diff --git a/game/tests/test_two_instances.gd b/game/tests/test_two_instances.gd
new file mode 100644
index 0000000..965dd2d
--- /dev/null
+++ b/game/tests/test_two_instances.gd
@@ -0,0 +1,65 @@
+extends TestCase
+## What milestone 1 promises: two instances of the game, each in a process of
+## its own, stay in lockstep. This process is the host; a second Godot
+## process plays a guest with tools/guest_bot.gd.
+
+## The tick at which the host stops and both compare their worlds.
+const UNTIL := 150
+## Milliseconds the test waits for the second instance.
+const PATIENCE_MSEC := 30000
+
+
+func test_a_guest_in_another_process_stays_in_lockstep() -> void:
+	var directory := TestFiles.temporary_dir("two-instances")
+	var host: Node = load("res://main.tscn").instantiate()
+	host.storage_dir = directory.path_join("host")
+	host.command_line = PackedStringArray()
+	tree.root.add_child(host)
+	host.options.port = 0
+	host.listen_address = "127.0.0.1"
+	check(host.play(true), "the world opens")
+
+	var report_path := directory.path_join("guest.json")
+	var arguments := PackedStringArray([
+		"--headless",
+		"--path", ProjectSettings.globalize_path("res://"),
+		"--log-file", ProjectSettings.globalize_path(directory.path_join("guest.log")),
+		"-s", "res://tools/guest_bot.gd",
+		"--",
+		"--join=127.0.0.1:%d" % host.link.link.local_port(),
+		"--storage=" + directory.path_join("guest"),
+		"--until=%d" % UNTIL,
+		"--out=" + report_path,
+	])
+	var process := OS.create_process(OS.get_executable_path(), arguments)
+	check(process > 0, "the second instance starts")
+	var started := Time.get_ticks_msec()
+	while OS.is_process_running(process) and Time.get_ticks_msec() - started < PATIENCE_MSEC:
+		if host.session.guest_count() > 0:
+			# Once the guest is there, the test runs the host's ticks itself,
+			# one per frame, instead of twenty a second, and stops at UNTIL.
+			# A guest's arrival ends a pause, so the clock is stopped here.
+			host.clock.paused = true
+			if host.session.tick() < UNTIL:
+				host._run_tick()
+		await tree.process_frame
+	if OS.is_process_running(process):
+		OS.kill(process)
+		check(false, "the second instance did not finish in time")
+
+	var report: Variant = JSON.parse_string(FileAccess.get_file_as_string(report_path))
+	check(report is Dictionary and not report.has("error"), "the guest reports: %s" % [report])
+	if report is Dictionary and not report.has("error"):
+		check_eq(int(report["tick"]), UNTIL, "the guest stopped where the host stopped")
+		check_eq(report["hash"], host.session.state_hash_hex(), "both worlds are the same")
+		check_eq(int(report["player"]), 1, "the guest is player 1")
+		check_eq(int(report["stones"]), 1, "the guest broke the rock")
+		check_eq(int(report["resyncs"]), 0, "without a divergence on the way")
+	var rock: Vector2i = host.session.spawn_point() + Vector2i(7, -2)
+	check_eq(host.session.terrain_at(rock.x, rock.y), GwSession.TERRAIN_GROUND, "the rock is gone in the host's world too")
+	check_eq(host.session.player_name(1), "Bot", "the host knows the guest")
+
+	tree.root.remove_child(host)
+	host.link.link.close()
+	host.free()
+	TestFiles.remove_dir(directory)
diff --git a/game/tests/test_two_instances.gd.uid b/game/tests/test_two_instances.gd.uid
new file mode 100644
index 0000000..7483170
--- /dev/null
+++ b/game/tests/test_two_instances.gd.uid
@@ -0,0 +1 @@
+uid://c2vju7aa5pgqm
diff --git a/game/tools/guest_bot.gd b/game/tools/guest_bot.gd
new file mode 100644
index 0000000..55df730
--- /dev/null
+++ b/game/tools/guest_bot.gd
@@ -0,0 +1,59 @@
+extends SceneTree
+## A second instance of the game for the test of two instances. It starts the
+## real main scene as a guest, hits the rock beside the spawn clearing until
+## it has a stone, follows the host until the host stops at a tick, and
+## writes what its world looks like then as JSON.
+##
+## The test of two instances starts it; by hand:
+##   godot --headless --path game -s res://tools/guest_bot.gd -- --join=ADDRESS
+##     --storage=FOLDER --until=TICK --out=FILE
+
+## Milliseconds the bot waits for everything before it gives up.
+const PATIENCE_MSEC := 20000
+
+
+func _initialize() -> void:
+	_run.call_deferred()
+
+
+func _run() -> void:
+	var settings := {}
+	for argument in OS.get_cmdline_user_args():
+		settings[argument.get_slice("=", 0)] = argument.get_slice("=", 1)
+	var until := int(settings.get("--until", "0"))
+	var report := {"error": "the host did not answer"}
+
+	var main: Node = load("res://main.tscn").instantiate()
+	main.storage_dir = settings.get("--storage", "user://bot")
+	main.command_line = PackedStringArray(["--join=" + settings.get("--join", ""), "--name=Bot"])
+	root.add_child(main)
+
+	var started := Time.get_ticks_msec()
+	while Time.get_ticks_msec() - started < PATIENCE_MSEC:
+		await process_frame
+		var session: GwSession = main.session
+		if session == null:
+			report = {"error": "the session ended: %s" % main.menu.status()}
+			break
+		if session.local_player() < 0:
+			continue
+		var rock := session.spawn_point() + Vector2i(7, -2)
+		if session.item_count(GwSession.ITEM_STONE) == 0:
+			main.dig_at(rock)
+		if session.tick() == until and session.pending_ticks() == 0:
+			report = {
+				"tick": session.tick(),
+				"hash": session.state_hash_hex(),
+				"player": session.local_player(),
+				"stones": session.item_count(GwSession.ITEM_STONE),
+				"resyncs": main.resyncs,
+			}
+			break
+
+	var file := FileAccess.open(settings.get("--out", "user://bot.json"), FileAccess.WRITE)
+	file.store_string(JSON.stringify(report))
+	file.close()
+	main.notification(Node.NOTIFICATION_WM_CLOSE_REQUEST)
+	root.remove_child(main)
+	main.free()
+	quit(0 if not report.has("error") else 1)
diff --git a/game/tools/guest_bot.gd.uid b/game/tools/guest_bot.gd.uid
new file mode 100644
index 0000000..78344e7
--- /dev/null
+++ b/game/tools/guest_bot.gd.uid
@@ -0,0 +1 @@
+uid://hvhqt2viwbjx