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To see how the 280 char limit feels, I tried to compose a tweet with the full 280 chars. Piping it through wc tells me it's actually 295 chars:

Maybe some chars or chars in links count for less then 1?

I believe links don’t directly count towards the character limit anymore, along with any mentions.

I might be wrong but I believe for a time (maybe still?) only the automatically shortened URL length (t.co URLs) counts

My empirical test shows that links are treated as if they had a fixed length of 23 characters (even if they are shorter or much longer)

No need for an empirical test - they have an endpoint that tells you.