Server variable SPAWN_MONSTERS does not work with MC version 25w35a and beyond

[ThimoNeubauer]

Describe the problem

I have a small minecraft world that I play in survival and SPAWN_MONSTERS false with my kids. Recently, we started the server again and were pretty surprised when suddenly a monster spawned! Daddy saved the world, kids are fine but afterwards I went digging. It turns out that the minecraft release 25w35a has an important change:

Removed server property spawn-monsters in favour of new game rule

Thus, the existing logic to turn the SPAWN_MONSTERS environment variable into a value in server.properties doesn't work anymore.

I now use the rcon startup as a workaround:

environment:
      RCON_CMDS_STARTUP:  |-
        gamerule minecraft:spawn_monsters false

which shows a change in the log:

[mc] | [18:30:58] [Server thread/INFO]: RCON running on 0.0.0.0:25575
[mc] | [Rcon loop] MCServer is listening, running startup
[mc] | [Rcon loop] running - gamerule minecraft:spawn_monsters false
[mc] | [18:31:01] [RCON Listener #1/INFO]: Thread RCON Client /0:0:0:0:0:0:0:1 started
[mc] | [18:31:01] [Server thread/INFO]: [Rcon: Gamerule spawn_monsters is now set to: false]
[mc] | [18:31:01] [RCON Client /0:0:0:0:0:0:0:1 #2/INFO]: Thread RCON Client /0:0:0:0:0:0:0:1 shutting down
[mc] | [Rcon loop] Gamerule spawn_monsters is now set to: false
[mc] | [Rcon loop] No addition rcon commands are given, stopping rcon cmd service

Now I'll need some time alone to double-check in-game that the monsters are indeed gone ;-)

Container definition

# https://docker-minecraft-server.readthedocs.io/
services:
  mc:
    image: docker.io/itzg/minecraft-server
    environment:
      EULA: "true"
      # TYPE: "PAPER"
      SPAWN_MONSTERS: "false"
    ports:
      - "25565:25565"
    volumes:
      - /containers/macht.org/minecraft:/data

# follow the advice in https://github.com/itzg/mc-image-helper/issues/498 to configure IPv6
# as described here: https://docs.docker.com/engine/daemon/ipv6/
networks:
   ip6net:
     enable_ipv6: true
     ipam:
       config:
         - subnet: 2001:db8::/64

Container logs

Original log showed that server.properties is generated:

[mc] | [init] Running as uid=1000 gid=1000 with /data as 'drwxr-xr-x 12 1000 1000 4096 Jan 18 13:14 /data'
[mc] | [init] Image info: buildtime=2026-01-13T02:57:01.149Z,version=java25,revision=0a98bee40ea9b86500ea7b6611784902a553803f
[mc] | [init] Starting RCON commands
[mc] | [init] Rcon cmds functionality enabled
[mc] | [init] Resolving type given VANILLA
[mc] | [init] Resolved version given LATEST into 1.21.11
[mc] | [init] Copying any configs from /config to /data/config
[mc] | [mc-image-helper] 18:33:19.482 INFO  : Created/updated 1 property in /data/server.properties
[mc] | [init] Setting initial memory to 1G and max to 1G
[mc] | [init] Starting the Minecraft server...
[mc] | Starting net.minecraft.server.Main

In-game we saw monsters, though...

[itzg]

Thanks for noting this. Without spending numerous days of implementation effort to enhance the image, there's nothing I can do to address this since the image has support all possible versions. It is unfortunately an exercise left for the user to determine what properties are applicable.

As for the game rule, the article says it is spawnMonsters and not minecraft:spawn_monsters

[itzg]

...hmm, the logs report the game rule was accepted. I suggest asking in the official Minecraft discord about this. Perhaps being a snapshot it is temporarily broken and that's the risk of running snapshots.

[ThimoNeubauer]

My point is that the documented server variable SPAWN_MONSTERS does NOT work as advertised. The log snipped above with the accepted game rule is the version where I use RCON_CMDS_STARTUP instead. Sorry, I could have phrased that more clearly...

[ThimoNeubauer]

... and to add more: I'm not using snapshots, we play the regular releases. With the copper golems they apparently also introduced a breaking change in server configuration

[ThimoNeubauer]

Minimal solution would be to remove SPAWN_MONSTERS from this table. A better solution could be an added comment that newer versions need a different setting.

[itzg]

Minimal solution would be to remove SPAWN_MONSTERS from this table. A better solution could be an added comment that newer versions need a different setting.

That's impractical for me. I am just one person working on this project in my spare time. Mojang has an entire division dedicated to creating, defining, and removing (apparently...they didn't use to) server properties. The environment variables that map to server properties are just a convenience I provide and the docs attempt to direct the reader to the official community managed wiki page for server properties.

I have to support Minecraft versions all over the place down to ones like 1.12.2 and older. As such I would have to document the applicability of each and every property. I don't even plan Minecraft anymore, so I don't have the ongoing exposure to these changes and can't monitor their blogs for all the changes.

The best I can do with what little time I have is put a disclaimer that users must consult the server properties for applicability of fill descriptions.

Even though you said "and beyond", you mentioned a snapshot version in the issue title. So my apologies for misconstruing this as a snapshot only version scenario. I overstepped on my feedback regarding that.

[itzg]

Thanks for pointing out the docs link. I have now removed all server properties from that variables table since I can see how that caused confusion about image supportability. That page now only lists variables directly used by the image.