Download stable paper version by default

[pgcudahy]

Describe the problem

Hi, I'm not sure if this is a bug or if I'm not understanding how this should work, but the experimental version of paper keeps getting downloaded. The latest stable version at https://papermc.io/downloads/paper is 1.21.8 but my docker logs keep showing it downloading 1.21.9-pre2-7

[init] Running as uid=1000 gid=1000 with /data as 'drwxrwxrwx 14 1000 1000 32 Sep 22 13:10 /data'
[init] Image info: buildtime=2025-09-21T14:11:02.982Z,version=java21,revision=d1da61d0deef83a1e04ddc7e0cd15e8fd01d0fbf
[init] Resolving type given PAPER
[mc-image-helper] 13:10:49.019 INFO  : Downloaded /data/paper-1.21.9-pre2-7.jar

I usually don't have PAPER_CHANNEL set, but I don't see any change with PAPER_CHANNEL=experimental or PAPER_CHANNEL=default.

I'm not sure if this is the issue, but it seems like they're using channel names of "ALPHA" and "STABLE"

curl -s https://fill.papermc.io/v3/projects/paper/versions/1.21.9-pre2/builds | jq -r '.. | .channel? // empty' | sort -u
ALPHA

while

curl -s https://fill.papermc.io/v3/projects/paper/versions/1.21.8/builds  |  jq -r '.. | .channel? // empty' | sort -u
STABLE

Container definition

services:
  minecraft:
    container_name: minecraft
    image: itzg/minecraft-server
    ports:
      - 25565:25565
      - 19132:19132/udp
    environment:
      - EULA=TRUE
      - INIT_MEMORY=1G
      - MAX_MEMORY=2G
      - TYPE=PAPER
      - PAPER_CHANNEL=default
      - TZ=Africa/Johannesburg
      - ENABLE_ROLLING_LOGS=true
      - >
        PLUGINS=https://download.geysermc.org/v2/projects/geyser/versions/latest/builds/latest/downloads/spigot,
        https://download.geysermc.org/v2/projects/floodgate/versions/latest/builds/latest/downloads/spigot,
        https://github.com/ViaVersion/ViaVersion/releases/download/5.4.2/ViaVersion-5.4.2.jar,
        https://github.com/ViaVersion/ViaBackwards/releases/download/5.4.2/ViaBackwards-5.4.2.jar
    tty: true
    stdin_open: true
    volumes:
      - /mnt/media/minecraft/data:/data
    restart: always

Container logs

[init] Running as uid=1000 gid=1000 with /data as 'drwxrwxrwx 14 1000 1000 32 Sep 22 13:20 /data'
[init] Image info: buildtime=2025-09-21T14:11:02.982Z,version=java21,revision=d1da61d0deef83a1e04ddc7e0cd15e8fd01d0fbf
[init] Resolving type given PAPER
[mc-image-helper] 13:20:30.176 INFO  : Downloaded /data/paper-1.21.9-pre2-7.jar
[mc-image-helper] 13:20:33.047 INFO  : The file /data/plugins/Geyser-Spigot.jar is already up to date
[mc-image-helper] 13:20:33.047 INFO  : The file /data/plugins/floodgate-spigot.jar is already up to date
[mc-image-helper] 13:20:33.167 INFO  : The file /data/plugins/ViaVersion-5.4.2.jar is already up to date
[mc-image-helper] 13:20:33.337 INFO  : The file /data/plugins/ViaBackwards-5.4.2.jar is already up to date
[init] Copying any plugins from /plugins to /data/plugins
[init] Copying any configs from /config to /data/config

[itzg]

Thank you for your investigation but neither of those are used to first identify the latest stable Minecraft version. It has to use

https://fill.papermc.io/v3/projects/paper/versions

and that API response doesn't identify the channel for each version so can be used to differentiate the "pre" versions. I can't even safely look at the version name since "pre" was a newly introduced convention and I don't know if they'll change it.

It would be great if you could report an issue with their API project. I don't have time to chase down third party issues.

[pgcudahy]

Oof, you're right. Thanks for helping me understand the issue. I'll follow it up there.

[itzg]

I'm going to reopen this and use to track changes needed if/when the channel field is introduced.

@pgcudahy did you get a chance to contact them? No worries if not and I might post an issue.

[pgcudahy]

Sorry, I haven't had a chance to contact them and I'm busy on a work trip at the moment

[itzg]

Thanks. I just wanted to check before I contacted them.

[LapinFou]

I encountered the same issue. As a workaround, I added the following environment variable:
VERSION=1.21.8
This way, the server will always update to the latest 1.21.8 release (build 60 at the time of writing).