Rack 3.1+ is breaking message_bus

[arrowcircle]

Hi! I tried to update my rails app to rack 3.1+ and got the log full of errors like this:

Started POST "/message-bus/3962c9a2-1f6a-4a7b-9958-ee74004764e0/poll" for ::1 at 2025-12-27 22:46:31 +0100
Delivering backlog [#<struct MessageBus::Message global_id=-1, message_id=-1, channel="/__status", data={"/channel" => 0}>] to client 3962c9a2-1f6a-4a7b-9958-ee74004764e0 for user
Started POST "/message-bus/3962c9a2-1f6a-4a7b-9958-ee74004764e0/poll" for ::1 at 2025-12-27 22:46:31 +0100
Delivering backlog [#<struct MessageBus::Message global_id=-1, message_id=-1, channel="/__status", data={"/channel" => 0}>] to client 3962c9a2-1f6a-4a7b-9958-ee74004764e0 for user

With rack < 3.1 I see this in logs and everything works perfectly:

Started POST "/message-bus/801112c9-3472-47d6-9bec-f13b88f5ab42/poll" for ::1 at 2025-12-27 22:45:13 +0100
Delivering messages [{"global_id":-1,"message_id":-1,"channel":"/__status","data":{"/channel":0}}] to client 801112c9-3472-47d6-9bec-f13b88f5ab42 for user  (chunked)

I used ruby client and also tried with python client.

How can I find out what is the problem and fix it?
PS. More curl details

Rack 3.1:

curl -v --no-buffer --http1.1 \
  -H "Content-Type: application/x-www-form-urlencoded" \
  --data "/__status=-1" \
  "http://localhost:3000/message-bus/TEST-CLIENT-ID/poll"
* Host localhost:3000 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:3000...
* Connected to localhost (::1) port 3000
> POST /message-bus/TEST-CLIENT-ID/poll HTTP/1.1
> Host: localhost:3000
> User-Agent: curl/8.7.1
> Accept: */*
> Content-Type: application/x-www-form-urlencoded
> Content-Length: 12
>
* upload completely sent off: 12 bytes
< HTTP/1.1 200 OK
< cache-control: must-revalidate, private, max-age=0
< content-type: application/json; charset=utf-8
< pragma: no-cache
< expires: 0
< access-control-allow-origin: *
< x-request-id: 264c6310-e28b-4648-86ac-91eeb5e4f975
< x-runtime: 0.005298
< server-timing:
< content-length: 79
<
[{"global_id":-1,"message_id":-1,"channel":"/__status","data":{"/__status":0}}]* Connection #0 to host localhost left intact

Rack 3:

curl -v --no-buffer --http1.1 \
  -H "Content-Type: application/x-www-form-urlencoded" \
  --data "/__status=-1" \
  "http://localhost:3000/message-bus/TEST-CLIENT-ID/poll"
* Host localhost:3000 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:3000...
* Connected to localhost (::1) port 3000
> POST /message-bus/TEST-CLIENT-ID/poll HTTP/1.1
> Host: localhost:3000
> User-Agent: curl/8.7.1
> Accept: */*
> Content-Type: application/x-www-form-urlencoded
> Content-Length: 12
>
* upload completely sent off: 12 bytes
< HTTP/1.1 200 OK
< Cache-Control: must-revalidate, private, max-age=0
< Pragma: no-cache
< Expires: 0
< Access-Control-Allow-Origin: *
< Connection: close
< Content-Type: text/plain; charset=utf-8
< Transfer-Encoding: chunked
< X-Content-Type-Options: nosniff
<
[{"global_id":-1,"message_id":-1,"channel":"/__status","data":{"/__status":0}}]
|

and keeps waiting for new data.

PS1. I tracked it down to rack 3.1 release, which deprecated chuned-encoding. https://github.com/rack/rack/blob/main/CHANGELOG.md#310---2024-06-11