generate-youtube-thumbnail

name: generate-youtube-thumbnail description: >- Generate high-CTR YouTube thumbnails using Nano Banana 2 via the KIE.ai API. Handles public URL reference images, character likeness alignment, proven CTR-tested prompt formulas, and parallel batch generation. Use when the user asks to create a YouTube thumbnail, video thumbnail, A/B test thumbnail variations, or refers to thumbnail design with their face, brand assets, or product photos.

Generate YouTube Thumbnail

A reusable workflow for creating YouTube thumbnails via KIE.ai's Nano Banana 2 image model with proper character likeness and proven CTR formulas.

When to use this skill

Trigger on phrases like:

• "make me a YouTube thumbnail"
• "create a thumbnail for this video"
• "I need thumbnail variations / A/B tests"
• "remake this thumbnail with my face"
• "generate 10 thumbnail concepts"
• "thumbnail with [me / my product / my brand]"

Read order

1. This file — workflow, decision tree, batch generation
2. shared/skills/generate-youtube-thumbnail/prompting/guide.md — likeness alignment, expressions cheat sheet, prompt structure (shared across all generative-AI APIs in this portfolio)
3. shared/skills/generate-youtube-thumbnail/prompting/formulas.md — 5 proven thumbnail formulas with templates (shared)
4. scripts/generate-batch.sh — KIE-specific batch script (URL-based references, jobs endpoint)

Prerequisites

• .env with KIE_API_KEY
• Reference images hosted at PUBLIC URLs before the batch runs. KIE has no presigned upload flow — reference images in input.image_input must be reachable URLs (your own CDN/bucket, an image host, GitHub raw, etc.). The batch script does NOT upload for you.
• Suggested reference categories (the script reads URLs from arrays you fill in):
  • face/ — 5+ photos of the subject (headshot + 3/4 angles + close-ups + expressions)
  • logos/ — brand logos
  • products/ — clean product shots
  • examples/ — real ad screenshots, comparison material
  • style/ — example thumbnails the user wants to match aesthetically

If references are missing or the user pastes images in chat instead of providing URLs, stop and ask the user for either (a) public URLs for each reference, or (b) file paths they can upload to a host themselves. Chat paste is not a URL.

Workflow

1. Gather requirements (in order)

Ask the user for any missing context, but only what you actually need:

1. Concept — what's the video about? Single concept, A/B variations, or specific recreation of an existing thumbnail style?
2. Subject — who is in the thumbnail (the user themselves, an AI character, no person)?
3. Brand assets — which logos / products / brand colors should appear?
4. Text — what should the title text say? Will text be baked in, or added in post (Canva/Photoshop)?
5. Comparison material — for "real vs AI" thumbnails, what real ad and what AI-generated ad?

2. Verify reference URLs are reachable

for url in "${REF_URLS[@]}"; do
  curl -I -s -o /dev/null -w "%{http_code} %{url_effective}\n" "$url"
done

Each URL must return 200. If any return 4xx/5xx, ask the user to re-host or share a new URL. Do not proceed with text-only descriptions for brand-specific items (logos, branded products, branded apparel) — you'll get generic AI approximations that don't match the brand. Generic descriptions are OK for backgrounds, expressions, and clothing.

3. Estimate cost and confirm

Always present cost as an estimate before firing:

"Estimated cost: N variations × {per-image rate from MASTER_CONTEXT.md} = X credits. Confirm exact pricing in kie.ai/logs after the run."

4. Pick a formula

See shared/skills/generate-youtube-thumbnail/prompting/formulas.md for the 5 proven formulas. Match the user's intent:

5. Compose prompts

Follow the template in shared/skills/generate-youtube-thumbnail/prompting/guide.md:

YouTube thumbnail, 16:9 landscape.
[SUBJECT — likeness block + clothing + framing + "no hands" if applicable]
Expression: [specific expression from expressions cheat sheet]
[LEFT visual element + reference]
[RIGHT visual element + reference]
Across the top in massive bold yellow block letters with thick black outline reads [TITLE].
Background: [color + glow]
Style: [aesthetic notes]
Avoid: distorted face, extra fingers, hands visible, blurry logos, generic face

Always include the CRITICAL CHARACTER LIKENESS block when the subject is a real person. See shared/skills/generate-youtube-thumbnail/prompting/guide.md.

6. Generate (use the batch script)

Copy scripts/generate-batch.sh to a new versioned script (scripts/generate-thumbnails-vN.sh) and modify:

1. Update COMMON_REF_URLS array with your public reference URLs (one per reference image)
2. Replace the PROMPTS array entries with your composed prompts
3. Run with bash scripts/generate-thumbnails-vN.sh > output/run.log 2>&1 &
4. Monitor with tail -F output/run.log | grep -E "DONE|FAILED|Task"

The script handles:

• Optional local Lanczos upsample (if user supplies local files they plan to host)
• Parallel firing of POST /api/v1/jobs/createTask with model: "nano-banana-2"
• Retry on failure with backoff
• Polling GET /api/v1/jobs/recordInfo?taskId=... until state: success or fail
• Parsing data.resultJson for result URLs and downloading
• Session folder organization under output/thumbnails-<timestamp>/

7. Review and present

After all generations complete, read each thumbnail with the Read tool and present:

• Brief verdict per thumbnail (likeness, readability, emotional impact)
• Top 3 picks ranked by CTR potential
• Specific reasons for the picks (which expression, which color contrast, which formula)
• Offer next-step refinements (different expression, background color, copy variation)

8. Mandatory disclosures

• Always label credit totals as estimates and direct the user to kie.ai/logs for exact charges
• Cost data: Record Nano Banana 2 per-image credits in MASTER_CONTEXT.md from first-run logs; confirm on the KIE marketplace page
• Generation time: typically 20-60 seconds per image on Nano Banana 2
• Parallel budget: KIE supports 100+ concurrent tasks with a 20 req/10s rate limit — 10 parallel is comfortable; add stagger/backoff for larger fan-outs

Quirks and pitfalls

Reference images must be PUBLIC URLs

KIE does not have a presigned-upload endpoint. Every entry in input.image_input is a URL string that KIE's fetchers must be able to reach. If you use a private bucket, either generate a temporary signed URL yourself and pass that, or use a public host.

Image preprocessing

Small or low-quality reference images produce weak likenesses. Before hosting, upsample with Lanczos to ~1080px longest side and flatten to RGB JPEG. The batch script includes an optional prepare_image() helper if you're hosting from this machine.

input.image_input is an array of plain URL strings

Not objects. Send ["https://.../a.jpg", "https://.../b.jpg"]. Up to 14 URLs supported per task.

Chat-pasted images are NOT URLs

If the user pastes an image directly in chat, you cannot pass it to the API. Ask them to upload the file to a host and share the URL.

Likeness drift without enough references

With 1-2 face references the AI generalizes to "generic bearded man with glasses." With 5+ face references from different angles it locks in the specific person. Always use 5+ face references for character work.

macOS bash 3.2

Default macOS bash doesn't support declare -A (associative arrays). The batch script uses indexed arrays and jq / python3 for JSON work.

Model variants on KIE

KIE offers multiple Nano Banana variants via the jobs endpoint:

• nano-banana-2 (default — current generation)
• nano-banana-pro (Gemini 3 Pro backbone, higher fidelity)
• nano-banana (legacy)
• nano-banana-edit (edit-focused)

The batch script defaults to nano-banana-2 and exposes MODEL as an env override so you can swap to nano-banana-pro for higher-stakes hero shots.

Brand-specific items need real reference URLs

Text descriptions of brand-specific items (logos, branded apparel, custom merchandise) will produce generic approximations. For pixel-accurate brand reproduction, host the actual brand asset on a public URL and pass it as a reference.

Cost reference

Record per-call credit costs in MASTER_CONTEXT.md after the first real run by checking kie.ai/logs. Typical patterns:

Always present as estimates, confirm exact in kie.ai/logs.

See also

• shared/skills/generate-youtube-thumbnail/prompting/guide.md — likeness alignment, expressions, prompt structure (shared across APIs)
• shared/skills/generate-youtube-thumbnail/prompting/formulas.md — 5 proven CTR formulas with prompt templates (shared)
• scripts/generate-batch.sh — KIE-specific bash batch generator (URL refs, jobs endpoint)
• kie-external-api skill — underlying API reference for Nano Banana 2 and the KIE jobs endpoint