Marker Command Engine (MCE)

A Fabric mod for Minecraft 1.20.6 that executes server commands on behalf of datapacks — no command blocks, no marker entities, no forceloaded chunks.

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Installation

Drop the .jar into your server's mods/ directory.
Requires Fabric Loader and Fabric API.

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commands.json

Located at data/mce/commands.json (inside the mod JAR; override with a datapack).

{
  "settings": {
    "require_op_level": 2,
    "log_executions": true,
    "allowed_datapacks": []
  },
  "denylist": {
    "prefixes": ["op", "deop", "ban", "stop"],
    "patterns": [".*\\$\\{.*\\}.*"]
  },
  "commands": [
    {
      "id": "greet",
      "aliases": ["g", "hello"],
      "command": "say Hello %mce:vars=\"player_name\"%!",
      "run_as": "console",
      "enabled": true
    },
    {
      "id": "start_event",
      "aliases": ["event"],
      "commands": [
        "title @a title {\"text\":\"Event Start!\",\"bold\":true}",
        "playsound minecraft:ui.toast.challenge_complete master @a",
        "say The event has begun."
      ],
      "run_as": "console",
      "enabled": true
    }
  ]
}

Entry fields

Field	Type	Required	Description
id	string	✓	Unique primary identifier
command	string	✓ (or commands)	Single command string
commands	string[]	✓ (or command)	Ordered command sequence (v1.0.1)
aliases	string[]	—	Alternative IDs for /mce run-id
run_as	"console" | "player"	—	Execution context (default: "console")
enabled	boolean	—	Disabled entries are ignored on load (default: true)

If both command and commands are present, commands takes priority.

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Commands

Command	Description
/mce run <command>	Execute a command string through the denylist (placeholders resolved)
/mce run-id <id|alias>	Run a named command from commands.json — tab-completed, aliases accepted
/mce reload	Reload commands.json from disk without restarting
/mce list	List all enabled commands with aliases and command count
/mce denylist	Show active denylist prefixes and patterns
/mce stats	Show execution statistics for the current session
/mce version	Print mod version

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Alias System

Each command entry can declare one or more aliases. They are treated identically to the primary id in all contexts: /mce run-id, tab-completion, and the event bus.

{
  "id": "test_command",
  "aliases": ["test", "tc"],
  "command": "say Hello!"
}

/mce run-id test_command   ✓
/mce run-id test           ✓
/mce run-id tc             ✓

All three names appear in tab-completion and are updated live after /mce reload.

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Multi-Command Sequences

An entry can run an ordered list of commands using the commands array (v1.0.1):

{
  "id": "start_event",
  "commands": [
    "title @a title {\"text\":\"Event!\",\"bold\":true}",
    "playsound minecraft:ui.toast.challenge_complete master @a",
    "say The event has started."
  ],
  "run_as": "console"
}

Commands execute in order. If any step is denied by the denylist or throws an exception, the sequence aborts immediately and the caller receives an [MCE] Sequence aborted at step N of M message. Each command in the sequence goes through the full denylist + placeholder pipeline independently.

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Storage Placeholders

Command strings can embed %namespace:path="key"% tokens. At execution time MCE reads the value from the Minecraft command storage and substitutes it before dispatch.

Syntax

%<storage-id>="<key>"%

• storage-id — Minecraft storage identifier (namespace:path), same as used in /data ... storage <id>
• key — Top-level key name inside that storage's NBT compound

Example

From a datapack function:

data modify storage mce:vars player_name set value "Legends11"
function #mce:run_id {id: "greet_player"}

In commands.json:

{
  "id": "greet_player",
  "command": "say Welcome %mce:vars=\"player_name\"%!",
  "run_as": "console"
}

Executed command:

say Welcome Legends11!

Type conversion

NBT type	Substituted as
NbtString	Raw string value (no SNBT quotes)
Numeric (int, long, float…)	Decimal string, SNBT type suffix stripped
NbtCompound, NbtList	SNBT representation
Missing key	Empty string "" (logged as warning)

Security

The denylist runs twice per command:

1. Before placeholder resolution — catches statically blocked templates
2. After placeholder resolution — catches values injected from storage that would form a denied command

This prevents a compromised datapack from writing op GrieferName into storage and having MCE execute it through an otherwise-allowed template.

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Event System

MceEventBus bus = MarkerCommandEngine.getEventBus();

bus.subscribe(MceEvent.COMMAND_ALLOWED, ctx -> {
    System.out.println("About to run: " + ctx.command());
});

bus.subscribe(MceEvent.COMMAND_EXECUTED, ctx -> {
    boolean ok = Boolean.TRUE.equals(ctx.success());
    System.out.println("Ran: " + ctx.command() + " success=" + ok);
});

Event	When fired	Context fields
COMMAND_ALLOWED	After denylist checks pass, before dispatch	source, command
COMMAND_DENIED	When denylist blocks a command	source, command, reason
COMMAND_EXECUTED	After every dispatch attempt (pass or fail)	source, command, success
CONFIG_RELOADED	After /mce reload	—

COMMAND_EXECUTED always fires if COMMAND_ALLOWED fired, even on dispatcher exception.
ctx.success() is true for a clean dispatch, false if the dispatcher threw.

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Stats

/mce stats shows in-session execution counters:

Counter	Meaning
allowed	Commands that passed all denylist checks
denied	Commands blocked by the denylist (pre- or post-expansion)
executed	Dispatched commands that ran without exception
failed	Dispatched commands that threw during execution

Counters reset automatically on /mce reload.

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Execution Flow

/mce run-id <id|alias>
        │
        ├─ Lookup by id OR any alias
        │
        └─ For each command in entry.commands():
                │
                ├─ 1. Strip leading '/'
                ├─ 2. Denylist check (raw template)
                ├─ 3. Resolve %ns:path="key"% placeholders → expanded
                ├─ 4. Denylist re-check (expanded)          ← injection guard
                ├─ 5. fireAllowed event
                ├─ 6. Dispatch via server command manager
                └─ 7. fireExecuted event (success or fail)

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Denylist

The denylist blocks commands by prefix or Java regex pattern. Both checks run on every execution, including post-placeholder expansion.

"denylist": {
  "prefixes": ["op", "deop", "ban", "stop"],
  "patterns": [".*\\$\\{.*\\}.*"]
}

• Prefix — the command must not start with the prefix (case-insensitive after stripping /)
• Pattern — the full expanded command is tested against each Java regex

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Changelog

v1.0.1

• Multi-command sequences — commands: [...] array in entries; fail-fast with step reporting
• Alias system — aliases: [...] per entry; tab-completed, accepted by /mce run-id
• Storage placeholders — %ns:path="key"% resolved from Minecraft command storage at runtime; double denylist pass for injection safety

v1.0.0

• /mce run <command> — execute raw command strings through the denylist
• /mce run-id <id> — execute named commands from commands.json with tab-completion
• /mce reload — hot-reload commands.json without server restart
• Event bus — COMMAND_ALLOWED, COMMAND_DENIED, COMMAND_EXECUTED, CONFIG_RELOADED
• /mce stats — in-session execution counters
• Denylist — prefix and regex pattern matching with post-expansion re-check