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What it checks

For every media item that carries altText: countGraphemes(altText) <= 1000. Counted with Intl.Segmenter, so a compound emoji (👨‍👩‍👧‍👦) is one grapheme — same counter the platform uses.

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Why

X’s v1.1 metadata endpoint (/1.1/media/metadata/create) caps alt text at 1000 characters. Beyond that the metadata write fails — and because alt-text writes are best-effort by design (they live on a separate deprecation track from /2/tweets), a silent failure means the tweet posts without accessibility metadata you intended to attach. Preflight makes that explicit.

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Failure response

{
  "error": {
    "code": "preflight_failed",
    "rule": "twitter.media.alt_text_max_graphemes",
    "platform": "twitter",
    "message": "Alt text is 1432 graphemes; X allows at most 1000.",
    "remediation": "Shorten alt text to 1000 graphemes or fewer."
  }
}

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Remediation

Trim the alt text. If you need more context, put it in the tweet body — alt text is for screen readers, the body is for everyone.