Dedicated servers

Multiplayer bugs (missing packets, client/server desync, code that only works in singleplayer because both sides share a JVM) only show up against a real dedicated server. MCTest runs on the server too: the same localRuntime dependency puts it on runServer, where it opens a second control API on ws://127.0.0.1:25598/ that the mc_server_* tools talk to.

./gradlew :fabric:runServer     # or :neoforge:runServer (first run: accept the EULA in runs/server/eula.txt)
./gradlew :fabric:runClient     # in a second terminal

Then the agent calls mc_server_wait_ready, mc_wait_ready and mc_connect (default localhost). From there client tools drive the player as usual, and the server tools show and change the authoritative side:

mc_server_command "setblock 10 64 10 mymod:grinder" → mc_connect → mc_use_on (open it) → mc_click_slot …
→ mc_server_block {x:10,y:64,z:10}  (server NBT)  vs  mc_block {…, source:"client"}  (what the client was sent)
→ mc_server_wait_for "log:Grinder finished" → mc_server_logs {level:"WARN"}

Comparing mc_server_block with mc_block {source: "client"} is the quickest way to find a block entity that never syncs its data to the client.

Dev-server conveniences

In a development environment MCTest makes a dev server usable by dev clients:

• Offline mode. Dev clients have no Mojang session, so the server skips authentication and does not require signed chat, whatever server.properties says. Opt out with -Dmctest.onlineMode=true.
• No whitelist. New 26.x servers whitelist by default. MCTest turns it off at startup (as /whitelist off would). /whitelist on still works afterwards. Keep it with -Dmctest.whitelist=true.
• Auto-op. Players are opped when they join, so mc_command works from the client. A test that deops a player is respected until they rejoin. Turn off with -Dmctest.autoOp=false for permission tests.

The server API is not started in the integrated server: in singleplayer the client API already reads the integrated server directly (mc_block, mc_time_*).

See Server tools for the full list.