Fix 1.21.10 spawn_entity and entity_velocity packet structures

[micahstubbs]

Summary

First, thank you to the PrismarineJS maintainers for this incredible project. The mineflayer/minecraft-protocol ecosystem has been invaluable for our work on AI-powered Minecraft bots.

Problem

When connecting a mineflayer bot to a 1.21.10 server, we encountered PartialReadError: Read error for undefined : undefined errors when parsing spawn_entity and entity_velocity packets. The errors occurred at the i16 reader for velocity fields.

Root Cause Investigation

We captured raw packet bytes and manually parsed them:

spawn_entity packet captured:
Total length: 50 bytes
After parsing entityId, UUID, type, x/y/z, pitch/yaw/headPitch, objectData:
  - Bytes consumed: 49
  - Remaining: 1 byte
  - Expected for velocity: 6 bytes (3x i16)

The packets are ~5 bytes shorter than expected, indicating the velocity fields (velocityX, velocityY, velocityZ) are no longer included in 1.21.10.

Changes

1. spawn_entity: Removed velocity fields, added restBuffer to handle remaining data
2. entity_velocity: Same treatment for consistency

Testing

• Tested with actual 1.21.10 server (protocol 773)
• Bot now connects, spawns, and operates without parse errors
• Used raw packet captures to verify the fix matches actual wire format

Note

We noticed PR #1118 has keep_alive at 0x2a, while our testing showed 0x2b works correctly. This might warrant further investigation, but we wanted to keep this PR focused on the structure fixes. Happy to help debug the packet ID mapping further if needed.

Related

• Fix 1.21.10 packet ID mappings for 4 missing debug packets #1118 - Packet ID mapping fixes (builds on this work)
• 🎈 Add Minecraft pc 1.21.10 data #1100 - Main 1.21.10 support PR

[micahstubbs]

Thanks for the feedback @rom1504. I've updated this PR to follow the proper workflow:

1. Created proto.yml for 1.21.10 based on 1.21.8
2. Made the packet structure fixes in the YAML:
   • Removed velocity fields from packet_spawn_entity (not sent in protocol 773)
   • Updated packet_entity_velocity to use restBuffer (format changed)
3. Updated dataPaths.json to point proto to pc/1.21.10
4. Regenerated protocol.json using npm run build in tools/js

The changes are now properly based on YAML source. Let me know if there's anything else that needs adjustment.

[micahstubbs]

Added packet ID mapping fixes:

• Added 5 placeholder packets (0x26-0x2a) to shift keep_alive to 0x2b and login to 0x30
• This matches the actual 1.21.10 protocol (773) behavior observed in testing
• Bot now connects and works without any runtime protocol fix scripts

Tested successfully with a mineflayer bot connecting to a Minecraft 1.21.10 server.

[ethavn]

is it able to open guis without crashing

[richnetdesign]

I believe 1.21.10 is just a minor hot fix release. The major protocol changes happened in 1.21.9

I am trying to fix here
#1120

[micahstubbs]

@chiterl, yes but still has warnings. trying to fix those warnings before marking this as ready for review.

@richnetdesign oh! will check your PR. thanks for pointing that out.

[extremeheat]

Done in #1096