Block Conditions (block_condition_type)

A block condition checks one block position in the world and returns true or false. Which level and which position get checked is decided by the definition that declares the condition, so a condition only ever describes what the block at that position has to look like.

Block conditions are a Java (built-in) registry, so type can only be one of the ids listed below, written with the mxt namespace. A data pack cannot add entries to this registry, and it cannot remove them either. Adding a custom type means writing Java, or going through the KubeJS bridge — see the KubeJS API.

Common Structure

A condition is a JSON object: type names the built-in type and every remaining key is a field of that type.

{
  "type": "mxt:block_tag",
  "tag": "minecraft:logs"
}

Conditions usually appear nested as a value inside other definitions, for example under a field such as block_condition:

"block_condition": {
  "type": "mxt:hardness",
  "comparison": ">=",
  "compare_to": 3.0
}

Anywhere a block condition is accepted, an array is accepted too. The array is shorthand for mxt:and and passes only when every entry passes:

"block_condition": [
  { "type": "mxt:movement_blocking" },
  { "type": "mxt:height", "comparison": "<", "compare_to": 64 }
]

An entry the array cannot decode is dropped and leaves one Ignoring invalid list element line in the log; the other entries are still evaluated normally. An empty array is legal as well, and mxt:and with an empty array is always true.

Comparison Fields

A group of types compares a value against a number. comparison (the operator) and compare_to (the number compared against) are always two separate keys written directly on the condition object, as in the examples above. The operators are ==, !=, <, <=, > and >=, and compare_to is always a plain number.

Datapack Visual Editor

The Datapack Visual Editor lists a type's fields interactively, which is handy for checking field names without scrolling through the tables on this page.

Meta Conditions

This group does not look at a block at all. They either assemble other block conditions or answer with a constant.

mxt:always

Always true. Takes no fields.

{ "type": "mxt:always" }

mxt:never

Always false. Takes no fields.

{ "type": "mxt:never" }

mxt:js

Hands the test to a block condition handler registered through the KubeJS bridge. The script side receives the Level, the BlockPos, params and the evaluation context when it is registered.

Field	Type	Default	Description
id	String	Required	The id the handler was registered under.
params	JSON object	Empty object	Passed to the handler as-is.

{
  "type": "mxt:js",
  "id": "example:my_check",
  "params": { "limit": 3 }
}

mxt:and

Passes only when every nested condition passes.

Field	Type	Default	Description
conditions	Block condition array	Required	The nested conditions to check one by one.

{
  "type": "mxt:and",
  "conditions": [
    { "type": "mxt:block_tag", "tag": "minecraft:logs" },
    { "type": "mxt:height", "comparison": ">", "compare_to": 60 }
  ]
}

mxt:or

Passes when at least one nested condition passes.

Field	Type	Default	Description
conditions	Block condition array	Required	The nested conditions to check one by one.

{
  "type": "mxt:or",
  "conditions": [
    { "type": "mxt:block_id", "block": "minecraft:water" },
    { "type": "mxt:block_id", "block": "minecraft:lava" }
  ]
}

An empty array is always false.

mxt:not

Inverts the result of the nested condition.

Field	Type	Default	Description
condition	Block condition	Required	The nested condition to invert.

{
  "type": "mxt:not",
  "condition": { "type": "mxt:block_tag", "tag": "minecraft:logs" }
}

mxt:chance

Passes randomly with a probability.

Field	Type	Default	Description
chance	Number	Required	The probability of passing, from 0 to 1.

{ "type": "mxt:chance", "chance": 0.25 }

A value outside 0..1 is refused at load time. The test is "a random number is less than chance", so even 1 is not a guaranteed pass; only 0 is a guaranteed failure.

Block Conditions

This group actually looks at the block or the environment at that position.

mxt:block_id

Matches the block at the position.

Field	Type	Default	Description
block	Block id	Required	The block to compare against.

{ "type": "mxt:block_id", "block": "minecraft:stone" }

Only one concrete block id is accepted here; a # tag does nothing, so use mxt:block_tag to match by tag.

mxt:block_tag

Matches the block at the position against a block tag. Both vanilla tags and tags added by data packs work.

Field	Type	Default	Description
tag	Block tag	Required	The tag id to compare against.

{ "type": "mxt:block_tag", "tag": "minecraft:logs" }

mxt:biome_tag

Matches the biome at the position against a biome tag.

Field	Type	Default	Description
tag	Biome tag	Required	The tag id to compare against.

{ "type": "mxt:biome_tag", "tag": "minecraft:is_forest" }

mxt:aura_range

Tests the aura concentration at the position against a per-aura requirement.

Field	Type	Default	Description
aura	Map from aura id to requirement object	Required	Keys are aura ids, values are { "min"?, "max" }.

{
  "type": "mxt:aura_range",
  "aura": {
    "example:azure_aura": { "min": 20, "max": 200 }
  }
}

In a requirement object max is required and min is optional (default 0), and both accept a number provider. The condition passes only when every listed aura's concentration falls inside that aura's own range. The server resolves the concentration at the position being evaluated.

Entries in the map whose key or value cannot be decoded are dropped with a log line rather than treated as an error.

mxt:offset

Tests a nested condition at a relative offset.

Field	Type	Default	Description
condition	Block condition	Required	The condition to test at the offset position.
x	Integer	0	Offset along X.
y	Integer	0	Offset along Y.
z	Integer	0	Offset along Z.

{
  "type": "mxt:offset",
  "condition": { "type": "mxt:block_tag", "tag": "minecraft:logs" },
  "y": -1
}

mxt:hardness

Compares the block's hardness.

Field	Type	Default	Description
comparison	Operator	Required	==, !=, <, <=, > or >=.
compare_to	Number	Required	The hardness to compare against.

{ "type": "mxt:hardness", "comparison": ">=", "compare_to": 3.0 }

mxt:height

Compares the Y coordinate of the position.

Field	Type	Default	Description
comparison	Operator	Required	==, !=, <, <=, > or >=.
compare_to	Number	Required	The Y coordinate to compare against.

{ "type": "mxt:height", "comparison": "<", "compare_to": 64 }

mxt:light_level

Compares the light level at the position.

Field	Type	Default	Description
light_type	String	none	Write a vanilla light layer name, for example block or sky, to compare one layer only.
comparison	Operator	Required	==, !=, <, <=, > or >=.
compare_to	Number	Required	The light level to compare against.

{ "type": "mxt:light_level", "light_type": "block", "comparison": "<=", "compare_to": 7 }

With no light_type the comparison uses the maximum raw brightness at the position. An unrecognised light layer name is refused at load time.

mxt:slipperiness

Compares the block's slipperiness.

Field	Type	Default	Description
comparison	Operator	Required	==, !=, <, <=, > or >=.
compare_to	Number	Required	The slipperiness to compare against.

{ "type": "mxt:slipperiness", "comparison": ">", "compare_to": 0.6 }

mxt:blast_resistance

Compares the block's blast resistance.

Field	Type	Default	Description
comparison	Operator	Required	==, !=, <, <=, > or >=.
compare_to	Number	Required	The blast resistance to compare against.

{ "type": "mxt:blast_resistance", "comparison": ">=", "compare_to": 10.0 }

This reads the block's base resistance value, with no explosion context attached.

mxt:movement_blocking

Passes when the block blocks motion and has a non-empty collision shape. Takes no fields.

{ "type": "mxt:movement_blocking" }

Both conditions have to hold at once: having a collision shape is not enough on its own, since cobwebs and bamboo saplings have one yet do not block motion.

mxt:adjacent

Counts the neighbouring blocks that match a nested condition and compares that count against a number.

Field	Type	Default	Description
adjacent_condition	Block condition	Required	The nested condition that filters the neighbouring blocks.
comparison	Operator	Required	==, !=, <, <=, > or >=.
compare_to	Number	Required	The number of neighbours to compare against.

{
  "type": "mxt:adjacent",
  "adjacent_condition": { "type": "mxt:block_tag", "tag": "minecraft:logs" },
  "comparison": ">=",
  "compare_to": 3
}

Each of the six directions counts once and adds one on a hit, so the count tops out at 6. Only positions in loaded chunks take part; an unloaded side is simply not counted, and it is not an error either.