Your first test

With the client running and the MCP server registered, describe the test in plain language:

Start the NeoForge client in the background, wait until it's ready, create a flat creative test world, give me a mymod:grinder, place it, put 8 iron ore in it, wait 200 ticks, and check that it output iron dust. Screenshot the GUI.

The agent turns that into tool calls like these:

mc_wait_ready → mc_world_create {name:"grinder-test", flat:true}
→ mc_command "give @s mymod:grinder" → mc_player (find the slot) → mc_select_slot
→ mc_use_on {x,y,z, face:"up"} (place) → mc_use_on (open GUI) → mc_wait_for "screen:GrinderScreen"
→ mc_snapshot → mc_click_slot … → mc_wait_for "ticks:200" → mc_block (block-entity NBT) / mc_snapshot
→ mc_screenshot → mc_logs {level:"WARN"} (anything go wrong?)

The usual loop

1. Set up. mc_world_create makes a new world and waits until you are in it. It defaults to creative with cheats on, and flat: true gives a clean test area. mc_command runs any command and returns its chat output.
2. Act. Use the GUI tools (mc_click_widget, mc_click_slot, mc_key) or the gameplay tools (mc_use_on, mc_break_block, mc_use_item, mc_keybind). They behave exactly like a player's input.
3. Wait. Many effects land a tick or more later. mc_wait_for checks a condition every client tick: screen:<Class>, slot:<i>=<item>, block:<x,y,z>=<id>, ticks:<n>, log:<text>, chat:<text>, entity:<type>.
4. Verify. mc_block reads block state and block-entity NBT from the integrated server, so it is authoritative. mc_player, mc_entities, mc_snapshot and mc_tooltip cover the rest. mc_screenshot when visuals matter.
5. Check the log. New ERROR lines are appended to every tool result automatically. mc_logs has the full stacks.

After changing your mod

Rebuilding needs a restart: mc_quit closes the game cleanly, then start runClient again and call mc_wait_ready. Asset-only changes don't need one: mc_reload_resources is F3+T.

If the game crashes, the next call fails with a hint to use mc_crash_report, which reads the newest crash report or JVM hs_err log even after the game has died.