What blocks access to custom HTTP header (Python+Flask+uwsgi+nginx)?

Question

Nginx

Hey! Tell me, who knows
There is a web application (Python + Flask + uwsgi + nginx)
The client sends the server in each request a session ID in the HTTP_token header.
I'm on the server, in the request handler I'm trying to read this header
token = request.headers.get('HTTP_token')
But there is no such header.
When I run the application locally (without uwsgi and nginx) everything works.
I can't figure out at what stage the headers are filtered and how can I pass my non-standard header through this filter?