Things worth knowing

Errors come to you

After every tool call the MCP server checks the game's log for new ERROR lines and appends them to the result, so an exception your mod throws shows up right after the action that caused it. mc_logs has the full stacks. If the game crashes, the next call fails with a hint to use mc_crash_report.

Logs from before MCTest loads

On Fabric MCTest attaches in preLaunch, before any mod initializes. On NeoForge, lines logged before MCTest is constructed are imported from logs/latest.log (flagged backfilled: true), so your mod's startup errors are visible on both loaders.

Screen class names

They are the real, unobfuscated ones (InventoryScreen, CreativeModeInventoryScreen, CraftingScreen, your mod's screen class). screen: in mc_wait_for also matches superclasses, for example screen:AbstractContainerScreen.

Two slot numberings

• Container slot numbers are menu indices from mc_snapshot (crafting table: 0 = result, 1–9 = grid, 10–45 = inventory). mc_click_slot, mc_hover and mc_tooltip (with a container open) use these.
• mc_player inventory slots are inventory indices (0–8 hotbar, 9–35 main, 36+ armor/offhand). mc_tooltip uses these when no container is open.

Keybind actions apply on the next tick

mc_key {key: "e"} returns before the inventory opens. Follow it with mc_wait_for {condition: "screen:InventoryScreen"}.

Input is SDL3 in 26.x

Raw key numbers for mc_key are SDL scancodes, not GLFW codes. Use names (e, escape, left_shift, f3).

Pause on lost focus is off

The game window is usually not focused while an agent drives it, and vanilla would otherwise open the pause menu every time a screen closes. MCTest turns pauseOnLostFocus off in memory. Launch with -Dmctest.pauseOnLostFocus=true to keep vanilla behaviour.

Your mouse is left alone

Vanilla captures the OS cursor whenever a screen closes in-world and warps it to the window centre whenever one opens, so an agent opening and closing screens would keep trapping or yanking your mouse while you work in another app. With MCTest, the game only captures or moves the cursor within 2 seconds of your own click or key press in its window; MCTest's synthetic input never counts. Click into the game to play by hand as usual. mc_status reports windowFocused and mouseGrabbed. Use -Dmctest.mouseWarp=true for vanilla behaviour.

Tutorial hints are skipped

Toasts like "Look around" would otherwise cover screenshots. Use -Dmctest.tutorial=true to keep them.