The description is plain text; the destination URL is a separate field on the Pin.
Traffic has to go through the bio link or a story sticker — there is no clickable link anywhere in a feed caption.
Captions are not clickable. Traffic has to go through the bio link, and the post needs a video, not an image.
Every URL counts as 23 characters no matter how long it is, because X rewrites it through t.co.
Links are only clickable if the posting tool attaches link facets. Some tools skip that and the URL renders as dead text.
500 characters is the common default, but each server sets its own — some allow far more.
Put the hook in the first two lines — that is all anyone sees before they have to choose to expand.
The character limit is generous, but each subreddit’s own rules on self-promotion are the real constraint.
Limits come from the same table seenpaid’s publisher enforces, so they do not drift from what actually posts. Nothing you type leaves your browser.