diff --git a/client/tests/test_dialogue_sprint18.gd b/client/tests/test_dialogue_sprint18.gd
index 977a4745a..025f3a30c 100644
--- a/client/tests/test_dialogue_sprint18.gd
+++ b/client/tests/test_dialogue_sprint18.gd
@@ -596,3 +596,77 @@ func test_is_dialogue_active_true_after_show_dialogue() -> void:
 	box.show_dialogue("NPC", "Speech.", _make_options(["Reply"]))
 	assert_bool(box.is_dialogue_active()).is_true()
 	box.queue_free()
+
+
+# ---------------------------------------------------------------------------
+# D-020 (#558): Signal decoupling — dialogue_box emits signals instead of
+# directly mutating GameState or calling AudioManager.
+# ---------------------------------------------------------------------------
+
+func test_dialogue_state_changed_emits_true_on_show() -> void:
+	## D-020: show_dialogue() must emit dialogue_state_changed(true).
+	var box := _make_dialogue_box()
+	if box == null: return
+	var received: Array = []
+	box.dialogue_state_changed.connect(func(active): received.append(active))
+	box.show_dialogue("NPC", "Speech.", _make_options(["Reply"]))
+	assert_bool(received.has(true)).override_failure_message(
+		"dialogue_state_changed(true) must be emitted on show_dialogue"
+	).is_true()
+	box.queue_free()
+
+
+func test_dialogue_state_changed_emits_false_on_hide() -> void:
+	## D-020: hide_dialogue() must emit dialogue_state_changed(false).
+	var box := _make_dialogue_box()
+	if box == null: return
+	var received: Array = []
+	box.dialogue_state_changed.connect(func(active): received.append(active))
+	box.show_dialogue("NPC", "Speech.", _make_options(["Reply"]))
+	box.hide_dialogue()
+	assert_bool(received.has(false)).override_failure_message(
+		"dialogue_state_changed(false) must be emitted on hide_dialogue"
+	).is_true()
+	box.queue_free()
+
+
+func test_audio_dip_requested_emits_dialogue_on_show() -> void:
+	## D-020: show_dialogue() must emit audio_dip_requested("dialogue").
+	var box := _make_dialogue_box()
+	if box == null: return
+	var received: Array = []
+	box.audio_dip_requested.connect(func(profile): received.append(profile))
+	box.show_dialogue("NPC", "Speech.", _make_options(["Reply"]))
+	assert_bool(received.has("dialogue")).override_failure_message(
+		"audio_dip_requested('dialogue') must be emitted on show_dialogue"
+	).is_true()
+	box.queue_free()
+
+
+func test_audio_dip_cleared_emits_on_hide() -> void:
+	## D-020: hide_dialogue() must emit audio_dip_cleared.
+	var box := _make_dialogue_box()
+	if box == null: return
+	var cleared := [false]
+	box.audio_dip_cleared.connect(func(): cleared[0] = true)
+	box.show_dialogue("NPC", "Speech.", _make_options(["Reply"]))
+	box.hide_dialogue()
+	assert_bool(cleared[0]).override_failure_message(
+		"audio_dip_cleared must be emitted on hide_dialogue"
+	).is_true()
+	box.queue_free()
+
+
+func test_no_direct_game_state_mutation() -> void:
+	## D-020: dialogue_box must not directly mutate GameState.dialogue_active.
+	## After show_dialogue, GameState.dialogue_active should remain unchanged
+	## (only the coordinator updates it via signal handler).
+	var box := _make_dialogue_box()
+	if box == null: return
+	GameState.dialogue_active = false
+	box.show_dialogue("NPC", "Speech.", _make_options(["Reply"]))
+	assert_bool(GameState.dialogue_active).override_failure_message(
+		"GameState.dialogue_active must NOT be mutated directly by dialogue_box"
+	).is_false()
+	box.queue_free()
+	GameState.dialogue_active = false
diff --git a/client/tests/test_dialogue_sprint20.gd b/client/tests/test_dialogue_sprint20.gd
new file mode 100644
index 000000000..b3f580dae
--- /dev/null
+++ b/client/tests/test_dialogue_sprint20.gd
@@ -0,0 +1,143 @@
+## Sprint 20 #558: dialogue_box.gd decoupling tests.
+##
+## Verifies D-020 compliance: dialogue_box.gd emits signals instead of mutating
+## GameState or calling AudioManager directly. main.gd wires the signal handlers.
+##
+## D-030: fixture-based, server-free, no subprocess required.
+class_name TestDialogueSprint20
+extends GdUnitTestSuite
+
+
+func _make_dialogue_box() -> Control:
+	if not ResourceLoader.exists("res://ui/dialogue_box.tscn"):
+		push_warning("TestDialogueSprint20: dialogue_box.tscn not found — scene tests skipped")
+		return null
+	var node: Control = load("res://ui/dialogue_box.tscn").instantiate()
+	add_child(node)
+	return node
+
+
+func before_test() -> void:
+	GameState.dialogue_active = false
+
+
+func after_test() -> void:
+	GameState.dialogue_active = false
+
+
+# -- dialogue_state_changed signal -------------------------------------------
+
+func test_show_dialogue_emits_dialogue_state_changed_true() -> void:
+	## show_dialogue() must emit dialogue_state_changed(true) not mutate GameState directly.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	var received: Variant = null
+	box.dialogue_state_changed.connect(func(active: bool): received = active)
+	box.show_dialogue("NPC", "Hello.", [])
+	assert_that(received).override_failure_message(
+		"show_dialogue() must emit dialogue_state_changed(true) (#558)"
+	).is_equal(true)
+
+
+func test_show_dialogue_does_not_mutate_game_state_directly() -> void:
+	## Without a connected handler, GameState.dialogue_active must stay false.
+	## Proves dialogue_box.gd has zero direct GameState mutation (D-020 #558).
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	GameState.dialogue_active = false
+	box.show_dialogue("NPC", "Hello.", [])
+	assert_bool(GameState.dialogue_active).override_failure_message(
+		"dialogue_box must not mutate GameState.dialogue_active directly (D-020 #558)"
+	).is_false()
+
+
+func test_hide_dialogue_emits_dialogue_state_changed_false() -> void:
+	## hide_dialogue() must emit dialogue_state_changed(false).
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	box.show_dialogue("NPC", "Hello.", [])
+	var received: Variant = null
+	box.dialogue_state_changed.connect(func(active: bool): received = active)
+	box.hide_dialogue()
+	assert_that(received).override_failure_message(
+		"hide_dialogue() must emit dialogue_state_changed(false) (#558)"
+	).is_equal(false)
+
+
+# -- audio_dip_requested / audio_dip_cleared signals -------------------------
+
+func test_show_dialogue_emits_audio_dip_requested_dialogue() -> void:
+	## show_dialogue() must emit audio_dip_requested("dialogue") not call AudioManager.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	var received_profile: Variant = null
+	box.audio_dip_requested.connect(func(profile: String): received_profile = profile)
+	box.show_dialogue("NPC", "Hello.", [])
+	assert_that(received_profile).override_failure_message(
+		"show_dialogue() must emit audio_dip_requested('dialogue') (#558)"
+	).is_equal("dialogue")
+
+
+func test_show_dialogue_does_not_call_audio_manager_directly() -> void:
+	## Without a connected handler, AudioManager state must be unchanged by show_dialogue().
+	## Verifies no direct AudioManager call in dialogue_box.gd (D-020 #558).
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	var dip_before := AudioManager.get_active_dip()
+	box.show_dialogue("NPC", "Hello.", [])
+	var dip_after := AudioManager.get_active_dip()
+	assert_str(dip_after).override_failure_message(
+		"dialogue_box must not call AudioManager.apply_dip() directly (D-020 #558)"
+	).is_equal(dip_before)
+
+
+func test_hide_dialogue_emits_audio_dip_cleared() -> void:
+	## Ending a conversation must emit audio_dip_cleared not call AudioManager directly.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	box.show_dialogue("NPC", "Hello.", [])
+	var cleared := false
+	box.audio_dip_cleared.connect(func(): cleared = true)
+	box.hide_dialogue()
+	assert_bool(cleared).override_failure_message(
+		"hide_dialogue() must emit audio_dip_cleared (#558)"
+	).is_true()
+
+
+func test_audio_dip_cleared_count_on_conversation_end() -> void:
+	## Verify audio_dip_cleared fires when conversation ends.
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	var cleared_count := 0
+	box.audio_dip_cleared.connect(func(): cleared_count += 1)
+	box.show_dialogue("NPC", "Speak.", [])
+	box.hide_dialogue()
+	assert_int(cleared_count).override_failure_message(
+		"audio_dip_cleared must fire at least once when conversation ends (#558)"
+	).is_greater_equal(1)
+
+
+# -- coordinator wiring verification -----------------------------------------
+
+func test_signal_handler_wires_game_state() -> void:
+	## Simulate main.gd: connect dialogue_state_changed to update GameState.dialogue_active.
+	## Verifies the coordinator pattern works end-to-end (D-020 #558).
+	var box := _make_dialogue_box()
+	if box == null: return
+	auto_free(box)
+	box.dialogue_state_changed.connect(func(active: bool): GameState.dialogue_active = active)
+	box.show_dialogue("NPC", "Hello.", [])
+	assert_bool(GameState.dialogue_active).override_failure_message(
+		"With handler wired, GameState.dialogue_active must be true after show_dialogue (#558)"
+	).is_true()
+	box.hide_dialogue()
+	assert_bool(GameState.dialogue_active).override_failure_message(
+		"With handler wired, GameState.dialogue_active must be false after hide_dialogue (#558)"
+	).is_false()
diff --git a/client/ui/dialogue_box.gd b/client/ui/dialogue_box.gd
index 54a0f5b87..c1b4f2ec1 100644
--- a/client/ui/dialogue_box.gd
+++ b/client/ui/dialogue_box.gd
@@ -15,6 +15,11 @@ signal dialogue_dismissed  # Walk-away or conversation end
 signal confrontation_monologue(text: String, duration: float)  # D-063: beat monologue
 signal pause_requested   # D-061: auto-pause — main.gd routes through input recording (#507)
 signal unpause_requested  # D-061: auto-unpause
+# D-020 (#558): Decoupled signals — dialogue_box emits, main.gd (coordinator) handles.
+# Replaces direct GameState.dialogue_active mutation and AudioManager calls.
+signal dialogue_state_changed(active: bool)
+signal audio_dip_requested(profile: String)
+signal audio_dip_cleared
 
 @onready var panel: PanelContainer = $PanelContainer
 @onready var dialogue_log: RichTextLabel = $PanelContainer/MarginContainer/VBoxContainer/DialogueLog
@@ -286,10 +291,10 @@ func show_dialogue(npc_name: String, speech: String, options: Array = []) -> voi
 	# Show panel
 	_ensure_visible()
 	mouse_filter = Control.MOUSE_FILTER_STOP
-	GameState.dialogue_active = true  # D-064: block movement while in conversation
+	dialogue_state_changed.emit(true)  # D-064: coordinator blocks movement
 
-	# D-069: Dialogue dip
-	AudioManager.apply_dip("dialogue")
+	# D-069: Dialogue dip — coordinator routes to AudioManager
+	audio_dip_requested.emit("dialogue")
 
 	# D-061: auto-pause — signal to main.gd for input recording (#507)
 	pause_requested.emit()
@@ -311,14 +316,14 @@ func _end_player_conversation() -> void:
 			entry.timestamp_msec = now
 	_log_dirty = true
 
-	# D-069: Clear dialogue/confrontation dip
-	AudioManager.clear_dip()
+	# D-069: Clear dialogue/confrontation dip — coordinator routes to AudioManager
+	audio_dip_cleared.emit()
 
 	# D-061: unpause — signal to main.gd for input recording (#507)
 	unpause_requested.emit()
 
-	# D-064: unblock movement immediately — log entries stay visible but don't block input.
-	GameState.dialogue_active = false
+	# D-064: unblock movement immediately — coordinator handles GameState update.
+	dialogue_state_changed.emit(false)
 
 	# If no entries remain, hide the panel with fade.
 	if _log_entries.is_empty():
@@ -331,11 +336,11 @@ func hide_dialogue() -> void:
 		_end_player_conversation()
 		return  # _end_player_conversation may call hide_dialogue if log is empty
 
-	GameState.dialogue_active = false
+	dialogue_state_changed.emit(false)  # D-020: coordinator handles GameState update
 
 
 func is_dialogue_active() -> bool:
-	return _in_player_conversation or GameState.dialogue_active
+	return _in_player_conversation
 
 
 func has_active_entries() -> bool:
@@ -428,12 +433,12 @@ func _start_confrontation_beat(response_id: String, text: String) -> void:
 	_active_tween.tween_property(panel, "modulate:a", CONFRONTATION_DIM_ALPHA, 0.2)
 
 	confrontation_monologue.emit(UIStrings.get_text(CONFRONTATION_MONOLOGUE_KEY), CONFRONTATION_BEAT_DURATION)
-	AudioManager.apply_dip("confrontation")
+	audio_dip_requested.emit("confrontation")  # D-069: coordinator routes to AudioManager
 
 	_beat_tween = create_tween()
 	_beat_tween.tween_interval(CONFRONTATION_BEAT_DURATION)
 	_beat_tween.tween_callback(func():
-		AudioManager.clear_dip()
+		audio_dip_cleared.emit()  # D-069: coordinator routes to AudioManager
 		option_selected.emit(response_id, text)
 		_end_player_conversation()
 	)
@@ -443,7 +448,7 @@ func _cancel_beat() -> void:
 	if _beat_tween and _beat_tween.is_valid():
 		_beat_tween.kill()
 		_beat_tween = null
-		AudioManager.clear_dip()
+		audio_dip_cleared.emit()  # D-069: coordinator routes to AudioManager
 
 
 # -- Log rendering --