Social preview 503s from repository-images CDN on one repo; other repo on same account works

[ChangkeunJ]

🏷️ Discussion Type

Bug

Body

Repository: https://github.com/ChangkeunJ/australian-law-mcp (public, id 1340182458)
Account: ChangkeunJ (Free plan)

Uploading a social preview image to this repository never becomes servable.
The upload is accepted in Settings > General > Social preview and the
repository's og:image meta tag updates to a new
repository-images.githubusercontent.com asset UUID. That URL then returns
HTTP 503 indefinitely. After some hours the custom preview is dropped
entirely and og:image silently reverts to the auto-generated
opengraph.githubassets.com card.

Latest attempt, uploaded 2026-08-20 ~21:50 KST:
https://repository-images.githubusercontent.com/1340182458/bee4545a-c1cf-4c91-8ae3-7e32d575ed3c
HTTP/2 503
server: Varnish
retry-after: 0
content-type: text/html; charset=utf-8

An earlier attempt behaved identically before being dropped:
https://repository-images.githubusercontent.com/1340182458/569eff6b-3cd2-4118-b750-7625e83253a3
HTTP/2 503, server: Varnish

Control - a different public repository on the same account works correctly:
https://github.com/ChangkeunJ/paperpack
https://repository-images.githubusercontent.com/1338110188/5a3ea3a1-ef29-4192-badc-98d86799a114
HTTP/2 200, content-type: image/png, cache-control: max-age=2592000

Ruled out:

• The image. 1280x640 PNG, 8-bit RGBA, non-interlaced, 60 KB, no embedded
  metadata and no ICC profile. Within the documented limits (min 640x320,
  max 1 MB).
• The encoding. PNG with alpha, PNG without alpha, and JPG were each tried;
  all three produce the same 503.
• Browser and session. Retried in a second browser after re-authenticating.
• Platform status. githubstatus.com was green throughout.
• Client caching. The 503 reproduces from curl on a different network and
  via a third-party image proxy, which sees 404.

A 503 from Varnish with content-type text/html looks like an origin or
processing failure rather than image validation, which would fail
synchronously at upload. Could you check whether the social-preview asset
pipeline is stuck for repository id 1340182458?

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[JonghoMoon]

I’m seeing the same issue on another public repository.
Social preview upload is accepted, but the generated repository-images.githubusercontent.com URL either returns 503 Service Unavailable from Varnish or later becomes 404 WebContentNotFound.
I also reproduced an S3 upload preflight failure (OPTIONS → CORS failed) in the browser network log.
Tested with PNG and JPG, including a minimal 1280×640 test image under 5 KB, with the same result.