Minecraft Datapack Skill

Inspect pack.mcmeta and the target Minecraft version before editing. Preserve an existing target unless migration is requested; metadata numbers alone do not make older command or registry schemas compatible. Check the edited files with the bundled validator and test in-game when available.

Skill Scope

Routing Boundaries

• Use when: the deliverable is datapack files (pack.mcmeta, data/...) and .mcfunction/JSON content.
• Do not use when: the request is command-only snippets not tied to a datapack file tree (minecraft-commands-scripting).
• Do not use when: the request requires loader APIs, Java code, or runtime mod behavior (minecraft-modding).

---

Pack Metadata

Minecraft Version	Preferred pack metadata
1.21 / 1.21.1	pack_format: 48
1.21.2 / 1.21.3	pack_format: 57
1.21.4	pack_format: 61
1.21.5	pack_format: 71
1.21.6	pack_format: 80
1.21.7 / 1.21.8	pack_format: 81
1.21.9 / 1.21.10	min_format: [88, 0], max_format: [88, 0]
1.21.11	min_format: [94, 1], max_format: [94, 1]
26.1	min_format: [101, 1], max_format: [101, 1]
26.2	min_format: [107, 1], max_format: [107, 1]

Use pack_format for a legacy-only target through data pack format 81. Starting with data pack format 82 in 1.21.9, define both explicit min_format and max_format values. A range that includes a legacy format below 82 must also retain pack_format and supported_formats; do not include supported_formats for a modern-only range. For a legacy-compatible range, supported_formats may be one integer, a two-integer inclusive range, or an object with integer min_inclusive and max_inclusive fields. For exact patch targeting, use [major, minor] arrays for both min_format and max_format, including .0 versions such as [88, 0]. A single integer is equivalent to [major, 0] for min_format, while a single integer in max_format allows any minor version on that major line. Do not write decimal JSON numbers such as 94.1.

Keep pack.mcmeta exact for the patch you target instead of trying to span the multiple Minecraft releases with one metadata block.

---

Directory Layout

my-datapack/
├── pack.mcmeta
└── data/
    ├── <namespace>/           ← use your pack's name (e.g., mypack)
    │   ├── function/
    │   │   ├── main.mcfunction
    │   │   └── tick.mcfunction
    │   ├── advancement/
    │   │   └── custom_advancement.json
    │   ├── recipe/
    │   │   └── custom_recipe.json
    │   ├── loot_table/
    │   │   └── custom_loot.json
    │   ├── predicate/
    │   │   └── is_night.json
    │   ├── item_modifier/
    │   │   └── add_name.json
    │   └── tags/
    │       ├── block/
    │       │   └── climbable.json
    │       ├── entity_type/
    │       │   └── bosses.json
    │       └── function/
    │           └── custom_flow.json  ← manually invoked tag
    └── minecraft/
        └── tags/function/
            ├── load.json             ← engine tag; runs on /reload
            └── tick.json             ← engine tag; runs every game tick

---

pack.mcmeta

1.21.8 and earlier

{
  "pack": {
    "pack_format": 81,
    "description": "My Custom Datapack v1.0"
  }
}

Deliberate legacy-to-modern compatibility range

Use this form only when the pack actually supports both sides of the format-82 boundary. Mojang requires the retained legacy fields for this range.

{
  "pack": {
    "pack_format": 81,
    "supported_formats": [81, 88],
    "min_format": [81],
    "max_format": [88],
    "description": "My compatible datapack"
  }
}

1.21.9 / 1.21.10

{
  "pack": {
    "min_format": [88, 0],
    "max_format": [88, 0],
    "description": "My Custom Datapack v1.0"
  }
}

1.21.11

{
  "pack": {
    "min_format": [94, 1],
    "max_format": [94, 1],
    "description": "My Custom Datapack v1.0"
  }
}

26.2

{
  "pack": {
    "min_format": [107, 1],
    "max_format": [107, 1],
    "description": "My Custom Datapack v1.0"
  }
}

---

Function Tags (load / tick)

The engine recognizes the minecraft:load and minecraft:tick tags, so these files must use the minecraft namespace. A load.json or tick.json in a custom namespace is a valid custom tag name, but it has no automatic behavior.

data/minecraft/tags/function/load.json

{
  "values": [
    "<namespace>:setup"
  ]
}

data/minecraft/tags/function/tick.json

{
  "values": [
    "<namespace>:tick"
  ]
}

data/<namespace>/function/setup.mcfunction

# Runs once on /reload
scoreboard objectives add deaths deathCount
scoreboard objectives add kills playerKillCount
tellraw @a {"text":"[MyPack] Loaded!","color":"green"}

data/<namespace>/function/tick.mcfunction

# Runs every tick — KEEP THIS SHORT
# Only put fast, targeted operations here
execute as @a[scores={deaths=1..}] run function mypack:on_death_check

---

Commands and Function Syntax

Execute subcommands (datapack-specific patterns)

# Chained execute — common datapack pattern for conditional per-player logic
execute as @a[gamemode=!spectator] at @s if block ~ ~-1 ~ #minecraft:logs run give @s minecraft:apple

# store result into score (bridge between NBT world and scoreboard state)
execute store result score @s mypack.health run data get entity @s Health

# in: run logic in another dimension
execute in minecraft:the_nether run say This runs in the Nether

Storage NBT (datapack-specific global state)

# Storage is the datapack-native key-value store — persists across /reload
data modify storage mypack:data config.difficulty set value "hard"
data get storage mypack:data config.difficulty

# Copy live entity data into storage for macro use or cross-function state
data modify storage mypack:log last_player_pos set from entity @s Pos

For full command syntax, selectors, and scoreboard operations see the Minecraft Wiki — Commands reference. The minecraft-commands-scripting skill covers command-only work in depth.

---

Macros (1.20.2+)

Macro functions let you pass dynamic arguments to a function.

Define a macro function (data/mypack/function/greet.mcfunction)

# Macro argument: $(name)
$tellraw @a {"text":"Welcome $(name)!","color":"gold"}
$scoreboard players set $(name) points 0

Call with run function + with

# Pass values from storage
data modify storage mypack:tmp input set value {name:"Steve"}
function mypack:greet with storage mypack:tmp input

# Pass values from entity NBT
function mypack:greet with entity @p {}

# Pass value from block NBT
function mypack:greet with block 0 64 0 {}

---

Registry data examples

Read references/data-examples.md when authoring advancements, recipes, loot tables, predicates, or tags. Load only the relevant section and keep existing namespaces and version targets.

Worldgen Overrides

Override biome noise (data/minecraft/worldgen/noise_settings/overworld.json)

Edit inside an existing copy — do NOT create from scratch without the full JSON. Get the vanilla version from the Minecraft jar: jar xf minecraft.jar data/.

Override a biome's spawn costs

{
  "spawn_costs": {
    "minecraft:zombie": {
      "energy_budget": 0.12,
      "charge": 0.7
    }
  }
}

---

Installation & Testing

Place the pack folder or ZIP under the world's datapacks/ directory, with pack.mcmeta at its root. Then use these in-game commands:

/datapack list
/datapack enable "file/my-datapack"
/datapack disable "file/my-datapack"
/reload

Development workflow

1. Edit .mcfunction or .json files
2. Run the bundled validator to catch JSON and path errors before loading:

   ./scripts/validate-datapack.sh --root /path/to/datapack
   

3. If errors, fix and re-validate until clean
4. Run /reload in-game (or /minecraft:reload if a mod intercepts it)
5. Test with target command (e.g., /function mypack:setup, trigger an advancement)
6. Check latest.log for runtime errors (missing references, bad selectors)

---

Common Errors

Error	Cause	Fix
Unknown or invalid command	Syntax error in function	Check whitespace, selector, trailing space
Datapack did not load	Invalid JSON in any file	Validate with jq . < file.json
pack metadata mismatch	Wrong pack_format or min_format / max_format values	Update pack.mcmeta for the exact 1.21.x patch
Function not running on tick	Missing engine tick tag or wrong namespace	Check data/minecraft/tags/function/tick.json
Macro error	$ line but no with	Provide with storage/entity/block

Validator Script

Use the bundled validator script before shipping a datapack update:

# Run from the installed skill directory (for example `.codex/skills/minecraft-datapack`):
./scripts/validate-datapack.sh --root /path/to/datapack

# Strict mode treats warnings as failures:
./scripts/validate-datapack.sh --root /path/to/datapack --strict

What it checks:

• JSON validity for pack.mcmeta and data/**/*.json
• Legacy pluralized path mistakes for loot tables, functions, and block/item/function tags
• data/minecraft/tags/function/load.json and tick.json references resolve to local .mcfunction files
• custom-namespace load.json and tick.json names, which are valid but do not run automatically

---

References

• Minecraft Wiki — Data Pack: https://minecraft.wiki/w/Data_pack
• Minecraft Java Edition 1.21.9 release notes: https://www.minecraft.net/en-us/article/minecraft-java-edition-1-21-9
• Minecraft Wiki — Function: https://minecraft.wiki/w/Function_(Java_Edition)
• Minecraft Wiki — Commands: https://minecraft.wiki/w/Commands
• Pack format history: https://minecraft.wiki/w/Pack_format
• NBT format: https://minecraft.wiki/w/NBT_format
• Predicate conditions: https://minecraft.wiki/w/Predicate
• Loot table format: https://minecraft.wiki/w/Loot_table