Attach an external website link card (social card preview) to a Bluesky post using app.bsky.embed.external, including an uploaded thumbnail image blob.

Documented steps

1. Fetch the target URL's HTML yourself (client-side) and parse Open Graph meta tags: og:title, og:description, and og:image. Bluesky's servers do not fetch or generate this card metadata for you - the embedding client is responsible for building it.
2. Build the required 'external' object fields: {"uri": "<the target URL>", "title": "<og:title or your own title>", "description": "<og:description or your own description>"}. uri, title, and description are all required (title/description may be empty strings if none found, but the keys must be present).
3. If an og:image was found, download it, then POST $PDSHOST/xrpc/com.atproto.repo.uploadBlob with Content-Type set to the image's MIME type, Authorization: Bearer <accessJwt>, and the raw image bytes as the body.
4. The thumbnail blob is capped at 1,000,000 bytes (1 MB) per the app.bsky.embed.external.external Lexicon schema (maxLength 1000000 on the 'thumb' field) - this is smaller than the 2 MB limit for post images, so resize/compress large thumbnails before upload.
5. Attach the resulting blob object as the 'thumb' field on the external object: {"uri": ..., "title": ..., "description": ..., "thumb": {"$type": "blob", "ref": {"$link": "<cid>"}, "mimeType": "image/png", "size": 23527}}.
6. Set the post record's embed field: {"$type": "app.bsky.embed.external", "external": {...the object from steps 2-5...}}, then createRecord the post as usual with 'text' and 'createdAt'.
7. 'thumb' is optional - you can create a valid external-embed post with only uri/title/description if no image is available or desired.