Configure and run the MCP server (Step 3 of 4)

This is Phase 3 of the four-phase Minecraft Java MCP setup. It assumes Phase 2 (setup-mod) is done: the MCP mod jar and Fabric API jar are in mods/. The MCP server is embedded in the mod, so "running the MCP server" means launching Minecraft (single-player) or the dedicated server.

Work interactively. Branch on the choice from Phase 1: single-player is mostly defaults; dedicated/remote needs a config file and a token.

Where config lives

The mod reads config/minecraft_fabric_mcp/config.json under the game directory:

• Single-player: <.minecraft>/config/minecraft_fabric_mcp/config.json.
• Dedicated server: <server>/config/minecraft_fabric_mcp/config.json.

The file is optional — it's only needed to override defaults. Any field can also be overridden by an environment variable named MCP_<FIELD> (e.g. MCP_PORT, MCP_AUTH_REQUIRED).

The defaults are deliberately safe: bind 127.0.0.1, port 8765, no auth, reject all cross-origin browser requests.

There are two servers. The above is the world server (minecraft-java, the main entrypoint). The same jar also has a client inspection server (minecraft-java-client, the client entrypoint) that runs inside a real, rendered Minecraft client and serves the client category — view_capture (a real first-person PNG) and sense_* / client_status. It reads a separate config config/minecraft_fabric_mcp/client.json (default port 8766, env prefix MCP_CLIENT_*) and defaults to exposing only the inspection tools. It needs a real client window with a GPU — it is not part of a headless dedicated server. This phase configures the world server; running the inspection server (and the server-only / client-only / combo patterns) is covered in setup-connect. Most fields below apply to both — only the file name, default port, and env prefix differ.

Single-player branch — defaults are enough

For Claude running on the same machine as Minecraft, you do not need a config file. The mod listens on http://127.0.0.1:8765/mcp with no token.

Step 1 — Launch

Have the user start the Fabric profile in the Minecraft Launcher and load any world. (At least one loaded world is needed — many tools act on the world or relative to a player.)

Step 2 — Verify it's listening

Check the game log for:

[minecraft_fabric_mcp] MCP server listening at http://127.0.0.1:8765 (host=127.0.0.1, port=8765, auth=false, tls=false)

Then the liveness probe (no auth required for /healthz):

curl http://localhost:8765/healthz
# → {"status":"ok"}

If /healthz answers, Phase 3 is done — skip to Wrap up. There is no token for this path.

Dedicated / remote branch — config + token

Use this when Claude connects from another machine, or you want LAN/internet access. (If Claude runs on the same host as the dedicated server, you can use the single-player defaults above and skip the config file.)

Step 1 — Write config.json

Create <server>/config/minecraft_fabric_mcp/config.json:

{
  "host": "0.0.0.0",
  "port": 8765,
  "allow_remote": true,
  "auth_required": true,
  "rate_limit_rpm": 120
}

Leave bearer_token unset — the mod generates a 256-bit token on first boot and logs it once.

The mod refuses unsafe bindings. Binding to a non-loopback host (0.0.0.0 or a LAN IP) requires both allow_remote: true and auth_required: true. If either is missing, the mod errors at startup rather than exposing an unauthenticated world to the network. This is intentional — don't work around it.

Optional fields the user may want:

• tls_cert_path / tls_key_path — PEM cert + PKCS8 key for TLS at the mod (set both or neither). For internet-facing servers, use TLS here or terminate it at a reverse proxy. See the mod's docs/security.md.
• Tool-surface controls (included_categories / excluded_categories / max_access) — see the section below; with no config the mod registers a lean ~104-tool surface.
• command_timeout_ms (default 15000) and rate_limit_rpm (default 60).

Step 2 — Launch and capture the token

Start the server:

java -Xmx4G -jar fabric-server-launch.jar nogui

In the log, find the generated token (shown once):

[minecraft_fabric_mcp] Generated bearer token for MCP server. Save this value — it is shown only once:
[minecraft_fabric_mcp]   Authorization: Bearer 9c1f9a…

Have the user copy the token somewhere safe — Phase 4 needs it. To rotate it later, delete bearer_token from config.json and restart. Never commit the token or the config file that contains it.

Step 3 — Run as a service (optional, recommended for 24/7)

For a deployment that survives logout and reboots, wrap the server as a service: systemd on Linux, NSSM or Task Scheduler on Windows. The mod's docs/setup-dedicated-server.md has ready-to-paste fabric-mcp.service and NSSM commands plus a locked-down minecraft user. Walk the user through it only if they want it now.

Step 4 — Verify it's listening

curl http://<host>:8765/healthz
# → {"status":"ok"}

Use localhost if testing on the server itself, otherwise the host's LAN IP/hostname. /healthz needs no auth even when auth_required is on, so a plain curl confirms reachability independent of the token.

Restricting the tool surface (optional)

The mod groups its world tools into ten domain categories and tags each tool with one access level. With no config, an operator gets a lean default: the seven default-on domains at write access, about 103 tools. This is plenty for the world builder and keeps the surface small. Touch this only if the user wants to widen the surface or lock it down further. (There is an eleventh category, client, but it is served only by the separate minecraft-java-client inspection endpoint — never the world server — and is configured there.)

The ten categories

Default-on (registered with no config): blocks, structures, world, entities, items, scripting, server. Opt-in (off until you add them): players, gameplay, registries.

The access axis

Every tool is read, write, or admin, ranked read < write < admin:

• read — does not change game or server state (*_get_*, *_list_*, *_describe, *_query, *_find_*, *_evaluate, …).
• write — the normal mutating case: place blocks, summon entities, set scores.
• admin — world-wide, destructive, or server-lifecycle ops, opt-in. The admin set is worldborder_set_* / worldborder_add_size, level_set_difficulty, level_set_game_rule, level_create_explosion, command_register, server_reload_resources, datapack_enable / datapack_disable, and player_kick.

max_access defaults to write, so admin tools stay out until you raise it. command_execute is write, not admin — it is the workhorse, even though it can technically run any command, including admin-equivalent ones. Treat the whole server as exposed once you allow it.

How the config resolves the surface

1. Start from included_categories if it's non-empty (an explicit allowlist); otherwise start from the seven default-on domains.
2. Subtract excluded_categories.
3. Drop any tool whose access rank is above max_access.

So to keep the lean default, set nothing. Common adjustments:

// Add the opt-in domains the builder occasionally reads from:
{ "included_categories": ["blocks","structures","world","entities","items",
                          "scripting","server","gameplay","registries"] }

// Or keep the defaults and just add what you need without re-listing all ten:
// (an empty included_categories means the default-on set; there is no
//  "default + extras" merge, so list the full set you want when you use it.)

// Raise the cap to expose admin ops (world border, difficulty, game rules, …):
{ "max_access": "admin" }

// Lock it down to read-only (equivalent to the legacy exclude_write_tools):
{ "max_access": "read" }

// Drop a default-on domain you don't want:
{ "excluded_categories": ["scripting"] }

exclude_write_tools: true is still honoured as a legacy alias for max_access: read.

Wrap up

Confirm with the user (the relevant set):

• Single-player: Minecraft launched with a world loaded; /healthz is OK on http://localhost:8765. No token.
• Dedicated/remote: config.json written with allow_remote + auth_required; server launched; bearer token captured; /healthz is OK on the host.

Hand off: Phase 3 is done — the world is reachable over MCP. The last step is connecting Claude to it. Offer to continue with the setup-connect skill now. For that step the user needs the /mcp URL (http://<host>:8765/mcp) and, for a remote setup, the bearer token.