diff --git a/client/scripts/autoloads/fog_state.gd b/client/scripts/autoloads/fog_state.gd
index 46f607c79..455621d14 100644
--- a/client/scripts/autoloads/fog_state.gd
+++ b/client/scripts/autoloads/fog_state.gd
@@ -29,6 +29,11 @@ var _width: int = 1
 var _height: int = 1
 var _prev_visible: Dictionary = {}  # Tiles visible last frame (for incremental decay)
 
+## Debug flag — when true, fog.gdshader renders raw exploration texture
+## as colored overlay (green=visible, blue=explored, red=unexplored).
+## Toggle via FogState.debug_exploration = true in the console.
+var debug_exploration: bool = false
+
 
 func _ready() -> void:
 	_resize(Rect2i(0, 0, 64, 64))
@@ -84,15 +89,17 @@ func update_from_state() -> void:
 	# Grow bounds to include newly visible tiles — never shrink, so explored
 	# tiles behind the player stay in the texture and render as deep fog
 	# instead of black. Exploration data is preserved across resizes.
-	var tiles := GameState.visible_tiles
-	if tiles.size() > 0:
-		var new_bounds := _grow_bounds(tiles)
+	# Use visible_positions (always populated from server snapshots) instead of
+	# visible_tiles, which stays empty in live server mode because the server
+	# sends tile_kind but game_state.gd's population check expects "type".
+	var positions: Dictionary = GameState.visible_positions
+	if positions.size() > 0:
+		var new_bounds := _grow_bounds_from_positions(positions)
 		if new_bounds != map_bounds:
 			_resize(new_bounds)
 
 	var ox: int = map_bounds.position.x
 	var oy: int = map_bounds.position.y
-	var positions: Dictionary = GameState.visible_positions
 
 	# TODO(v0.2): gradual decay over game-time instead of immediate EXP_VISIBLE→EXP_EXPLORED
 
@@ -130,6 +137,27 @@ func update_from_state() -> void:
 	_prev_visible = positions.duplicate()
 
 
+func _grow_bounds_from_positions(positions: Dictionary) -> Rect2i:
+	## Compute bounds from visible_positions (Dictionary[Vector2i, bool]).
+	## Equivalent to _grow_bounds() but reads from the always-populated
+	## positions dict instead of the legacy visible_tiles Array.
+	var min_x := 999999
+	var min_y := 999999
+	var max_x := -999999
+	var max_y := -999999
+	for pos in positions:
+		min_x = mini(min_x, pos.x)
+		min_y = mini(min_y, pos.y)
+		max_x = maxi(max_x, pos.x)
+		max_y = maxi(max_y, pos.y)
+	if min_x > max_x:
+		return map_bounds
+	var tile_bounds := Rect2i(min_x - 8, min_y - 8, max_x - min_x + 17, max_y - min_y + 17)
+	if map_bounds.size.x <= 1 and map_bounds.size.y <= 1:
+		return tile_bounds
+	return map_bounds.merge(tile_bounds)
+
+
 func _grow_bounds(tiles: Array) -> Rect2i:
 	## Compute bounds that include all currently visible tiles, merged with
 	## existing bounds so the texture only grows — never shrinks. Explored
@@ -148,8 +176,11 @@ func _grow_bounds(tiles: Array) -> Rect2i:
 	# Guard: all tiles invalid (no x/y) — sentinels would produce negative Rect2i
 	if min_x > max_x:
 		return map_bounds
-	# Margin for fog gradient bleed at edges
-	var tile_bounds := Rect2i(min_x - 4, min_y - 4, max_x - min_x + 9, max_y - min_y + 9)
+	# Margin for fog gradient bleed at edges.
+	# The Gaussian blur in fog.gdshader samples at 2-texel intervals across a 7x7 kernel,
+	# reaching ±6 tiles from the fragment position. Margin must be >= 8 to avoid
+	# clamping artifacts at texture boundaries.
+	var tile_bounds := Rect2i(min_x - 8, min_y - 8, max_x - min_x + 17, max_y - min_y + 17)
 	# Merge with existing bounds — grow only
 	if map_bounds.size.x <= 1 and map_bounds.size.y <= 1:
 		return tile_bounds
diff --git a/client/scripts/rendering/fog_shader.gd b/client/scripts/rendering/fog_shader.gd
index dab5b8677..97586aeb9 100644
--- a/client/scripts/rendering/fog_shader.gd
+++ b/client/scripts/rendering/fog_shader.gd
@@ -69,3 +69,4 @@ func update_fog() -> void:
 	_shader_mat.set_shader_parameter("map_offset", Vector2(FogState.map_bounds.position))
 	_shader_mat.set_shader_parameter("map_size", Vector2(FogState.map_bounds.size))
 	_shader_mat.set_shader_parameter("time", Time.get_ticks_msec() / 1000.0)
+	_shader_mat.set_shader_parameter("debug_exploration", FogState.debug_exploration)
diff --git a/client/shaders/fog.gdshader b/client/shaders/fog.gdshader
index 467858a93..16c89b18d 100644
--- a/client/shaders/fog.gdshader
+++ b/client/shaders/fog.gdshader
@@ -2,11 +2,11 @@ shader_type canvas_item;
 
 // D-059: 3-layer fog shader. Composites over world content.
 // Layer 1: Clear (forward cone) — transparent, soft gradient edge
-// Layer 2: Explored fog — light overlay indicating "not fresh", art preserved
+// Layer 2: Explored fog — light overlay (~25-30%), art fully preserved
 // Layer 3: Unexplored — solid near-black #12141a
 
 uniform sampler2D visibility_tex : filter_linear, repeat_disable;
-uniform sampler2D exploration_tex : filter_linear, repeat_disable;
+uniform sampler2D exploration_tex : filter_nearest, repeat_disable;
 uniform sampler2D zone_tint_tex : filter_nearest, repeat_disable;
 uniform sampler2D noise_tex : filter_linear, repeat_enable;
 uniform vec2 rect_pos;       // World-space position of the ColorRect (pixels)
@@ -15,14 +15,17 @@ uniform vec2 map_offset;     // map_bounds.position (tiles)
 uniform vec2 map_size;       // map_bounds.size (tiles)
 uniform float tile_size;     // Pixels per sim tile
 uniform float time;          // Seconds since start
+uniform bool debug_exploration = false; // When true, render raw exploration texture
 
 const vec3 UNEXPLORED_COLOR = vec3(0.071, 0.078, 0.102);  // #12141a
-const vec3 DARK_OVERLAY = vec3(0.02, 0.02, 0.05);
+const vec3 FOG_OVERLAY_COLOR = vec3(0.06, 0.07, 0.12);    // Dark blue-grey fog tint
 
-// Soft gradient via 7x7 Gaussian blur on visibility (sigma 2.0).
-// Spreads the cone boundary into a 3-4 tile radius gradient for soft edges.
+// D-066: 6-8 sim tile soft gradient at cone boundary.
+// 7x7 Gaussian kernel sampling at 2-texel intervals spreads across ±6 tiles.
+// Effective sigma = 4 tiles in world space (sigma_kernel=2.0 * step=2.0).
+// At 2-sigma (8 tiles): weight drops to 0.14, giving ~6-8 tile visible gradient.
 float sample_visibility(vec2 uv) {
-    vec2 t = 1.0 / map_size;
+    vec2 t = 2.0 / map_size;
     float sum = 0.0;
     float weight = 0.0;
     for (float dy = -3.0; dy <= 3.0; dy += 1.0) {
@@ -44,39 +47,54 @@ void fragment() {
     // Outside known map -> unexplored
     if (tex_uv.x < 0.0 || tex_uv.x > 1.0 || tex_uv.y < 0.0 || tex_uv.y > 1.0) {
         COLOR = vec4(UNEXPLORED_COLOR, 1.0);
-    } else {
-        float vis_raw = texture(visibility_tex, tex_uv).r;
-        float vis = sample_visibility(tex_uv);
-        float explored = texture(exploration_tex, tex_uv).r;
-
-        // Don't bleed gradient into never-explored tiles
-        if (explored < 0.01 && vis_raw < 0.01) {
-            vis = 0.0;
-        }
-
-        if (explored < 0.01 && vis < 0.01) {
-            // Unexplored: solid near-black
-            COLOR = vec4(UNEXPLORED_COLOR, 1.0);
-        } else {
-            // Continuous blend: clear vision (vis=1) -> light fog (vis=0).
-            // The Gaussian blur creates a smooth 3-4 tile soft gradient
-            // at the cone edge — no hard boundary.
-
-            // Light fog: subtle animated overlay, preserves all art/info
-            float noise_val = texture(noise_tex, tile * 0.03 + vec2(time * 0.11, time * 0.07)).r;
-            float fog_alpha = 0.28 + noise_val * 0.04;
-
-            // Clarity ramp from the blurred visibility
-            float clarity = smoothstep(0.0, 0.85, vis);
-            float alpha = mix(fog_alpha, 0.0, clarity);
-            vec3 color = mix(DARK_OVERLAY, vec3(0.0), clarity);
-
-            // Soft edge between explored and unexplored
-            float exp_fade = smoothstep(0.0, 0.3, explored);
-            alpha = mix(1.0, alpha, exp_fade);
-            color = mix(UNEXPLORED_COLOR, color, exp_fade);
-
-            COLOR = vec4(color, alpha);
-        }
+        return;
     }
+
+    float vis = sample_visibility(tex_uv);
+    float explored = texture(exploration_tex, tex_uv).r;
+
+    // Debug mode: render raw exploration texture (bypass fog rendering).
+    // Green = EXP_VISIBLE (255), blue = EXP_EXPLORED (128), red = EXP_UNEXPLORED (0).
+    if (debug_exploration) {
+        if (explored > 0.9) {
+            COLOR = vec4(0.0, explored, 0.0, 0.8);       // Green: currently visible
+        } else if (explored > 0.1) {
+            COLOR = vec4(0.0, 0.0, explored * 2.0, 0.8);  // Blue: explored
+        } else {
+            COLOR = vec4(0.5, 0.0, 0.0, 0.8);             // Red: unexplored
+        }
+        return;
+    }
+
+    // D-066: 6-8 sim tile gradient at the cone boundary.
+    // smoothstep(0.02, 0.95, vis) maps the Gaussian-blurred visibility into a
+    // smooth clarity ramp spanning the full blur radius — no tile-stepping.
+    float clarity = smoothstep(0.02, 0.95, vis);
+
+    if (explored < 0.01) {
+        // Unexplored: smooth fade from transparent (inside cone) to solid black (beyond).
+        // The Gaussian vis gradient drives alpha — same ramp as explored fog — so the
+        // cone edge looks seamless regardless of whether adjacent tiles are explored.
+        // World art is NOT preserved in this region: alpha approaches 1.0 outside the cone.
+        COLOR = vec4(UNEXPLORED_COLOR, 1.0 - clarity);
+        return;
+    }
+
+    // Explored fog: light overlay preserving all art and information (D-059).
+    // fog_alpha ~0.27-0.31 -> world appears at ~70% brightness with blue-grey tint.
+    // 8-10s Perlin noise adds atmosphere without obscuring content.
+    float noise_val = texture(noise_tex, tile * 0.03 + vec2(time * 0.11, time * 0.07)).r;
+    float fog_alpha = 0.27 + noise_val * 0.04;
+
+    // Blend from fog (clarity=0, out of cone) to clear (clarity=1, in cone)
+    float alpha = mix(fog_alpha, 0.0, clarity);
+    vec3 color = mix(FOG_OVERLAY_COLOR, vec3(0.0), clarity);
+
+    // Soft transition at the explored/unexplored tile boundary.
+    // exploration_tex uses filter_nearest: explored is exactly 0.0, ~0.5, or 1.0.
+    float exp_fade = smoothstep(0.0, 0.2, explored);
+    alpha = mix(1.0, alpha, exp_fade);
+    color = mix(UNEXPLORED_COLOR, color, exp_fade);
+
+    COLOR = vec4(color, alpha);
 }
diff --git a/client/tests/test_fog_sprint22.gd b/client/tests/test_fog_sprint22.gd
new file mode 100644
index 000000000..fb71b84da
--- /dev/null
+++ b/client/tests/test_fog_sprint22.gd
@@ -0,0 +1,517 @@
+## Sprint 22 — Fog system acceptance tests (#569)
+##
+## Validates FogState data management against the Sprint 22 acceptance criteria:
+## - Explored tiles never revert to unexplored black (EXP_EXPLORED persistence)
+## - Bounds grow-only invariant (explored tiles behind player stay in texture)
+## - All visible tiles written as Forward (server simplified to Forward-only)
+## - Exploration data survives texture resize (grow-only bounds copy)
+## - Shader file present with correct fog_alpha constant
+##
+## Spec: D-059 (fog shader), D-015 (vision cone), D-066 (dual-scale grid, 6-8 tile gradient)
+## Ticket: #569
+class_name TestFogSprint22
+extends GdUnitTestSuite
+
+
+func _get_fog_state() -> Node:
+	var node = get_node_or_null("/root/FogState")
+	if node == null:
+		push_warning("TestFogSprint22: FogState autoload not found — test skipped (awaiting #569)")
+	return node
+
+
+func before_test() -> void:
+	GameState.visible_positions.clear()
+	GameState.visible_tiles.clear()
+	GameState.visibility_sectors.clear()
+
+
+func after_test() -> void:
+	GameState.visible_positions.clear()
+	GameState.visible_tiles.clear()
+	GameState.visibility_sectors.clear()
+
+
+# -- Spec constants (D-059) ---------------------------------------------------
+
+func test_exp_explored_constant_is_128() -> void:
+	# EXP_EXPLORED = 128 → shader reads this as ~0.502.
+	# smoothstep(0.0, 0.2, 0.502) = 1.0 → exp_fade fully applied.
+	# If EXP_EXPLORED were 0, explored tiles would render as solid unexplored black.
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	assert_int(fog_state.EXP_EXPLORED).override_failure_message(
+		"EXP_EXPLORED must be 128 — shader exp_fade requires explored value > 0.2 to avoid unexplored-black rendering"
+	).is_equal(128)
+
+
+func test_exp_unexplored_constant_is_0() -> void:
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	assert_int(fog_state.EXP_UNEXPLORED).is_equal(0)
+
+
+func test_exp_visible_constant_is_255() -> void:
+	# EXP_VISIBLE = 255 → shader reads 1.0, full art visibility (currently in LOS)
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	assert_int(fog_state.EXP_VISIBLE).is_equal(255)
+
+
+func test_vis_forward_constant_is_255() -> void:
+	# D-059: VIS_FORWARD = 255 → clear vision, nearly transparent fog overlay
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	assert_int(fog_state.VIS_FORWARD).is_equal(255)
+
+
+func test_vis_hidden_constant_is_0() -> void:
+	# D-059: VIS_HIDDEN = 0 → no vision, fog fully opaque
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	assert_int(fog_state.VIS_HIDDEN).is_equal(0)
+
+
+func test_unexplored_color_spec_value() -> void:
+	# D-059: Unexplored = solid near-black #12141a
+	# Verify the hex value decodes to the expected channel values.
+	var c := Color("#12141a")
+	assert_float(c.r).is_equal_approx(18.0 / 255.0, 0.003)
+	assert_float(c.g).is_equal_approx(20.0 / 255.0, 0.003)
+	assert_float(c.b).is_equal_approx(26.0 / 255.0, 0.003)
+	# Sanity: it IS very dark (all channels < 0.12)
+	assert_float(c.r).is_less(0.12)
+	assert_float(c.g).is_less(0.12)
+	assert_float(c.b).is_less(0.12)
+
+
+# -- Acceptance: explored tiles persist after leaving LOS (criterion 3) ------
+
+func test_explored_tile_becomes_exp_explored_after_leaving_los() -> void:
+	# ACCEPTANCE CRITERION: "Explored tiles never revert to unexplored black"
+	# When tile (5,5) was in LOS (frame 1) and then leaves LOS (frame 2),
+	# its exploration byte must be EXP_EXPLORED (128), not EXP_UNEXPLORED (0).
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		push_warning("TestFogSprint22: update_from_state missing — skipped")
+		return
+
+	# Frame 1: tile (5,5) is visible
+	GameState.visible_positions = {Vector2i(5, 5): true}
+	GameState.visible_tiles = [{"x": 5, "y": 5, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+
+	# Frame 2: tile (5,5) leaves LOS
+	GameState.visible_positions.clear()
+	GameState.visible_tiles = []
+	fog_state.update_from_state()
+
+	# Internal state check: _exp_bytes[tile(5,5)] must be EXP_EXPLORED (128)
+	var exp_bytes = fog_state.get("_exp_bytes")
+	if exp_bytes == null:
+		push_warning("TestFogSprint22: _exp_bytes not accessible — data path untestable headlessly")
+		return
+	var ox: int = fog_state.map_bounds.position.x
+	var oy: int = fog_state.map_bounds.position.y
+	var w: int = fog_state.get("_width") if fog_state.get("_width") != null else -1
+	if w <= 0:
+		push_warning("TestFogSprint22: _width inaccessible — data path untestable")
+		return
+	var px := 5 - ox
+	var py := 5 - oy
+	if px < 0 or py < 0 or px >= w:
+		push_warning("TestFogSprint22: tile (5,5) out of bounds after update — check grow_bounds margin")
+		return
+	var idx := py * w + px
+	if idx < 0 or idx >= exp_bytes.size():
+		push_warning("TestFogSprint22: idx %d out of exp_bytes range %d" % [idx, exp_bytes.size()])
+		return
+	assert_int(exp_bytes[idx]).override_failure_message(
+		"Tile (5,5) must be EXP_EXPLORED=128 after leaving LOS — not EXP_UNEXPLORED=0 (#569 regression)"
+	).is_equal(fog_state.EXP_EXPLORED)
+
+
+func test_explored_tile_is_exp_visible_while_in_los() -> void:
+	# While in LOS, tile exploration byte must be EXP_VISIBLE (255)
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		return
+
+	GameState.visible_positions = {Vector2i(3, 3): true}
+	GameState.visible_tiles = [{"x": 3, "y": 3, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+
+	var exp_bytes = fog_state.get("_exp_bytes")
+	if exp_bytes == null:
+		return
+	var ox: int = fog_state.map_bounds.position.x
+	var oy: int = fog_state.map_bounds.position.y
+	var w: int = fog_state.get("_width") if fog_state.get("_width") != null else -1
+	if w <= 0:
+		return
+	var px := 3 - ox
+	var py := 3 - oy
+	if px < 0 or py < 0 or px >= w:
+		return
+	var idx := py * w + px
+	if idx >= 0 and idx < exp_bytes.size():
+		assert_int(exp_bytes[idx]).is_equal(fog_state.EXP_VISIBLE)
+
+
+func test_unexplored_tile_stays_exp_unexplored() -> void:
+	# Tile (7, 8) was never seen — must remain EXP_UNEXPLORED (0)
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		return
+
+	# See only (5, 5) — tile (7, 8) is not in LOS
+	GameState.visible_positions = {Vector2i(5, 5): true}
+	GameState.visible_tiles = [{"x": 5, "y": 5, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+
+	var exp_bytes = fog_state.get("_exp_bytes")
+	if exp_bytes == null:
+		return
+	var ox: int = fog_state.map_bounds.position.x
+	var oy: int = fog_state.map_bounds.position.y
+	var w: int = fog_state.get("_width") if fog_state.get("_width") != null else -1
+	if w <= 0:
+		return
+	var px := 7 - ox
+	var py := 8 - oy
+	if px < 0 or py < 0 or px >= w:
+		return
+	var idx := py * w + px
+	if idx >= 0 and idx < exp_bytes.size():
+		assert_int(exp_bytes[idx]).is_equal(fog_state.EXP_UNEXPLORED)
+
+
+# -- Acceptance: bounds grow-only invariant ------------------------------------
+
+func test_bounds_never_shrink() -> void:
+	# ACCEPTANCE CRITERION: "Explored tiles never revert to unexplored black"
+	# Requires grow-only bounds: once a tile is in the texture, it stays there.
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		return
+
+	# Frame 1: see (10, 10) → establishes initial bounds
+	GameState.visible_positions = {Vector2i(10, 10): true}
+	GameState.visible_tiles = [{"x": 10, "y": 10, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+	var b1: Rect2i = fog_state.map_bounds
+
+	# Frame 2: see (30, 30) → bounds must expand to include both
+	GameState.visible_positions = {Vector2i(30, 30): true}
+	GameState.visible_tiles = [{"x": 30, "y": 30, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+	var b2: Rect2i = fog_state.map_bounds
+
+	# Frame 3: back to (10, 10) → bounds must NOT shrink
+	GameState.visible_positions = {Vector2i(10, 10): true}
+	GameState.visible_tiles = [{"x": 10, "y": 10, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+	var b3: Rect2i = fog_state.map_bounds
+
+	assert_bool(b2.size.x >= b1.size.x).override_failure_message(
+		"Bounds must grow when player moves to larger region"
+	).is_true()
+	assert_bool(b2.size.y >= b1.size.y).is_true()
+	assert_bool(b3.size.x >= b2.size.x).override_failure_message(
+		"Bounds must not shrink when player returns to previous position (grow-only invariant)"
+	).is_true()
+	assert_bool(b3.size.y >= b2.size.y).is_true()
+
+
+func test_bounds_include_margin_for_gradient_bleed() -> void:
+	# D-066: 6-8 tile gradient at cone edge requires texture margin.
+	# _grow_bounds adds 8-tile margin on each side (accommodates 7x7 Gaussian
+	# kernel at 2-texel intervals = ±6 tile reach). After seeing (10,10),
+	# bounds should extend at least 4 tiles beyond the visible tile.
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		return
+
+	GameState.visible_positions = {Vector2i(10, 10): true}
+	GameState.visible_tiles = [{"x": 10, "y": 10, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+
+	var b: Rect2i = fog_state.map_bounds
+	# With 4-tile margin: bounds.position.x <= 10 - 4 = 6
+	assert_bool(b.position.x <= 6).override_failure_message(
+		"FogState bounds must include 4-tile margin for gradient bleed (D-066 gradient spec)"
+	).is_true()
+	assert_bool(b.position.y <= 6).is_true()
+
+
+# -- Acceptance: Forward-only visibility (Sprint 22 server simplification) ----
+
+func test_visible_tiles_written_as_vis_forward() -> void:
+	# Sprint 22: server sends only Forward tiles (Peripheral sector removed).
+	# FogState writes VIS_FORWARD (255) for all tiles in visible_positions.
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		return
+
+	GameState.visible_positions = {Vector2i(5, 5): true, Vector2i(6, 5): true}
+	GameState.visible_tiles = [
+		{"x": 5, "y": 5, "z": 0, "visibility": "Forward"},
+		{"x": 6, "y": 5, "z": 0, "visibility": "Forward"},
+	]
+	fog_state.update_from_state()
+
+	var vis_bytes = fog_state.get("_vis_bytes")
+	if vis_bytes == null:
+		return
+	var ox: int = fog_state.map_bounds.position.x
+	var oy: int = fog_state.map_bounds.position.y
+	var w: int = fog_state.get("_width") if fog_state.get("_width") != null else -1
+	if w <= 0:
+		return
+	for pos in [Vector2i(5, 5), Vector2i(6, 5)]:
+		var px := pos.x - ox
+		var py := pos.y - oy
+		if px < 0 or py < 0 or px >= w:
+			continue
+		var idx := py * w + px
+		if idx >= 0 and idx < vis_bytes.size():
+			assert_int(vis_bytes[idx]).override_failure_message(
+				"All visible tiles should be VIS_FORWARD=255 — server is Forward-only in Sprint 22"
+			).is_equal(fog_state.VIS_FORWARD)
+
+
+func test_tiles_outside_los_written_as_vis_hidden() -> void:
+	# Tiles in bounds but not in visible_positions must be VIS_HIDDEN (0)
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		return
+
+	GameState.visible_positions = {Vector2i(5, 5): true}
+	GameState.visible_tiles = [{"x": 5, "y": 5, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+
+	# (5, 7) is inside the padded bounds but not visible — must be VIS_HIDDEN
+	var vis_bytes = fog_state.get("_vis_bytes")
+	if vis_bytes == null:
+		return
+	var ox: int = fog_state.map_bounds.position.x
+	var oy: int = fog_state.map_bounds.position.y
+	var w: int = fog_state.get("_width") if fog_state.get("_width") != null else -1
+	if w <= 0:
+		return
+	var px := 5 - ox
+	var py := 7 - oy
+	if px >= 0 and py >= 0 and px < w:
+		var idx := py * w + px
+		if idx >= 0 and idx < vis_bytes.size():
+			assert_int(vis_bytes[idx]).is_equal(fog_state.VIS_HIDDEN)
+
+
+# -- Acceptance: exploration survives texture resize --------------------------
+
+func test_exploration_data_preserved_across_bounds_growth() -> void:
+	# D-059: Texture resize must copy old exploration bytes into new texture.
+	# Without this, tiles seen before a resize appear as EXP_UNEXPLORED (black).
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		return
+
+	# Frame 1: see (5, 5), then leave
+	GameState.visible_positions = {Vector2i(5, 5): true}
+	GameState.visible_tiles = [{"x": 5, "y": 5, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+	GameState.visible_positions.clear()
+	GameState.visible_tiles = []
+	fog_state.update_from_state()  # (5,5) → EXP_EXPLORED
+
+	# Frame 2: move far away — forces bounds growth (resize)
+	GameState.visible_positions = {Vector2i(80, 80): true}
+	GameState.visible_tiles = [{"x": 80, "y": 80, "z": 0, "visibility": "Forward"}]
+	fog_state.update_from_state()
+
+	# (5,5) must still be EXP_EXPLORED after the resize
+	var exp_bytes = fog_state.get("_exp_bytes")
+	if exp_bytes == null:
+		return
+	var ox: int = fog_state.map_bounds.position.x
+	var oy: int = fog_state.map_bounds.position.y
+	var w: int = fog_state.get("_width") if fog_state.get("_width") != null else -1
+	if w <= 0:
+		return
+	var px := 5 - ox
+	var py := 5 - oy
+	if px < 0 or py < 0 or px >= w:
+		push_warning("TestFogSprint22: (5,5) not in bounds after resize — is copy-on-resize working?")
+		return
+	var idx := py * w + px
+	if idx >= 0 and idx < exp_bytes.size():
+		assert_int(exp_bytes[idx]).override_failure_message(
+			"Exploration data at (5,5) must survive bounds growth — EXP_EXPLORED (128) expected after resize"
+		).is_greater_equal(fog_state.EXP_EXPLORED)
+
+
+# -- Shader file checks (D-059) -----------------------------------------------
+
+func test_fog_gdshader_exists() -> void:
+	assert_bool(ResourceLoader.exists("res://shaders/fog.gdshader")).override_failure_message(
+		"fog.gdshader must exist — fog rendering requires this shader file (#569)"
+	).is_true()
+
+
+func test_fog_shader_defines_fog_alpha() -> void:
+	# D-059: explored fog overlay must be ~25-30% opacity.
+	# fog_alpha constant controls this. Verify the shader defines it.
+	if not ResourceLoader.exists("res://shaders/fog.gdshader"):
+		push_warning("TestFogSprint22: fog.gdshader not found — shader check skipped")
+		return
+	var source := FileAccess.get_file_as_string("res://shaders/fog.gdshader")
+	if source.is_empty():
+		push_warning("TestFogSprint22: fog.gdshader is empty or unreadable")
+		return
+	assert_bool(source.contains("fog_alpha")).override_failure_message(
+		"fog.gdshader must define fog_alpha for the 25-30%% explored-tile overlay (D-059)"
+	).is_true()
+
+
+func test_fog_shader_defines_smoothstep_clarity_ramp() -> void:
+	# D-059/D-066: smooth gradient requires a clarity ramp (smoothstep).
+	# The blurred visibility → clarity ramp must use smoothstep for smooth gradients.
+	if not ResourceLoader.exists("res://shaders/fog.gdshader"):
+		push_warning("TestFogSprint22: fog.gdshader not found — skipped")
+		return
+	var source := FileAccess.get_file_as_string("res://shaders/fog.gdshader")
+	if source.is_empty():
+		return
+	assert_bool(source.contains("smoothstep")).override_failure_message(
+		"fog.gdshader must use smoothstep for the clarity ramp — hard steps violate D-066 gradient spec"
+	).is_true()
+
+
+func test_fog_shader_defines_unexplored_color() -> void:
+	# D-059: unexplored = solid near-black #12141a.
+	if not ResourceLoader.exists("res://shaders/fog.gdshader"):
+		push_warning("TestFogSprint22: fog.gdshader not found — skipped")
+		return
+	var source := FileAccess.get_file_as_string("res://shaders/fog.gdshader")
+	if source.is_empty():
+		return
+	assert_bool(source.contains("UNEXPLORED_COLOR")).override_failure_message(
+		"fog.gdshader must define UNEXPLORED_COLOR constant (D-059 #12141a spec)"
+	).is_true()
+
+
+func test_fog_shader_uses_gaussian_blur_for_gradient() -> void:
+	# D-066: 6-8 tile soft gradient requires Gaussian blur on visibility texture.
+	# Current implementation: 7x7 kernel at 2-texel intervals (±6 tiles), sigma 2.0
+	# in kernel space = 4.0 tiles effective. At 2-sigma (8 tiles), weight drops to 0.14.
+	# This covers the D-066 "6-8 tile" gradient spec.
+	if not ResourceLoader.exists("res://shaders/fog.gdshader"):
+		push_warning("TestFogSprint22: fog.gdshader not found — skipped")
+		return
+	var source := FileAccess.get_file_as_string("res://shaders/fog.gdshader")
+	if source.is_empty():
+		return
+	# 7x7 Gaussian uses dy from -3 to 3
+	assert_bool(source.contains("sample_visibility")).override_failure_message(
+		"fog.gdshader must call sample_visibility() for Gaussian-blurred visibility (D-066 gradient)"
+	).is_true()
+	assert_bool(source.contains("-3.0")).override_failure_message(
+		"fog.gdshader sample_visibility must use 7x7 kernel (±3 tiles) for 6-tile gradient coverage (D-066)"
+	).is_true()
+
+
+# -- Regression: GameState visible_positions (existing contract) ---------------
+
+func test_visible_positions_derived_from_visible_tiles_in_server_mode() -> void:
+	# D-020: In real server mode, visible_positions derives from visible_tiles.
+	# Fog rendering depends on this derivation being correct.
+	GameState.apply_snapshot({
+		"tick": 10,
+		"visible_tiles": [
+			{"x": 7, "y": 7, "z": 0, "visibility": "Forward"},
+			{"x": 8, "y": 7, "z": 0, "visibility": "Forward"},
+		],
+	})
+	assert_bool(GameState.visible_positions.has(Vector2i(7, 7))).override_failure_message(
+		"visible_positions must be derived from visible_tiles when no explicit visible_positions key"
+	).is_true()
+	assert_bool(GameState.visible_positions.has(Vector2i(8, 7))).is_true()
+
+
+func test_visibility_sectors_populated_forward_only() -> void:
+	# D-015: visibility_sectors must be populated from visible_tiles.
+	# In Forward-only mode, all sectors are "Forward".
+	GameState.apply_snapshot({
+		"tick": 11,
+		"visible_tiles": [
+			{"x": 4, "y": 4, "z": 0, "visibility": "Forward"},
+		],
+	})
+	assert_bool(GameState.visibility_sectors.has(Vector2i(4, 4))).is_true()
+	assert_str(GameState.visibility_sectors[Vector2i(4, 4)]).is_equal("Forward")
+
+
+func test_visible_positions_cleared_on_new_snapshot() -> void:
+	# Old positions from tick N must not persist to tick N+1
+	GameState.apply_snapshot({
+		"tick": 1,
+		"visible_tiles": [{"x": 5, "y": 5, "z": 0, "visibility": "Forward"}],
+	})
+	assert_int(GameState.visible_positions.size()).is_equal(1)
+	GameState.apply_snapshot({
+		"tick": 2,
+		"visible_tiles": [{"x": 10, "y": 10, "z": 0, "visibility": "Forward"}],
+	})
+	assert_bool(GameState.visible_positions.has(Vector2i(5, 5))).override_failure_message(
+		"Old visible positions must be cleared when new visible_tiles arrive"
+	).is_false()
+	assert_bool(GameState.visible_positions.has(Vector2i(10, 10))).is_true()
+
+
+# -- Performance (D-059) -------------------------------------------------------
+
+func test_fog_state_update_under_2ms_for_400_tiles() -> void:
+	# D-059: <1ms/frame CPU budget for fog update. Allow 2x margin for test env.
+	var fog_state = _get_fog_state()
+	if fog_state == null:
+		return
+	if not fog_state.has_method("update_from_state"):
+		return
+
+	var positions: Dictionary = {}
+	var tiles: Array = []
+	for x in range(20):
+		for y in range(20):
+			positions[Vector2i(x, y)] = true
+			tiles.append({"x": x, "y": y, "z": 0, "visibility": "Forward"})
+	GameState.visible_positions = positions
+	GameState.visible_tiles = tiles
+
+	var start := Time.get_ticks_usec()
+	fog_state.update_from_state()
+	var elapsed_ms := (Time.get_ticks_usec() - start) / 1000.0
+
+	assert_float(elapsed_ms).override_failure_message(
+		"FogState.update_from_state() must complete in <2ms for 400 tiles (spec: <1ms D-059)"
+	).is_less(2.0)