Spawner blocks and minecarts will unsuccessfully attempt to summon hostile entities when the world difficulty is set to peaceful
Steps to Reproduce:
Place down/summon one of the following:
/summon spawner_minecart ~ ~ ~ {SpawnData:{entity:{id:"minecraft:slime"}},SpawnPotentials:[{weight:1,data:{entity:{id:"minecraft:slime"}}}]}
/give @p spawner{BlockEntityTag:{SpawnData:{entity:{id:"minecraft:slime"}},SpawnPotentials:[{weight:1,data:{entity:{id:"minecraft:slime"}}}]}} 1
Expected & Observed Results:
❌ - The spawner will display cloud particles and a slime will appear for a brief moment, then despawn.
✔ - The spawner would not try and summon entities, as it cannot.
Screenshots/Videos:
[media]
Notes:
This does not affect trial spawners, which are only active in gamemodes above peaceful.
Linked issues
is duplicated by
relates to
Attachments
Comments

Can confirm in 1.20.4 and 24w03b. I second @unknown's suggestion.
Along with my confirmation, I've made a new description and title to better describe the issue.
I still think it makes the most sense to resolve MC-266949 and MC-257309 into this, but until then I would just suggest a 'caused by' relation.
Title:
Spawners unsuccessfully try to summon hostile entities on peaceful difficulty
Description:
Encase the commands in the steps to reproduce in a code block after copy + pasting)
Spawner blocks and minecarts will unsuccessfully attempt to summon hostile entities when the world difficulty is set to peaceful
h3. Steps to Reproduce:
# Place down/summon one of the following:
/summon spawner_minecart ~ ~ ~ {SpawnData:{entity:{id:"minecraft:slime"}},SpawnPotentials:[{weight:1,data:{entity:{id:"minecraft:slime"}}}]}
/give @p spawner{BlockEntityTag:{SpawnData:{entity:{id:"minecraft:slime"}},SpawnPotentials:[{weight:1,data:{entity:{id:"minecraft:slime"}}}]}} 1
h3. Expected & Observed Results:
(x) - The spawner will display cloud particles and a slime will appear for a brief moment, then despawn.
(/) - The spawner would not try and summon entities, as it cannot.
h3. Screenshots/Videos:
[^SpawnerSummonPeaceful.mp4]
h3. Notes:
# This does not affect trial spawners, which are only active in gamemodes above peaceful.
# Relates to:
MC-65065 MC-110427 MC-779 MC-32110
# Causes:
MC-266949 MC-257309
This is essentially MC-266949 and MC-257309, but for all hostile mobs.
These 3 reports should all be merged into 1.