Large and medium sized magma cube spawn in less than 3x3x2 areas

Not sure if related, but "large slimes" will spawn in a 2-block high area. When that happens they appear stuck in the floor and can't seem to move until attacked enough to break into medium slimes.

I will add further information to this bug, it has to do something with what the magma cubes are checking for while spawning, they are ignoring spawning rules based on surrounding blocks (therefore they can spawn in a 1x1 spot with other blocks around them) but you can block their spawns to their correct 3x2.5x3 spawning requirement with armor stands, or lava. You should be able to block their spawns with almost any other block in those spaces, such as buttons, signs, trap doors, or other solid blocks.

I believe that spiders are also effected by this bug, as you can block off a 2x2 spawning location, and if you have a valid 1x1 spot, the spiders will still spawn. here is a screen shot of a design I was attempting, 3x3 platforms with the middle block covered by a button, and also I put signs in the gaps, the spiders seem to spawn on the outer edges of the blocks, partially inside the signs.

Spider spawning is not affected by this bug because the code that allows magacubes to spawn in small areas is specific to magma cubes.

Buttons only stop spider spawning if the spider's hitbox overlaps the button's collision box. In Prowl's far design, the buttons are not positioned in a way to stop all spider spawns.

Back to this bug report. Magma Cubes are one of the monsters that use their own function to check for block collisions when they spawn. They do not use Monster::checkSpawnRules() that Skeletons, Creepers, Spiders, etc. use. In fact, LavaSlime::checkSpawnRules() only checks for collisions with liquid blocks or if lava slime is obstructed by existing entities.

I don't know if this is WAI, but large Magma Cubes spawning in 2x2 tunnels is really annoying for players building through basalt deltas.

One solution to this bug is for LavaSlime::checkSpawnRules() to also call Monster::checkSpawnRules() so the standard block checks are done in addition to the Magma Cube specific ones.

Please note that if Monster::checkSpawnRules() is used, then LavaSlime::isDarkEnoughToSpawn() will also need to be defined to always return TRUE, otherwise magma cubes will not spawn in high light levels.

Large Magma Cube spawning in a 2 high space...either magma blocks are not full blocks and/or Magma Cubes have too small of hit box.

Tested it is the hit box as I have had magma cubes spawn in 2 high space between sea lantern.

Affects 1.16.100, or is this working as intended?

Any word on if this has been fixed with regular slimes? This is certainly a thing that also applies to the regular ones, you can find large ones stuck into the floor in 2 tall areas!

This doesn't seem to be fixed. I have a large 2-tall area dug out in the Nether, and large magma cubes still spawn there all the time. Happy to post screencaps if that helps, but I haven't seen any change in this behavior before/after the release of 1.17.30.

I can confirm from testing that it's not fully fixed in 1.17.41. Magma cubes can still spawn on platforms with 2-block high ceilings (large magma cubes spawn in the floor, or clip through the floor and fall down if the floor is only one block thick). They also spawn in 2-block wide areas with slabs on opposing sides.

Code diggers have confirmed for me that the magma cube spawn code was in fact updated in 1.17.30 so that magma cubes use the same block collision check function as other mobs.

The problem now is in the magma_cube.json entity file. The collision box for each size magma cube is given in the component group for that size. However, the spawning function does not check component groups. It only checks the mob data based on the main component section. In that section magma cubes do not have any collision box assigned, so the game uses a hidden default that lets them spawn in areas >= 1.8 high.

The bug can be fixed by adding the following component to the main "components" section of the magma_cube.json.

"minecraft:collision_box": {
        "width": 2.08,
        "height": 2.08
      },

Please add the tag "vanilla-parity" to this issue. In Java Edition, slimes and magma cubes always check for the space of a large slime/magma cube, so they never spawn in 2-high areas.

Reply from @unknown:

This report does not meet the requirements for the vanilla-parity label. See https://feedback.minecraft.net/hc/en-us/articles/360015877192-Parity-Requests-and-You-A-Guide

Is this being addressed? I’ve spent hours mob proofing my nether hub and highways only to find I’m still having to deal with magma cubes. This makes my ice highway useless. I am on Bedrock 1.19.22 Xbox edition.