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What it checks

countGraphemes(item.altText) <= 2000 for every media item that supplies altText.

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Why

Bluesky’s app.bsky.embed.images#image schema’s alt field is documented at 2000 graphemes. Same Unicode segmentation as the post-text rule.

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Failure response

{
  "error": {
    "code": "preflight_failed",
    "rule": "bluesky.media.alt_text_max_graphemes",
    "platform": "bluesky",
    "message": "Alt text is 2400 graphemes; Bluesky allows at most 2000.",
    "remediation": "Shorten alt text to 2000 graphemes or fewer."
  }
}

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Remediation

Tighten the description. Alt text is for accessibility — it should describe the image’s content concisely, not act as a hidden caption.