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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.1.251

Custom thumbnails when publishing an artifact

Use it now
Useful4 Signal3
Artifacts Notable not in their notes

You can publish an artifact with your own light and dark thumbnail images.

thumbnailthumbnailDark
What

Publishing an artifact can now carry your own thumbnail images, one for light and one for dark. They must be PNG or JPG and under a size cap, and if the server refuses the custom image the publish is retried automatically with the auto-generated screenshot and a warning.

Details
  • Thumbnail and thumbnailDark are validated by a new check on both the image link and its content, limited to PNG/JPG with size caps.
  • A server rejection comes back as a distinct thumbnail_rejected reason, which triggers the retry rather than failing the publish.
Evidence

The custom thumbnail was not accepted by the server

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

See this entry in the whole of v2.1.251 →