In the documentation we can read :
@mentions: a limit of 50 @mentions per Tweet in the hidden region. This is enforced on the server side, so that users cannot exceed this number.
If a user adds more than 50 @mentions, will the Tweet be rejected or will @mention 51 considered as being part of the tweet content?
I personally understand it as the query being rejected.
Anything added in the text region is counted in the Tweet body. It is ONLY if the Tweet call uses the server-side lookup option that @mentions are prepended.
If the number is exceeded i.e. a Tweet with 50 existing @mentions is replied to via the server-side lookup option, then additional @mentions may be ignored or dropped from the body.
Thanks andy, so if we use the lookup options and set 51 @mentions, the the mention number 51 will not be included in the content and the update_status will not fail.
Also I realized that
The REST API endpoints that create new Tweets (statuses/update) will accept a new boolean parameter when a Tweet is sent as a reply to a conversation: auto_populate_reply_metadata (true to enable, false to disable, false being the default). The existing in_reply_to_status_id must also be set.
It means that if we create a new tweet, that is not a direct reply to another one but we do use @mention at the beginning of the Tweet, it will be counted as part of the Content (140 characters).
If this is true, why such decision?
That’s correct. The new behaviour is tied to the idea of Tweets being part of a conversation and the reply-to Tweet being there to lookup the @mentions.