Updated the SKILL.md documentation to include critical steps for generating server reports and validating decompiled source against server data, emphasizing the importance of using server data since MC 1.21.9. Enhanced the diff_registries.py script to support cross-validation with server registries.json, allowing for accurate palette generation. Added new scripts for generating block and entity palettes from server data, ensuring completeness and correctness of entries.
This update improves the workflow for adapting to new Minecraft versions and ensures that palette generation reflects the latest changes in item and block registration.
Made-with: Cursor
MC 1.21.9 changed how some items and blocks are registered — 24 block
items (copper bars/chains/lanterns variants, potted azalea renames) and
additional blocks are now registered outside of Items.java/Blocks.java
field declarations, making the previous source-field-order-based palette
generation produce incorrect protocol IDs.
Regenerated ItemPalette1219.cs using server registries.json (1488 items,
up from 1464) and Palette1219.cs using blocks.json (1166 blocks with
correct state IDs). Added 37 new enum values to ItemType.cs (35 new +
DryShortGrass/DryTallGrass for backward compat) and 9 new values to
Material.cs.
Verified all item/block/entity identification against a 1.21.10 server.
Made-with: Cursor
Minecraft 1.21.9 changed the SpawnEntity (Add Entity) packet layout:
the velocity/movement field was moved before the angle fields and
switched from 3 x Short to a new variable-length LpVec3 encoding.
Add ReadNextLpVec3() to consume the LpVec3 wire format (1 byte header,
optionally 5+ more bytes with a continuation VarInt), and update
ReadNextEntity() to use the new field order for protocol >= 773.
Made-with: Cursor
- Route block/entity/item/packet/metadata palettes to new 1219 variants
for protocol >= 773, and raise upper-bound guards from MC_1_21_7 to
MC_1_21_9 so terrain, inventory, and entity handling are enabled.
- Add DataTypes readers for three new entity metadata serializer types:
CopperGolemState and WeatheringCopperState (both VarInt), and
ResolvableProfile (composite: Either<GameProfile, Partial> with
optional name/UUID/properties + PlayerSkin.Patch with 4 optional
fields for body/cape/elytra texture ResourceLocations and model type).
Made-with: Cursor
- Palette1219.cs: 1053 blocks with new copper chests, copper golem
statues, copper torches, shelves, oxidized lightning rods, iron chain
- EntityPalette1219.cs: 153 entities (+copper_golem at 27, mannequin at 82)
- ItemPalette1219.cs: 1464 items (generated via gen_item_palette.py)
- EntityMetadataPalette1219.cs: 37 serializers (COMPOUND_TAG removed,
+CopperGolemState, WeatheringCopperState, ResolvableProfile)
- PacketPalette1219.cs: updated clientbound IDs for 4 new debug packets
and GameTestHighlightPos, plus config CodeOfConduct/AcceptCodeOfConduct
Also update gen_entity_metadata_palette.py FIELD_TO_ENUM with the three
new serializer type mappings.
Made-with: Cursor
Register protocol 773 for Minecraft 1.21.9 and 1.21.10 (which share the
same protocol as a hotfix release). Update MCHighestVersion to 1.21.10.
Add 49 new item types (copper tools/armor, shelves, copper chests,
copper golem statue variants, oxidized lightning rods, iron chain, etc.),
2 new entity types (CopperGolem, Mannequin), 38 new block materials,
3 new entity metadata serializer types (CopperGolemState,
WeatheringCopperState, ResolvableProfile), and 6 new packet types
(DebugBlockValue, DebugChunkValue, DebugEntityValue, DebugEvent,
GameTestHighlightPos, CodeOfConduct, AcceptCodeOfConduct).
Chain item/block renamed to IronChain in 1.21.9; old enum values
retained for backward compatibility with older palettes.
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1.21.7 and 1.21.8 share protocol 772. The only registry change from
1.21.6 is one new item (music_disc_lava_chicken). All other palettes
(blocks, entities, packets, entity metadata, structured components)
are unchanged and reuse 1.21.6 versions.
Changes:
- Add MC_1_21_7_Version (772) constant
- Add "1.21.7" / "1.21.8" version mappings in ProtocolHandler
- Add MusicDiscLavaChicken to ItemType enum
- Generate ItemPalette1217 (1416 items) for the new item palette
- Update all version upper-bound checks from MC_1_21_6 to MC_1_21_7
- Update MCHighestVersion to "1.21.8"
Made-with: Cursor
The biome PalettedContainer data array length was calculated as
ceil(64 * bitsPerEntry / 64) which is incorrect for non-power-of-2
bit widths. The correct calculation uses SimpleBitStorage's formula:
valuesPerLong = 64 / bitsPerEntry, then ceil(64 / valuesPerLong).
For example, with bitsPerEntry=3: old formula gave 3 longs but the
actual data contains 4 longs (valuesPerLong=21, ceil(64/21)=4).
This bug was masked in 1.21.5 by excess padding bytes in chunk buffers
(due to PalettedContainer.Data.getSerializedSize over-counting). MC
1.21.6 fixed the size calculation server-side, removing the padding
and exposing this pre-existing MCC bug.
Made-with: Cursor
- ItemPalette1216: 1415 items (generated from decompiled Items.java)
- EntityPalette1216: 151 entities (HappyGhast at index 56, all after +1)
- Palette1216: block states (DriedGhast 32 states at 13826-13857, all after +32)
- PacketPalette1216: clientbound +3 (Waypoint, ClearDialog, ShowDialog),
serverbound +2 (ChangeGameMode at 0x04 shifting all after, CustomClickAction at end),
config clientbound +2 (ClearDialog, ShowDialog),
config serverbound +1 (CustomClickAction)
- Version routing in Protocol18.cs, PacketType18Handler.cs, EntityMetadataPalette.cs
updated to select 1216 palettes for protocol >= 771
- EntityMetadataPalette reuses 1215 (EntityDataSerializers unchanged)
Made-with: Cursor
1.21.5 changed LastSeenMessages.Update to include a trailing checksum
byte (0 = skip verification). This affects serverbound chat and signed
chat command packets.
Additionally, the clientbound PlayerChat packet now has a globalIndex
VarInt prepended before the sender UUID.
Without these fixes:
- Sending plain chat messages causes DecoderException on the server
- Receiving player chat messages causes Queue empty crash in MCC
Made-with: Cursor
In 1.21.5, two wire format changes in level chunk packets:
1. Heightmaps changed from NBT CompoundTag to map<VarInt, long[]> encoding
2. PalettedContainer data arrays no longer have VarInt length prefix
(uses writeFixedSizeLongArray instead of writeLongArray)
Both changes affect ChunkData (level_chunk_with_light) packet parsing.
Without this fix, MCC crashes with "Queue empty" when processing chunks.
Made-with: Cursor
Added entries to the .gitignore file to exclude possible debug files related to Minecraft, including language files, input configurations, and backup files.
Made-with: Cursor
Added a new entry to the .gitignore file to exclude the directory for decompiled Minecraft official source code. Also ensured that the .vscode/launch.json file is not ignored.
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The PickItem packet was split into PickItemFromBlock (0x22) and
PickItemFromEntity (0x23) in 1.21.4. Mapping both to the same
PacketTypesOut.PickItem enum caused a duplicate key error in the
reverse mapping. Added PickItemFromEntity enum to resolve this.
Made-with: Cursor
Add complete protocol 769 support for Minecraft 1.21.4:
- Version constants: Add 769 to supported versions, MC_1_21_4_Version constant,
and version string mappings (including 1.21.3 -> 768 compatibility)
- Item palette: 10 new items (Resin series + Eyeblossom), generated ItemPalette1214
- Entity palette: Remove CreakingTransient (149 entities, down from 150)
- Block palette: 10 new blocks with correct blockstate ID ranges from server data
- Packet palette: Serverbound packet ID reshuffling - PickItem split into
PickItemFromBlock/PickItemFromEntity, new PlayerLoaded packet inserted after
PlayerInput, subsequent IDs shifted accordingly. Clientbound unchanged.
- PlayerLoaded: Send empty PlayerLoaded packet after JoinGame processing (>= 1.21.4)
- EntityMetadata/DataComponents: Reuse 1.20.6 palettes (unchanged registries)
- Update all version guard checks from MC_1_21_2 to MC_1_21_4
Made-with: Cursor
Update submodule to commit a045f7e which adds net8.0 target framework,
fixing NETSDK1005 build error in CI when publishing with -f net8.0.
Made-with: Cursor
The previous commits added palette files, packet IDs, and structured components
for MC 1.21.2 (protocol 768), but terrain/inventory/entity features were still
disabled at runtime because the version guards in the constructor checked
> MC_1_21_Version (767) instead of > MC_1_21_2_Version (768).
This commit completes the 1.21.2 adaptation with the following changes:
- Update feature-disable guards from > MC_1_21_Version to > MC_1_21_2_Version
so terrain, inventory, and entity handling are enabled for protocol 768
- Update healthField metadata index guard to > MC_1_21_2_Version
- Handle container ID encoding change: byte -> VarInt for 1.21.2+ in both
clientbound reads (CloseWindow, WindowItems, WindowProperty, SetSlot) and
serverbound sends (ClickWindow, CloseWindow)
- Handle EntityTeleport format change: 1.21.2 uses PositionMoveRotation
(pos + delta + float angles) + relative flags bitmask (int) + onGround
- Handle TimeUpdate format change: 1.21.2 appends a tickDayTime boolean
- Add handlers for new 1.21.2 packets: EntityPositionSync, PlayerRotation,
SetCursorItem, SetPlayerInventory, MoveMinecartAlongTrack, and
RecipeBookAdd/Remove/Settings (ignored, MCC doesn't track recipes)
Tested: successful connection to 1.21.2 vanilla server with inventory,
entity tracking, and chat all working correctly.
Made-with: Cursor
- SpawnEntity packet handler now registers non-player entities via OnSpawnEntity
for protocol >= 1.20.2 (previously only players were tracked, causing 'entity near'
to find nothing)
- PlaceBlock gains lookAtBlock option that sends a position/rotation update before the
block placement packet, fixing containers not opening via useblock
- Enchantment registry IDs are now dynamically parsed from server RegistryData
(minecraft:enchantment), fixing incorrect enchantment name display in 1.21
- AttributeModifiersComponent uses base SubComponent type to avoid InvalidCastException
when parsing 1.21-specific attribute subcomponents
Made-with: Cursor
Add tools/ directory with Python scripts for comparing Minecraft version
registries and generating MCC palette files:
- diff_registries.py: Compare Items/EntityTypes/Blocks/DataComponents/
EntityDataSerializers between two decompiled MC versions, reporting
which palettes need updating with ID shift analysis.
- gen_item_palette.py: Generate ItemPaletteXXX.cs from Items.java field
declaration order, with name validation against ItemType.cs.
- gen_entity_metadata_palette.py: Generate EntityMetadataPaletteXXX.cs
from EntityDataSerializers.java registration order.
- README.md: Usage documentation for all scripts.
Made-with: Cursor
1.20.6 introduced three new EntityDataSerializer types compared to 1.20.4:
- PARTICLES (id 18) - list of particles, inserted after PARTICLE
- WOLF_VARIANT (id 23) - wolf variant holder, inserted after CAT_VARIANT
- ARMADILLO_STATE (id 28) - armadillo state, inserted after SNIFFER_STATE
These insertions shifted subsequent serializer IDs, causing the 1.19.4
palette (EntityMetadataPalette1194) to misidentify metadata types on
1.20.6+ servers. This could lead to incorrect byte consumption and
potential packet parse failures when entities with affected metadata
types (e.g. wolves, armadillos, area effect clouds with particles)
were present.
Changes:
- Add Particles, WolfVariant, ArmadilloState to EntityMetaDataType enum
- Create EntityMetadataPalette1206 with correct 31-entry ID mapping
- Route 1.20.6+ to the new palette in EntityMetadataPalette.GetPalette()
- Add read logic for the three new types in DataTypes.cs
Verified on 1.21.1 vanilla server: cat, wolf, frog, armadillo, painting
entities all spawn without metadata parse errors.
Made-with: Cursor
Fix the Explosion packet handler that was truncating reads at the
knockback fields, leaving BlockInteraction, particles, and SoundEvent
bytes unconsumed for 1.20.4+. The old commented-out code had three bugs:
conditional particle read (should always read both small and large),
reading SoundEvent as a plain string (it's a Holder<SoundEvent> encoded
as VarInt id + optional inline DIRECT_STREAM_CODEC), and an incorrect
fixedRange version gate. Verified against decompiled ClientboundExplodePacket
from both 1.20.6 and 1.21.1 — the wire format is identical across versions.
Update LoadDefaultAttributes() fallback to match the 1.21.1 registry
order (31 attributes), adding 9 new entries: burning_time,
explosion_knockback_resistance, mining_efficiency, movement_efficiency,
oxygen_bonus, sneaking_speed, submerged_mining_speed,
sweeping_damage_ratio, and water_movement_efficiency. This fallback is
only used when the server omits the attribute RegistryData packet.
Made-with: Cursor
- Add AttributeSubComponent121 that uses ResourceLocation(string) instead
of UUID+Name, matching the 1.21 attribute modifier wire format change.
Register it in SubComponentRegistry121 via new ReplaceSubComponent method.
- Add ProjectilePower packet handler: reads 1 double (accelerationPower)
for 1.21+, or 3 doubles (xPower/yPower/zPower) for 1.20.6.
- Add CustomReportDetails and ServerLinks packet handlers in both Play
and Configuration phases, consuming all fields to prevent byte offset
errors on 1.21 servers.
Made-with: Cursor
Add ItemPalette121.cs with item ID mappings for MC 1.21 (protocol 767).
Add three new music disc entries to ItemType enum: MusicDiscCreator,
MusicDiscCreatorMusicBox, and MusicDiscPrecipice, introduced in 1.21.
Made-with: Cursor
Audited batch 5 components (ChargedProjectiles, BundleContents, Container,
WritableBookContent, BlockState, PotDecorations) against official 1.20.6
decompiled STREAM_CODEC definitions. All network encodings were correct.
Removed redundant NumberOfItems/NumberOfPages/NumberOfProperties fields from
ContainerComponent, WritableBlookContentComponent, BlockStateComponent, and
PotDecorationsComponent. Serialize now uses the actual collection .Count
instead of a potentially stale cached value, matching the pattern already
used by ContainerComponent's Serialize and other components.
Also modernized loop style (foreach with deconstruction where applicable)
and fixed a typo in an exception message ("setialize" -> "serialize").
Made-with: Cursor
In 1.20.6+, ComponentSerialization.STREAM_CODEC uses
ByteBufCodecs.fromCodecWithRegistries (NBT tag format), not plain string.
The previous implementation incorrectly used ReadNextString/GetString
for custom_name (5), item_name (6), and lore (7) components.
Fixed all three to use ReadNextNbt/GetNbt, preserving raw NBT data for
round-trip serialization while still extracting readable text via
ChatParser.ParseText(Dictionary).
Other batch 4 components (custom_data, entity_data, bucket_entity_data,
block_entity_data, debug_stick_state, map_decorations, recipes, lock,
container_loot, intangible_projectile — all NBT; hide_additional_tooltip,
hide_tooltip, fire_resistant, creative_slot_lock — all Unit/Empty;
note_block_sound — ResourceLocation string) were verified correct.
Made-with: Cursor
Audited all 14 simple binary components (batch 3): max_stack_size,
max_damage, damage, unbreakable, rarity, custom_model_data, repair_cost,
enchantment_glint_override, ominous_bottle_amplifier, dyed_color,
map_color, map_id, map_post_processing, base_color.
Found and fixed 1 bug:
- EnchantmentGlintOverrideComponent: was reading/writing VarInt but the
official STREAM_CODEC uses ByteBufCodecs.BOOL (single byte boolean).
Changed property type from int to bool, Parse from ReadNextVarInt to
ReadNextBool, and Serialize from GetVarInt to GetBool.
All other 13 components matched the official 1.20.6 STREAM_CODEC
definitions exactly.
Verified in-game: connected to 1.20.6 vanilla server, received items
with enchantment_glint_override=true/false, dyed_color, map_color,
map_id, base_color, unbreakable, rarity, custom_model_data, repair_cost,
damage, max_damage. All parsed and serialized correctly with no errors.
Made-with: Cursor
Audited all 8 batch-2 components (enchantments, stored_enchantments,
can_place_on, can_break, lodestone_tracker, firework_explosion, fireworks,
banner_patterns, suspicious_stew_effects, bees) against official 1.20.6
decompiled STREAM_CODEC definitions.
Found and fixed 3 bugs in BlockPredicate/PropertySubComponent:
1. BlockPredicateSubcomponent.Serialize(): missing HasNbt bool write.
Parse reads the bool but Serialize skipped writing it, causing all
subsequent fields to be offset by one byte.
2. BlockPredicateSubcomponent.Serialize(): missing Properties list count
VarInt write. Parse reads VarInt count before iterating, but Serialize
only wrote the elements without the preceding count.
3. PropertySubComponent: RangedMatcher min/max values must use Optional
encoding (Bool prefix + conditional String), matching the official
ByteBufCodecs.either(ExactMatcher, RangedMatcher) where RangedMatcher
uses ByteBufCodecs.optional(STRING_UTF8) for both min and max fields.
Previously read/wrote plain Strings unconditionally.
Remaining 6 components (enchantments, stored_enchantments, lodestone_tracker,
firework_explosion, fireworks, banner_patterns, suspicious_stew_effects, bees)
verified correct — no changes needed.
Made-with: Cursor
Audited all 8 high-complexity structured components against official 1.20.6
decompiled STREAM_CODEC definitions. Found and fixed bugs in 3 components
plus a systemic NBT serialization issue:
TrimComponent (ID 35):
- Serialize had TrimPatternType and ShowInTooltip incorrectly nested inside
the TrimMaterialType==0 branch; moved them outside to match Parse logic
- Description fields (TrimMaterial.description, TrimPattern.description) were
read/written as String but official codec uses ComponentSerialization
(NBT Tag format); changed to ReadNextNbt/GetNbt
ProfileComponent (ID 46):
- Serialize was missing the HasUniqueId Bool prefix before UUID
- Serialize only wrote properties when count > 0 but omitted the VarInt count
prefix entirely when empty; now always writes VarInt count
WrittenBookContentComponent (ID 34):
- Page content uses Filterable<Component> where Component is NBT-encoded via
ComponentSerialization.STREAM_CODEC, not plain String; changed Parse to use
ReadNextNbt and Serialize to use GetNbt
- Added RawContentNbt/FilteredContentNbt fields to BookPage record for
round-trip NBT preservation
- Removed unnecessary ChatParser.ParseText on title (it's a plain string)
DataTypes.GetNbt:
- Added TAG_String root support for 1.20.4+ (chat components like "Page 1"
are encoded as TAG_String, not TAG_Compound)
- Fixed root name handling: versions >= 1.20.2 omit the root compound name,
but GetNbt was unconditionally writing it
Components confirmed correct (no changes needed):
- FoodComponentComponent (ID 20), ToolComponent (ID 22),
InstrumentComponent (ID 40), PotionContentsComponent (ID 31),
AttributeModifiersComponent (ID 12)
Made-with: Cursor
EnchantmentsComponent (used by both regular and stored enchantments)
was directly casting the registry VarInt ID to the Enchantments enum
via (Enchantments)id. However, the Enchantments enum is ordered
alphabetically (AquaAffinity=0, BaneOfArthropods=1, ..., Sharpness=32)
while the 1.20.6 registry uses a completely different order
(protection=0, fire_protection=1, ..., sharpness=13). This caused all
enchantment names to display incorrectly (e.g. Sharpness V shown as
"Unknown Enchantment with ID: 32").
Changes:
- Parse now uses EnchantmentMapping.GetEnchantmentByRegistryId1206()
to properly map registry IDs to enum values via the existing
1.20.6+ mapping table
- Serialize now uses EnchantmentMapping.GetRegistryId1206ByEnchantment()
to convert enum values back to registry IDs (reverse lookup)
- Fixed translation key prefix: "Enchantments.minecraft." (wrong) ->
"enchantment.minecraft." (matches en_us.json resource keys)
- Fixed 3 long-standing typos in the Enchantments enum that prevented
translation lookup from matching resource keys:
- DepthStrieder -> DepthStrider (depth_strieder vs depth_strider)
- Efficency -> Efficiency (efficency vs efficiency)
- Loyality -> Loyalty (loyality vs loyalty)
Verified on vanilla 1.20.6 server: items with sharpness, efficiency,
unbreaking, fortune, mending, and bane_of_arthropods all display
correct localized names (锋利, 效率, 耐久, 时运, 经验修补, 节肢杀手).
Made-with: Cursor
Both components had incorrect Parse/Serialize implementations that would
cause packet deserialization misalignment when encountered in-game.
PotionContentsComponent (3 bugs):
- Serialize unconditionally wrote VarInt(PotionId) and Int(CustomColor)
even when HasPotionId/HasCustomColor was false. The official format
(PotionContents.STREAM_CODEC) uses Optional encoding: Bool(hasValue)
followed by the value only when true. The extra bytes caused all
subsequent fields in the packet to be read at wrong offsets.
- Serialize omitted the VarInt(count) prefix for the custom effects list.
The official codec uses ByteBufCodecs.list() which always writes a
VarInt count header before the list elements.
- Also fixed typo: PotiononId -> PotionId.
InstrumentComponent (3 bugs):
- The official Instrument.STREAM_CODEC uses ByteBufCodecs.holder() which
encodes as VarInt(holderId): 0 = inline data, N>0 = registry ref (N-1).
The SoundEvent field inside uses the same holder pattern. The old code
unconditionally read SoundName (ResourceLocation) and HasFixedRange/
FixedRange even when SoundEventHolderId != 0 (registry reference case
has no inline data).
- UseDuration was read/written as Float, but the official codec uses
ByteBufCodecs.VAR_INT. This caused a 4-byte vs variable-length
mismatch that would shift all subsequent data.
- HasFixedRange was read unconditionally when SoundEventHolderId == 0,
but FixedRange was also read unconditionally. The official SoundEvent
DIRECT_STREAM_CODEC uses Optional<Float> encoding: Bool(hasValue)
followed by Float only when true.
These components are used for potion items and goat horns respectively.
Verified against official 1.20.6 decompiled source:
- net.minecraft.world.item.alchemy.PotionContents (STREAM_CODEC)
- net.minecraft.world.item.Instrument (STREAM_CODEC/DIRECT_STREAM_CODEC)
- net.minecraft.sounds.SoundEvent (STREAM_CODEC/DIRECT_STREAM_CODEC)
- net.minecraft.network.codec.ByteBufCodecs (holder/optional/list)
Made-with: Cursor
Audited all 58 StructuredComponent subclasses against the official Minecraft
1.20.6 decompiled source to verify Parse()/Serialize() symmetry. Found and
fixed four bugs across four components:
1. ContainerComponent: Parse() skipped null item slots (empty slots in a
container) but Serialize() looped NumberOfItems times using Items[i],
causing IndexOutOfRangeException when any slot was empty. The official
ItemContainerContents uses OPTIONAL_STREAM_CODEC which serializes empty
slots as VarInt(0). Fixed: Parse now stores all slots including nulls,
Serialize uses Items.Count and iterates all entries. GetItemSlot(null)
correctly writes VarInt(0) for empty slots.
2. ChargedProjectilesComponent: Used Items.OfType<Item>() in Serialize()
which silently dropped null entries, causing the serialized count to
differ from the written VarInt header. The official ChargedProjectiles
uses STREAM_CODEC (non-optional, no empty slots allowed). Fixed: Items
list is now List<Item> (non-nullable), Parse defensively skips nulls,
Serialize writes Items.Count matching the actual list.
3. BundleContentsComponent: Same issue as ChargedProjectilesComponent.
Applied the same fix pattern.
4. FoodComponentComponent: Two type mismatches vs the official
FoodProperties.DIRECT_STREAM_CODEC:
- Saturation was declared as bool and read with ReadNextBool (1 byte),
but the protocol sends it as float (4 bytes). This caused all
subsequent fields in the component to be read at wrong offsets,
corrupting CanAlwaysEat, SecondsToEat, and the effects list.
- NumberOfEffects was serialized with GetFloat() instead of GetVarInt(),
writing 4 bytes of IEEE 754 float instead of a variable-length integer.
Fixed both Parse and Serialize to use correct types.
Also removed redundant NumberOfItems/NumberOfEffects fields from components
where the count is derivable from the list length, and replaced
ArgumentNullException with cleaner patterns.
Tested end-to-end on vanilla 1.20.6 server: item receiving (diamond_sword,
golden_apple, diamond_pickaxe), inventory slot movement (click to pick up
and place), and inventory listing all work correctly with no server-side
protocol errors.
Made-with: Cursor
After the previous commit (99ac3d0) moved attribute lookup from a hardcoded
dictionary to the dynamic RegistryData, MCC would crash immediately upon
joining a vanilla 1.20.6 server with:
System.ArgumentException: An item with the same key has already been added.
Key: unknown
Root cause: When KnownDataPacks negotiation tells the server that MCC already
has the "minecraft" data pack, the server skips sending RegistryData for
registries it considers "known" — including minecraft:attribute. This left
the dynamic attribute map empty, so every VarInt attribute ID resolved to
"unknown". The EntityProperties packet often contains multiple attributes
(e.g. armor, max_health, movement_speed), and `keys.Add("unknown", ...)` on
the second "unknown" attribute threw ArgumentException.
Two fixes applied:
1. World.GetAttributeNameById(): When the dynamic attribute map is empty
(server didn't send the registry), automatically load the vanilla 1.20.6
default attribute order (22 entries matching Attributes.java registration
order). This mirrors the pattern used for dimensions where defaults are
loaded when RegistryData is not sent. If a modded server sends a custom
attribute registry, the dynamic map takes precedence.
2. Protocol18.cs EntityProperties handler: Change `keys.Add(propertyKey,
propertyValue2)` to `keys[propertyKey] = propertyValue2` to tolerate
duplicate keys defensively, in case an unknown attribute ID still appears.
Tested: MCC now connects to a vanilla 1.20.6 offline-mode server, stays
online for 6+ minutes with no crashes or disconnections. Verified: chat
messages received, inventory listing (item names/counts correct), entity
detection, TPS query, and health query all work correctly.
Made-with: Cursor
