YouTube Thumbnail Brief

This skill does not generate an image. It writes the brief a human designer (or an image tool) executes, grounded in what actually drives the click in 2026, and documents how a custom thumbnail gets attached when you auto-publish.

When to use

• User says "what should the thumbnail be for this video"
• User wants a brief to hand to a designer or paste into an image generator
• User wants 2 to 3 thumbnail concepts to A/B test
• User is auto-publishing and needs the thumbnail upload steps

What the brief contains

For each concept the skill outputs:

• Concept name and the angle it sells (and how it complements the title).
• Focal subject: a face plus the specific emotion, or the object/result.
• Text overlay: the exact words (4 or fewer), and where they sit. Never the same words as the title.
• Background and contrast: color, lighting, and how the subject separates.
• Composition: rule-of-thirds placement, gaze direction, crop.
• 120-pixel test note: what must still read when shrunk to mobile size.

Steps

1. Gather inputs. The title (so the thumbnail complements, never repeats it), the video's payoff and emotional tone, whether there is a face available, the single most visual moment or result, and long-form vs Short. If a URL is given, parse it with lib/url_parser.py.
2. Split the load with the title. Decide what the title already says and make the thumbnail carry the rest (the face/emotion, the result, the object). List the split explicitly.
3. Write 2 to 3 concepts. Vary one big element between them (face vs object, text vs no text, color), not five small things, so the A/B test is readable. Each concept fills the six fields above.
4. Apply the non-negotiables. One focal point, a legible real emotion, high contrast, 4 words or fewer of text, and it must read at 120 px. Flag any concept that risks failing the shrink test.
5. Recommend the lead concept for YouTube's Test & Compare and say why.
6. Hand off the brief. The user gives it to a designer or an image tool. This skill writes words, not pixels.
7. If auto-publishing, give the upload flow (see below). Otherwise the user sets the thumbnail in YouTube Studio.

The thumbnail flow (auto-publish)

A custom thumbnail cannot be set on create-post because the thumbnail upload needs a postGroupId first. The order is:

1. Create the video post as a draft (create-post, no scheduledTime) -> returns the postGroupId.
2. Upload the thumbnail image out of band. Publora requires the thumbnail to be a tracked media asset from its dedicated YouTube thumbnail endpoint (JPEG or PNG, max 2 MB, 1280x720 recommended). That upload is not wired into this bundle: do it in the Publora dashboard (or via the dedicated endpoint once available). The generic get-upload-url media flow is not accepted for thumbnails. The upload returns a mediaId and url.
3. Attach it: call update-post with platformSettings.youtube.thumbnail = {mediaId, url} (the lib.PubloraClient.set_thumbnail(post_group_id, media_id=, url=) helper wraps this attach step). This is the part the bundle automates.
4. Schedule the post.

So this skill outputs the thumbnail brief plus the attach instructions, not a one-call thumbnail upload. If you do not have a tracked {mediaId, url} yet, ship the video without the custom thumbnail and set it in YouTube Studio.

Constraints: the channel must be verified; the apply is best-effort (a rejected thumbnail does not block the video, it just ships without the custom one); and you cannot change the thumbnail while the post is processing.

Hard rules

Global voice rules: see root SKILL.md Voice rules. Additional skill-specific rules:

• Text overlay is 4 words or fewer, and never repeats the title's words.
• One focal point per concept. Two competing subjects halve the click.
• The emotion on a face must match the video. A shocked face on a calm tutorial is a broken promise the retention graph punishes.
• Every concept must pass the 120-pixel mobile read test.
• Always produce 2 to 3 concepts, each varying one big element.

Anti-patterns (skill will refuse)

• Repeating the full title as the thumbnail text.
• A face with a fake exaggerated expression that the video does not earn.
• A busy collage with no single focal point.
• Tiny text or full sentences that vanish at 120 px.
• A thumbnail showing a moment that is not in the video.
• Punctuation in overlay text, em dashes included (a 3-word overlay is read at thumbnail size; it carries words, not clauses).

Resources

• ../../references/thumbnail-principles.md - the full design language and the upload flow
• ../../references/algorithm-heuristics.md - packaging, CTR, Test & Compare
• references/brief-template.md - the fill-in brief a designer or image tool can execute

Related skills

• yt-title-optimizer - the other half of the click; build them as a pair
• yt-hook-scripter - the spoken open should match the thumbnail's emotion
• yt-content-planner - pair titles and thumbnails across a week