Commit ec82d85 breaks --write-thumbnail.

[Hajitorus]

If I check out commit cfb56d1, --write-thumbnail will correctly record thumbnails, as in:

% ~git/youtube-dl/youtube-dl --write-thumbnail --download-archive=/dev/null KAZrNrcgrRc
[youtube] KAZrNrcgrRc: Downloading webpage
[youtube] KAZrNrcgrRc: Extracting video information
[youtube] KAZrNrcgrRc: Downloading DASH manifest
[youtube] KAZrNrcgrRc: Downloading thumbnail ...
[youtube] KAZrNrcgrRc: Writing thumbnail to: Handmade Hero Day 053 - Environment Elements as Entities-KAZrNrcgrRc.jpg
[download] Handmade Hero Day 053 - Environment Elements as Entities-KAZrNrcgrRc.mp4 has already been downloaded
[ffmpeg] Not converting video file Handmade Hero Day 053 - Environment Elements as Entities-KAZrNrcgrRc.mp4 - already is in target format mp4

However, reverting to any commit more recent than that, (like HEAD) will not write thumbnails:

% ~git/youtube-dl/youtube-dl --write-thumbnail --download-archive=/dev/null KAZrNrcgrRc
[youtube] KAZrNrcgrRc: Downloading webpage
[youtube] KAZrNrcgrRc: Extracting video information
[youtube] KAZrNrcgrRc: Downloading DASH manifest
[download] Handmade Hero Day 053 - Environment Elements as Entities-KAZrNrcgrRc.mp4 has already been downloaded
[ffmpeg] Not converting video file Handmade Hero Day 053 - Environment Elements as Entities-KAZrNrcgrRc.mp4 - already is in target format mp4

This appears to be directly related to the changes made in ec82d85.

[jaimeMF]

Thansk for the report, it will work again in the next version (or if you want, you can just use the HEAD version).

[phihag]

I believe this issue to be fixed in youtube-dl version 2015.01.30.1 and newer. See our FAQ if you need help updating.