The biggest difference is that MessagePack extensibility (which is not yet widely implemented) is based upon binary blobs, whereas Transit defines extensions in terms of other Transit types. Also, Transit can reach the browser via JSON. And Transit has caching...

Okay, thanks for edifying. As someone else said (to the creators) it would be nice to see a "why I would use this" blurb.

MessagePack implementations in JavaScript get trounced by JSON for read/write performance and JavaScript is a pretty important part of the puzzle for many people building systems these days. Transit on the other hand can best JSON on more recent JS engines and also in a bind I'd rather debug Transit verbose JSON output than MessagePack :)

I wouldn't say that it's a reinvention of MessagePack. Indeed, Transit uses MessagePack at the bottom (when specified) to provide a level of extensibility and richer types.

The reasons why it's better or why it's not solving the same problem should probably be listed on that page. There is an oblique reference to MessagePack but no clear comparison with it.