diff --git a/client/tests/test_monologue_display.gd b/client/tests/test_monologue_display.gd
index 2370e449a..8b1c7a07c 100644
--- a/client/tests/test_monologue_display.gd
+++ b/client/tests/test_monologue_display.gd
@@ -27,6 +27,22 @@ func _label_text(d: Node) -> String:
 	return (slot_node.get_child(0) as RichTextLabel).text
 
 
+# ---------------------------------------------------------------------------
+# Lifecycle — ensure clean GameState between every test
+# ---------------------------------------------------------------------------
+
+func before_test() -> void:
+	## Reset GameState fields touched by this suite so tests don't bleed into each other.
+	## lattice_profile: tests that care about colour set it explicitly — default to baseline.
+	## current_monologue: GameState integration tests need null as start state.
+	GameState.current_monologue = null
+	GameState.lattice_profile = "lattice_baseline"
+
+func after_test() -> void:
+	GameState.current_monologue = null
+	GameState.lattice_profile = "lattice_baseline"
+
+
 # ---------------------------------------------------------------------------
 # Initial state
 # ---------------------------------------------------------------------------
@@ -370,3 +386,80 @@ func test_expire_timer_decrements_in_process() -> void:
 	d._process(1.5)
 	assert_float(d._visible[0].expire_timer).is_less(10.0)
 	d.queue_free()
+
+
+# ---------------------------------------------------------------------------
+# GameState integration — current_monologue field (v5, #414)
+# These tests do not instantiate the scene — they verify apply_snapshot()
+# correctly populates and clears current_monologue so the display layer
+# receives valid data (or null) every tick.
+#
+# Joint test plan (sprint-14/joint.md §Test Plan Alignment):
+#   "Verify current_monologue: None produces no display (no ghost text from previous tick)"
+# ---------------------------------------------------------------------------
+
+func test_gamestate_monologue_null_when_snapshot_omits_field() -> void:
+	## D-016: Snapshot without current_monologue must clear the field.
+	## Prevents a stale line from a previous tick persisting as ghost text.
+	GameState.current_monologue = {"id": "stale", "text": "Old.", "duration_seconds": 3.0}
+	GameState.apply_snapshot({"tick": 2})
+	assert_that(GameState.current_monologue).is_null()
+
+
+func test_gamestate_monologue_set_from_snapshot() -> void:
+	## apply_snapshot populates current_monologue when the field is a dict.
+	GameState.apply_snapshot({
+		"tick": 1,
+		"current_monologue": {"id": "m001", "text": "A thought.", "duration_seconds": 5.0},
+	})
+	assert_that(GameState.current_monologue).is_not_null()
+	assert_that(GameState.current_monologue.get("text")).is_equal("A thought.")
+
+
+func test_gamestate_monologue_null_when_field_is_non_dict() -> void:
+	## Defensive: a non-dictionary value from the server must be rejected (null).
+	GameState.apply_snapshot({"tick": 1, "current_monologue": "not-a-dict"})
+	assert_that(GameState.current_monologue).is_null()
+
+
+func test_gamestate_monologue_duration_seconds_survives_round_trip() -> void:
+	## duration_seconds feeds show_monologue()'s duration param — must not be lost.
+	GameState.apply_snapshot({
+		"tick": 1,
+		"current_monologue": {"id": "m1", "text": "Test.", "duration_seconds": 7.5},
+	})
+	var dur: float = float(GameState.current_monologue.get("duration_seconds"))
+	assert_that(dur).is_equal_approx(7.5, 0.001)
+
+
+func test_gamestate_monologue_id_survives_round_trip() -> void:
+	## The id field is used by SimBridge carry-forward deduplication (#477).
+	GameState.apply_snapshot({
+		"tick": 1,
+		"current_monologue": {"id": "enter_cargo_bay_001", "text": "Hmm.", "duration_seconds": 3.0},
+	})
+	assert_that(GameState.current_monologue.get("id")).is_equal("enter_cargo_bay_001")
+
+
+func test_gamestate_monologue_replaced_by_next_snapshot() -> void:
+	## Each snapshot with a monologue replaces the previous value — no accumulation.
+	GameState.apply_snapshot({
+		"tick": 1,
+		"current_monologue": {"id": "first", "text": "First.", "duration_seconds": 3.0},
+	})
+	GameState.apply_snapshot({
+		"tick": 2,
+		"current_monologue": {"id": "second", "text": "Second.", "duration_seconds": 3.0},
+	})
+	assert_that(GameState.current_monologue.get("id")).is_equal("second")
+
+
+# ---------------------------------------------------------------------------
+# D-049 Z-layer / canvas scope compliance
+# Monologue display must be parented to CANVAS_UI (CanvasLayer 20), not the
+# world layer. This constant check ensures it hasn't silently drifted.
+# ---------------------------------------------------------------------------
+
+func test_canvas_ui_constant_is_20() -> void:
+	## D-049: CANVAS_UI = 20 is the agreed HUD layer for monologue display.
+	assert_that(Constants.CANVAS_UI).is_equal(20)