Mirror a LinkedIn post to Bluesky and Threads

The X→Bluesky migration left a lot of people wanting “post once, mirror everywhere.” This workflow does it the clean way — driven by Posta’s outbound webhooks, not a leaky cron poll. When a LinkedIn post goes live, Posta sends a signed post.published webhook; the workflow verifies it, fetches the caption, and uses an LLM to compress it into a Bluesky and a Threads draft for you to review.

How it works

1. Posta webhook — receives post.published (point a Posta outbound webhook scoped to LinkedIn at this node’s URL).
2. Verify signature (Code) — checks HMAC-SHA256(secret, "{X-Posta-Timestamp}.{rawBody}") in constant time and stops on mismatch.
3. Get LinkedIn post — fetches the published post’s caption from the Posta API.
4. Compress per network (LLM) — returns a Bluesky variant (≤300 chars) and a Threads variant, links preserved, LinkedIn opener dropped.
5. Create Bluesky / Threads drafts (Posta) — created as drafts so nothing publishes without your say-so.
6. Slack review ping — posts both Posta dashboard links so you can review and publish from one place.

Verifying the webhook

Posta signs each delivery as HMAC-SHA256(secret, "{timestamp}.{body}") and sends X-Posta-Signature, X-Posta-Timestamp, and X-Posta-Event headers. The Code node enables the Webhook’s Raw Body option and verifies the bytes that were signed — never a re-serialized copy.

Tips

• Set the Bluesky and Threads Posta account ids on the two Create-post nodes (grab them from the Get many social accounts Posta node).
• Want it fully hands-off? Drop the “draft” intent so the mirrored posts publish immediately instead of waiting for review.
• Add more targets by duplicating a Create-post node and pointing it at another account id.