Repository navigation
Slime block piston elevator no longer launches players upward in Paper 26.3 #14327
Description
Expected behavior
When a slime block is pushed upward by a piston underneath a player, the player should be launched/bounced upward.
This behavior worked correctly on Paper 26.2.
Observed/Actual behavior
After upgrading the server to Paper 26.3, the piston and slime block still move normally and the player is physically pushed by the moving slime block, but the player is no longer launched upward.
The upward bounce/automatic jump effect does not occur.
Steps/models to reproduce
Start a Paper 26.3 server.
Place a piston facing upward.
Place a slime block on top of the piston.
Stand a player on or directly above the slime block.
Activate the piston.
Observe that the player is pushed by the slime block but is not launched upward.
The exact same contraption works correctly on Paper 26.2.
Plugin and Datapack List
Plugin and Datapack List: None
Paper version
This server is running Paper version 26.3-140-main@977da0d (2026-09-29T16:11:19Z) (Implementing API version 26.3.build.140-beta)
You are 1 version(s) behind
Download the new version at: https://papermc.io/downloads/paper
Previous version: 26.2-129-9240f58 (MC: 26.2)
Other
Relevant Paper configuration currently uses:
Piston duplication/headless piston settings do not appear to affect the issue.
The regression occurred specifically when upgrading from Paper 26.2 to Paper 26.3.
The important detail is that player collision with the moving slime block still works, but the vertical bounce/launch velocity is missing.
Activity
Doc94 commentedon Oct 2, 2026
I test in a clean paper server and cannot notice difference using piston+slime block
pc1412 commentedon Oct 2, 2026
Additional information:
I forgot to include the plugin and datapack lists in the original report.
I retested the issue with my current setup, and it is still reproducible.
Plugins:
Datapacks:
There are no additional custom datapacks installed.
The piston and slime block work normally and push the player, but the upward launch/bounce still does not occur.
RVSkeLe commentedon Oct 2, 2026
Can you share a spark report so that we can check your config values? Run
/spark healthreport --uploadand share the link pleasepc1412 commentedon Oct 2, 2026
Here is the requested spark health report:
https://spark.lucko.me/cJQQKs4yVy
The upload briefly triggered spark's 5-second network timeout warning, but the report completed and uploaded successfully.
pc1412 commentedon Oct 2, 2026
pc1412 commentedon Oct 2, 2026
I retested this issue on Paper 26.3 build 143.
The slime block piston elevator is now working correctly again.
The player is launched upward as expected.
Plugins:
Datapacks:
The issue appears to be resolved in build 143.
[05:03:49 INFO]: Checking version, please wait...
[05:03:49 INFO]: This server is running Paper version 26.3-143-main@ff3655a (2026-10-02T14:13:32Z) (Implementing API version 26.3.build.143-beta)
You are running the latest version
Previous version: 26.3-142-1c92a6c (MC: 26.3)
pc1412 commentedon Oct 3, 2026
Update:
The issue has reproduced again.
After updating to build 143, the slime-block piston elevator initially appeared to work correctly, so I thought the issue had been resolved.
However, after leaving the server running idle for about 6 hours and then testing the same elevator again, the problem returned.
The piston and slime block still move normally and the player is pushed, but the player is no longer launched upward by the moving slime block.
So the issue appears to be intermittent or dependent on server uptime/state rather than fully resolved.
I can reproduce it again after the server has been running for several hours.