd, **k):
         raise mf.subprocess.CalledProcessError(