How to parse JSON request body in Sinatra just once and expose it to all routes?

Use a sinatra before handler:

before do
  request.body.rewind
  @request_payload = JSON.parse request.body.read
end

this will expose it to the current request handler. If you want it exposed to all handlers, put it in a superclass and extend that class in your handlers.

You can also use Rack Middleware to parse it. See https://github.com/rack/rack-contrib Just use Rack::PostBodyContentTypeParser when initializing your Sinatra class.

Like this working for sinatra 1.4.5

before do
  if request.body.size > 0
    request.body.rewind
    @params = ActiveSupport::JSON.decode(request.body.read)
  end
end