diff --git a/YTDL_OPTIONS-Cookbook.md b/YTDL_OPTIONS-Cookbook.md
index 7349c41..52f5058 100644
--- a/YTDL_OPTIONS-Cookbook.md
+++ b/YTDL_OPTIONS-Cookbook.md
@@ -9,7 +9,7 @@ Embeds English subtitles and chapter markers (for videos that have them), change
       - 'YTDL_OPTIONS={"writesubtitles":true,"subtitleslangs":["en","-live_chat"],"updatetime":false,"postprocessors":[{"key":"Exec","exec_cmd":"chmod 0664","when":"after_move"},{"key":"FFmpegEmbedSubtitle","already_have_subtitle":false},{"key":"FFmpegMetadata","add_chapters":true}]}'
 ```
 
-### Embed metadata
+### Embed metadata and a cover image in **video** downloads
 
 Contributed by [@PikuZheng](https://github.com/PikuZheng).
 
@@ -20,6 +20,12 @@ NOTE: May cause errors for non-Youtube videos.
       - 'YTDL_OPTIONS={ "ignoreerrors":true, "writethumbnail":true, "postprocessors":[  {"key":"FFmpegMetadata","add_metadata":true,"add_chapters":true,"add_infojson":"if_exists"}, {"key":"EmbedThumbnail"}]}'
 ```
 
+> ⚠️ **Do not apply this to audio downloads.** Picking Audio in the UI already
+> embeds a cover image, and the `writethumbnail` above *switches that off* — see
+> [What MeTube already sets for you](#what-metube-already-sets-for-you). To use
+> this for video only, define it as a named preset via `YTDL_OPTIONS_PRESETS`
+> instead of setting it globally.
+
 ### Automatically marking MeTube downloads with SponsorBlock chapters
 
 Contributed by [@loomweaver](https://github.com/loomweaver).
@@ -89,3 +95,77 @@ To convert the video container instead (e.g. everything as MKV):
 (Note: yt-dlp's option is really spelled `preferedformat`, with one `r`.)
 Define these as named presets via `YTDL_OPTIONS_PRESETS` to pick them
 per-download in the UI instead of applying them globally.
+
+## What MeTube already sets for you
+
+MeTube builds part of the yt-dlp configuration itself from what you pick in the
+UI, then merges your `YTDL_OPTIONS` on top. Options it already handles are best
+left out of `YTDL_OPTIONS` — setting them again is at best redundant, and in one
+case actively harmful.
+
+Picking **Audio** (any format except `wav`) makes MeTube add all of this:
+
+| Option | Purpose |
+|---|---|
+| `writethumbnail` | download the cover image |
+| `FFmpegExtractAudio` | extract to your chosen format and quality |
+| `FFmpegThumbnailsConvertor` | convert the cover to jpg **before** embedding |
+| `FFmpegMetadata` | write tags |
+| `EmbedThumbnail` | embed the cover |
+
+Picking **Video** adds no postprocessors at all, which is why the metadata
+recipe above is useful there and unnecessary for audio.
+
+> ⚠️ **The `writethumbnail` trap.** MeTube adds that audio chain only when
+> `writethumbnail` is *absent* from your options. The intent was to let you turn
+> cover art off with `"writethumbnail": false`, but the check looks at whether the
+> key is present rather than at its value — so setting it to **`true`** switches
+> the chain off just the same:
+>
+> ```
+> Audio + mp3, no YTDL_OPTIONS        -> ExtractAudio, ThumbnailsConvertor, Metadata, EmbedThumbnail
+> Audio + mp3, "writethumbnail": true -> ExtractAudio
+> ```
+>
+> Without `FFmpegThumbnailsConvertor` the cover stays in its original format
+> (webp, from YouTube). Embedding that into an mp3 fails with *"Supported
+> filetypes for thumbnail embedding are: mp3..."*, or leaves the image sitting
+> next to the audio file as a separate `.webp`. If cover art has stopped working
+> on your audio downloads, this is almost always why — remove `writethumbnail`
+> from `YTDL_OPTIONS`.
+
+## Converting a yt-dlp command line into JSON
+
+yt-dlp ships [`devscripts/cli_to_api.py`](https://github.com/yt-dlp/yt-dlp/blob/master/devscripts/cli_to_api.py),
+which turns command-line flags into the API options that `YTDL_OPTIONS` expects.
+It is the right starting point, but **do not paste its output in wholesale** — it
+emits everything, including the keys MeTube manages from the UI. Delete those and
+keep only what remains.
+
+For example, a square-cropped cover image:
+
+```
+yt-dlp -x --audio-format mp3 --embed-thumbnail --convert-thumbnail jpg \
+  --ppa "EmbedThumbnail+ffmpeg_o:-c:v mjpeg -vf crop=\"'if(gt(ih,iw),iw,ih)':'if(gt(iw,ih),ih,iw)'\""
+```
+
+The converter turns that into a block containing `format`, `writethumbnail`,
+`final_ext`, `FFmpegExtractAudio`, `FFmpegThumbnailsConvertor` and
+`EmbedThumbnail` — every one of which MeTube already does when you pick Audio +
+mp3, and one of which (`writethumbnail`) would break it. The only part MeTube
+does not provide is the postprocessor argument, so that is all you need:
+
+```json
+{
+  "postprocessor_args": {
+    "embedthumbnail+ffmpeg_o": [
+      "-c:v", "mjpeg",
+      "-vf", "crop='if(gt(ih,iw),iw,ih)':'if(gt(iw,ih),ih,iw)'"
+    ]
+  }
+}
+```
+
+The `--ppa` prefix becomes the lowercased key `embedthumbnail+ffmpeg_o`, and the
+arguments become a list rather than a single string, which removes the shell
+quoting problem entirely.