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What it checks

byteLength <= 1_000_000 per image item.

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Why

Bluesky’s PDS blob upload caps at ~976 KB (1,000,000 bytes — Bluesky uses base-10, not base-2). Larger blobs are rejected with BlobTooLarge. The 976 KB number is the “post” you want; the constant is the byte limit you should compare to.

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Failure response

{
  "error": {
    "code": "preflight_failed",
    "rule": "bluesky.media.image_size_max",
    "platform": "bluesky",
    "message": "Image is 1450000 bytes; Bluesky allows at most 1000000.",
    "remediation": "Re-encode the image under 1000000 bytes (Bluesky's blob limit is ~976 KB)."
  }
}

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Remediation

Re-encode at lower quality or smaller dimensions. For JPEGs, quality 80–85 + max-dimension 2048 px is usually enough. 
# ImageMagick
magick input.jpg -quality 82 -resize 2048x2048\> output.jpg

# sharp (Node)
await sharp(input).resize({ width: 2048, withoutEnlargement: true }).jpeg({ quality: 82 }).toFile("output.jpg");