MCFunction Debug Toolkit

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A Fabric debugging mod for Minecraft Java commands, functions, and datapacks. Add #! lines to .mcfunction files to inspect the real execution context, scoreboards, and NBT. The optional MCP bridge also lets AI agents run commands, read output, operate the game, and inspect screenshots.

Development preview for Minecraft Java 26.2.

Replace a verbose state dump with:

#! player: \{
#!   name: {@s},
#!   health: {entity @s Health: .1f},
#!   position: [{position:.2f}],
#!   inventory: [{storage demo:showcase inventory[]: {}, ...}],
#!   stats: \{kills: {@s kills}, deaths: {@s deaths}\}
#! \}

It immediately becomes this in chat:

Example

# Current execution context
#! [{fname}] {@s} is at {position:.2f}, facing {rotation:.1f}
#! selected targets={@e[tag=target]}
#! dimension={dimension} anchor={anchor} stack={fstack}

# Single scoreboard values
#! own score={@s points}
#! global fake player={#total stats}

# Dynamically list every selected score
#! all scores={@e[tag=test] points: {"{display_name} [{holder}]={score:04d}"}, ...}

# Read a unit vector from Storage
#! direction={storage demo:debug direction[]: {value:.4f}, ...}

# Read entity UUID and health
#! own UUID={entity @s UUID}
#! own health={entity @s Health}
#! nearest target={entity @e[tag=target,sort=nearest,limit=1] UUID}
#! nearest target health={entity @e[tag=target,sort=nearest,limit=1] Health}

# Block-entity NBT
#! chest items={block ~ ~ ~ Items[]: {}, ...}

# Group values in pairs; /no_strip keeps unused template text
#! groups={storage demo:debug values[]: {}, {}, ... /no_strip}

# Consecutive #! lines join while braces remain open
# The inner ... repeats per coordinate and the outer ... per entity
#! player positions={
#! entity @a Pos[]: {{entity}: {}, ...}\n ...
#! }

# Literal braces and newline
#! raw=\{{storage demo:debug values[]: {}, ...}\}\nanchor={anchor}

When the function runs, the result is sent to every player as a native Minecraft text component. Invalid #! lines produce reload warnings and are skipped without breaking other commands. Runtime failures show a short message and retain the complete diagnostic.

Without MCP, this is a human-facing debugging mod. With MCP, an AI can also validate and execute commands, incrementally read chat and debug events, control movement and GUIs, and capture JPEG screenshots.

Run demo:setup and read the new #! output. If it has no errors, walk forward
for half a second, right-click, then take a screenshot and explain what changed.

Documentation

• Changelog
• Install the mod and connect an AI agent
• Complete #! syntax and formatting guide
• MCP and HTTP protocol
• Architecture
• Real-world datapack tests

Safety

• Use disposable worlds for automation and keep backups.
• Keep bridge ports on 127.0.0.1.
• Treat the bearer token like privileged command access.
• allow_dangerous is a mistake-prevention guard, not a security sandbox.
• Never automatically retry timeout_unknown_outcome; the command may have run.

License

MIT