YouTube thumbnail design with specific dimensions, contrast rules, and mobile preview optimization. Covers safe zones, text placement, face expression psychology, and A/B testing.
SkillScore96Maintained
Automatic
◆Marketing & Content pack
Last verified 8/21/2026
What it changes
Passed 24 of 25 test cases with the skill, and 13 of 25 without it.
Without skill52%
With skill96%
Checks it passed that the model alone missed
Executes belt app run falai/flux-dev-lora
Output uses belt app run falai/flux-dev-lora
Executes belt app run bytedance/seedream-4-5
and 10 more in the full report
11 fixed13 passed either way1 still failing0 broke
Pass-rate lift
+44 pp
Tokens
+94%
Agent turns
0%
Response time
-43%
Model gemini-3.6-flashMeasured 8/21/2026v124 of 25 cases passed with skill
Tokens, turns and time describe resource use, not a dollar price.
See it in action
One of 11 test cases the skill fixed. Same model, same prompt, with and without the skill.
1 of 11
Prompt
When running concurrent image generation requests via belt CLI to create multiple A/B test thumbnail options without waiting for each job to complete, what specific CLI flag must be included?
What was checked, without and with the skill
Checked
Without skill
With skill
Includes the --no-wait flag in the CLI command
Missed
Passed
case-17 · gemini-3.6-flash · 8/21/2026 · answers are capped and redacted
What's inside
The instructions your model reads when the skill loads.
SKILL.md246 linessingle file
Install the belt CLI skill:npx skills add belt-sh/cli
YouTube Thumbnail Design
Create high-CTR YouTube thumbnails with AI image generation via inference.sh CLI.
belt login
# Generate a thumbnail
belt app run falai/flux-dev-lora --input '{
"prompt": "YouTube thumbnail style, close-up of a person with surprised excited expression looking at a glowing laptop screen, vibrant blue and orange color scheme, dramatic studio lighting, shallow depth of field, high contrast, cinematic",
"width": 1280,
"height": 720
}'
Specifications
Spec
Value
Dimensions
1280 x 720 px (minimum)
Recommended
1920 x 1080 px
Aspect ratio
16:9
Max file size
2 MB
Formats
JPG, GIF, PNG
The 120px Test
Your thumbnail appears at roughly 120px wide on mobile — that's how most viewers first see it.
This skill provides comprehensive patterns for using Zod validation library in TypeScript applications. It ensures input validation is done correctly, securely, and consistently across the codebase.