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Your X ad has 257 characters, not 280

X states every link costs 23 characters. Every ad has a destination, so copy written to the advertised limit doesn't fit.

X's creative specification is blunt about it: "280 characters. (Note: each link used reduces character count by 23 characters, electing 257 characters for X copy.)"

Every ad this side of a brand-awareness buy has a destination. So 257 is the number, and a variant written to 280 gets rejected or truncated at the point you least want to find out.

Hashtags come out of the same budget

X specifies a hashtag at "21 characters, including the hashtag character". That's counted against your 257 too. Two hashtags and a link leave you about 215 characters of actual sentence.

The headline is a separate, tighter field

The website card title takes 70 characters. X notes that "up to two lines of text are rendered on the card title", with anything beyond truncated by an ellipsis, and suggests keeping it to 50 to be safe across devices.

What this changes about testing

X advises keeping 3 to 5 creative options in rotation rather than one tightly isolated variable. Combined with no minimum campaign spend, the cheap move here is more creative variety rather than a precisely controlled matrix. That's a different philosophy from Meta's, and worth deciding on deliberately instead of importing your Meta habits.

See the X ad specs, quoted with sources.