MundoKit

Title & Thumbnail Test Planner for YouTube

Prepare explicit title-thumbnail combinations and review their asset metadata. Export the plan as JSON before setting up the experiment in Studio.

Prepare a local compatibility report. No files are uploaded.

How it works

Each active row is one intended title-thumbnail pair, with two or three rows per plan. Title-only mode shares the first thumbnail and its metadata across all titles; thumbnail-only mode shares the first title across all thumbnails. Redundant controls are hidden and ignored. Exact duplicate pairs are flagged. Local image uploads use SHA-256 identifiers; manually entered identifiers are compared literally.

Titles are trimmed and counted as Unicode code points, against the documented 100-character limit; angle brackets are flagged. Studio remains authoritative for unusual Unicode. Byte sizes and dimensions are exact integers, with no rounding. Local JPG/PNG reading is limited to 30 MB and 40 megapixels per image, one at a time; larger assets can be described manually.

YouTube recommends 16:9 thumbnails and lists a 640-pixel minimum width and a desktop limit under 50 MB. Its experiment help says any asset below 1280 × 720 causes all test thumbnails to be reduced to 854 × 480. This is reported as a warning. The JSON includes issues and metadata, not image files or predicted results.

Worked examples

Two HD pairs

Two distinct pairs with 1280 × 720 thumbnails of 1,000,000 bytes each produce 2 rows and no downscale warning, with the eligibility checklist confirmed.

One smaller thumbnail

If the second thumbnail is 854 × 480, the plan still contains 2 rows but flags the experiment-wide 854 × 480 downscale.

Frequently asked questions

Does this run a YouTube experiment?
No. Set up the reviewed pairs in desktop Studio with advanced features enabled. The tool only prepares a local plan.
Can I test Shorts or private videos?
YouTube excludes Shorts, private, made-for-kids and age-restricted videos, scheduled lives and active Premieres. Live archives and completed Premieres may qualify.
What selects the winner?
YouTube uses watch time. Tests may take up to two weeks; this tool does not predict statistical significance or a winner.
How do single-variable modes share fields?
Title-only uses the shared thumbnail in variant 1 for every title. Thumbnail-only uses the shared title in variant 1 for every thumbnail. Switching back restores the separate fields.
Can I export a plan with warnings?
Yes. Exports retain the issues for review. Correct outstanding errors before configuring Studio.

Sources

Last reviewed 2026-11-14; next review due by 2026-12-14. The method is written out above so you can check our math.