understudy-client

Born from needing reliable automated testing at blocktopia.world.

A headless Minecraft client, written in Go, that speaks the Java Edition wire protocol directly — no game, no rendering, no mod loader. It connects as an ordinary offline-mode player and can mine, place, craft, eat, shoot, fall and fight.

It exists for automated in-game testing. If you maintain a server plugin or datapack and want to assert that playing the game produces the effect you intended, you need something that plays the game. A bot that issues commands proves nothing: /setblock does not fire the events a player mining a block fires, and a statistic that only a real interaction increments will not move.

              ┌─ your test suite ─┐
              │  HTTP: /dig /place│
              └─────────┬─────────┘
                        │
              ┌─────────▼─────────┐        TCP, Minecraft wire protocol
              │ understudy-client │◄──────────────────────────────────►  server
              └───────────────────┘        (Fabric, Paper, vanilla…)

Because it is a real connection, everything the server does in response is real: statistics accrue, advancements fire, plugins see the events they would see from a person.

Supported versions

Minecraft 1.21.11 and newer, on Paper, Fabric and vanilla. One binary carries every supported version and picks by the server-list ping, so there is nothing to configure. understudy-client -versions lists what your build speaks.

Support is measured per version rather than assumed to carry across, because payload shapes move between releases. A version whose encodings have not been measured refuses to decode rather than reading them at the wrong offsets and desynchronising quietly — which is why 1.21.4 tables ship but 1.21.4 is not a supported target.

Install

go install github.com/blocktopiaworld/understudy-client/cmd/understudy-client@latest

Go 1.26 or newer. No cgo, no runtime dependencies, and the module needs only the standard library — go.sum is empty and there is no supply chain to audit.

Run it

# Connect, stay 15s, leave.
understudy-client -addr localhost:25565 -username Probe

# Stay until interrupted, and expose the control API on :8181.
understudy-client -addr localhost:25565 -username Probe -hold 0 -control 8181

flag	meaning
-addr	server host:port (default 127.0.0.1:25565)
-username	offline-mode player name (default Understudy)
-version	protocol version to speak; default auto-detects by pinging
-versions	list supported versions and exit
-hold	how long to stay after joining; 0 means until interrupted
-control	serve the HTTP control API on this port or host:port
-no-idle-position	stop reporting position while standing still
-no-respawn	stay dead instead of respawning
-debug, -trace	debug logging; log every clientbound packet ID

Drive it over HTTP

With -control, a test suite in any language can drive the bot over JSON.

curl localhost:8181/state
curl -X POST localhost:8181/hold -d '{"item":"diamond_pickaxe"}'
curl -X POST localhost:8181/dig  -d '{"X":10,"Y":64,"Z":10,"hold_ms":1500}'

Reading, movement, digging, placing, combat, containers and workstations are all covered — 53 endpoints, each documented with its parameters and a request and response captured from a live server.

Full API reference → · Runnable examples →

Both read better on the site: the guide and the OpenAPI reference, the latter generated from the route table.

There is no authentication. Bind it to loopback.

Use it as a library

c, err := understudy.New(understudy.Options{
	Addr:     "127.0.0.1:25565",
	Username: "Probe",
})
if err != nil {
	log.Fatal(err)
}
defer c.Close()

ctx := context.Background()
if err := c.Connect(ctx); err != nil {
	log.Fatal(err)
}
if _, err := c.HoldItem("minecraft:diamond_pickaxe"); err != nil {
	log.Fatal(err)
}
err = c.DigBlock(ctx, 10, 64, 10, 1, 1500*time.Millisecond)

The Client refuses what a real client could not do rather than sending a packet the server will ignore: an out-of-reach block, a swing at something behind a wall, an attack on a version with no attack packet. Those come back as errors that name the distance or the obstruction, because a silent no-op is the single most expensive failure mode in a test harness.

Performance

A bot is cheap enough that the number you can run is a question about your server, not about this.

resident memory, idle	25 MB
resident memory, 470 chunks loaded	34 MB
CPU, standing still	0.5% of one core
CPU, mining and walking	~1% of one core
binary	8.6 MB, no dependencies

Chunk storage is the part that could grow badly and does not: 137 more loaded chunks cost about 4 MB, so roughly 30 KB each with palette compaction intact.

Control API latency splits in two. Reads answer from the client's own state and never touch the network, so they cost what an HTTP round trip on loopback costs. Actions that wait for the world cost what the world costs.

	median	p95
GET /state, /inventory, /block, /ground, /entities	0.35 ms	0.43 ms
POST /look, /slot, /swing — one packet, no wait	0.38 ms	0.45 ms
POST /place with verify:true	6.4 ms	6.7 ms
POST /dig one block, efficient tool	49 ms	54 ms
POST /walk four blocks	901 ms	902 ms

Walking is walking speed and digging is the block's hardness; neither is overhead this could remove. Below the instant-break threshold a dig costs the full hold instead — 0.55s rather than 0.058s on obsidian — so an unenchanted tool is the first thing to check when a mining suite is slow.

Caveats

• Movement is dead reckoning. WalkTo walks a straight line and knows nothing about walls, drops or water. There is no pathfinding.
• Teleports are treated as absolute. The relative-teleport flags in the position packet are ignored.
• Offline mode only. No Mojang authentication and no encryption, so it cannot join an online-mode server.
• The control API is unauthenticated. Loopback only.

Contributing

make check     # fmt, vet, lint, and the tests under -race

Tests are hermetic: sessions run against a net.Pipe fake server, so the suite needs no Minecraft server and no network. See CONTRIBUTING.md, and Adding a version for the generator runs a new Minecraft release needs.

How this is built

This project is written with heavy use of AI, under human direction and review. We think that is worth saying plainly rather than leaving you to guess.

It also shapes how the code is checked. Nothing version-specific here is written from memory: protocol tables are generated from the server's own output, payload encodings are measured against a running server, and the status table above comes from a suite that asks the server what happened rather than asking the client what it thinks it did.

Licence

Apache License 2.0 — see LICENSE and NOTICE.

Commercial use is explicitly allowed, including in closed-source products. The grant covers use, modification, sublicensing and distribution, with no obligation to publish your changes. Keep the licence text, the NOTICE and the copyright notices, and say if you significantly changed a file.

It does not grant rights to the Blocktopia name or marks (section 6). Build what you like on this; just do not brand it as ours.

Credits

Protocol tables are generated from minecraft-data (MIT) and from the Minecraft server's own --reports output — see NOTICE for exactly which parts come from where.

Minecraft is a trademark of Mojang Synergies AB. This project is not affiliated with, endorsed by, or connected to Mojang or Microsoft.