diff --git a/docs/BUILD_IN_PUBLIC_PLAN.md b/docs/BUILD_IN_PUBLIC_PLAN.md
index 307c9b5..c4d3c3a 100644
--- a/docs/BUILD_IN_PUBLIC_PLAN.md
+++ b/docs/BUILD_IN_PUBLIC_PLAN.md
@@ -623,11 +623,16 @@ Completed:
 6. `JajceWorld` runtime integration with bindings and regression coverage.
 7. Bounded Jajce beauty baseline with six terrain layers, water, atmosphere,
    foliage motion, building blockouts, and smoke.
+8. Base Terrain3D collision/navigation guardrails: runtime tests now keep
+   Terrain3D collision enabled, keep the old greybox ground out of physics, and
+   verify required navigation paths track the authored height field.
 
 Next:
 
-1. Capture and visually review “Jajce Lookdev 01.”
-2. Add the cinematic/debug toggle and repeatable demo reset.
+1. Continue replacing the temporary greybox navigation source with baked
+   Terrain3D-derived navigation for the sculpted playable loop.
+
+2. Capture and visually review “Jajce Lookdev 01.”
 3. Capture “Simulation Garden 01.”
 
 Do not start with GIS data, a full city, a large asset pack, or more NPC
diff --git a/docs/LEARNING_ROADMAP.md b/docs/LEARNING_ROADMAP.md
index d6dae2e..a82bcfd 100644
--- a/docs/LEARNING_ROADMAP.md
+++ b/docs/LEARNING_ROADMAP.md
@@ -670,6 +670,10 @@ Recently completed:
 - Activity-site capacity enforcement for rest, study, and patrol target
   resolution, derived from NPC target claims rather than presentation-only
   counters.
+- Terrain3D collision/navigation hardening began: scaffold and runtime tests
+  now assert Terrain3D collision remains enabled, the hidden greybox ground no
+  longer provides physics collision, and tested navigation routes stay close to
+  the authored Terrain3D height field.
 
 This order strengthens the simulation while regularly producing visible
 progress suitable for public development updates.
diff --git a/docs/PROJECT_CONTEXT.md b/docs/PROJECT_CONTEXT.md
index 11b58f0..2552a53 100644
--- a/docs/PROJECT_CONTEXT.md
+++ b/docs/PROJECT_CONTEXT.md
@@ -193,7 +193,8 @@ study, and patrol target typed activity sites. The migration is documented in
 - **Main scene:** `res://main.tscn`
 - **Terrain:** Terrain3D 1.0.2 is installed and enabled
 - **Jajce runtime:** reusable 512 m Terrain3D seed, greybox landmarks, stable
-  ResourceNode placement, lookdev camera, and tested runtime navigation
+  ResourceNode placement, lookdev camera, tested runtime navigation, and base
+  Terrain3D collision/navigation guardrails
 - **Terrain compatibility floor:** Godot 4.4 according to the bundled extension
 - **Version control:** Git
 - **Primary branch:** `main`
@@ -328,6 +329,9 @@ distance with resource `safety_risk`, `comfort_distance`, and
 - eight ResourceNodes preserving stable food/wood discovery IDs;
 - typed village pantry storage and typed guard, study, and rest activity sites;
 - a temporary baked navigation loop with 24 tested routes;
+- base collision/navigation guardrails that keep Terrain3D collision enabled,
+  keep the hidden greybox ground out of runtime physics, and compare required
+  navigation paths against the Terrain3D height field;
 - a look-development scene and camera.
 
 This is a runtime integration proof, not the final terrain composition.
@@ -534,6 +538,9 @@ These are expected prototype constraints, not necessarily isolated bugs:
   mutation APIs, but selection, execution, target resolution, and active-world
   queries now have focused collaborators.
 - Path failure and interruption emit a `navigation_failed` signal and send the NPC to wander; this is functional but not yet polished.
+- The old greybox navigation source remains temporary; current tests now guard
+  against collision/nav regressions, but the next terrain base phase is a
+  Terrain3D-derived bake for the sculpted playable loop.
 - Terrain3D and the bounded Jajce beauty baseline now run in the main game
   scene; final visual review and character/profession readability remain.
 - Combat, companions, factions, politics, trade, rumours, quests, persistence,
diff --git a/tests/jajce_runtime_integration_test.gd b/tests/jajce_runtime_integration_test.gd
index 5c6070e..e9b0e74 100644
--- a/tests/jajce_runtime_integration_test.gd
+++ b/tests/jajce_runtime_integration_test.gd
@@ -26,6 +26,12 @@ func _run() -> void:
 		terrain.get_camera() == main_scene.get_node("CameraRig/Camera3D"),
 		"Runtime Terrain3D should bind to the gameplay camera"
 	)
+	_check(terrain.collision_mode != 0, "Runtime Terrain3D collision should be enabled")
+	var greybox_ground := main_scene.get_node("JajceWorld/NavigationRegion3D/GreyboxGround") as StaticBody3D
+	_check(
+		greybox_ground.collision_layer == 0 and greybox_ground.collision_mask == 0,
+		"Runtime GreyboxGround should not provide physics collision"
+	)
 	_check(
 		(
 			not main_scene.has_node("World")
@@ -73,6 +79,9 @@ func _run() -> void:
 				not path.is_empty(),
 				"Navigation path should exist from %s to %s" % [origin, destination]
 			)
+			_check_path_tracks_terrain(
+				terrain, path, "Runtime navigation path should track Terrain3D height"
+			)
 
 	var village := SimVillage.new()
 	var worker := SimNPC.new(100, "BaselineWorker", SimulationIds.PROFESSION_SCHOLAR, 5.0, 5.0)
@@ -126,3 +135,20 @@ func _wait_for_navigation_map(navigation_map: RID) -> void:
 func _check(condition: bool, message: String) -> void:
 	if not condition:
 		failures.append(message)
+
+
+func _check_path_tracks_terrain(terrain: Terrain3D, path: PackedVector3Array, message: String) -> void:
+	if path.is_empty():
+		return
+
+	for point in path:
+		var terrain_height: float = terrain.data.get_height(point)
+		if is_nan(terrain_height):
+			failures.append("%s: missing terrain height at %s" % [message, point])
+			return
+		if absf(point.y - terrain_height) > 0.85:
+			failures.append(
+				"%s: path point %s is too far from terrain height %.2f"
+				% [message, point, terrain_height]
+			)
+			return
diff --git a/tests/jajce_world_scaffold_test.gd b/tests/jajce_world_scaffold_test.gd
index d305f00..cdf5009 100644
--- a/tests/jajce_world_scaffold_test.gd
+++ b/tests/jajce_world_scaffold_test.gd
@@ -26,6 +26,12 @@ func _run() -> void:
 		terrain.data_directory == "res://terrain/jajce/data",
 		"Terrain3D should use dedicated Jajce data"
 	)
+	_check(terrain.collision_mode != 0, "Terrain3D collision should be enabled for gameplay")
+	var greybox_ground := world.get_node("NavigationRegion3D/GreyboxGround") as StaticBody3D
+	_check(
+		greybox_ground.collision_layer == 0 and greybox_ground.collision_mask == 0,
+		"GreyboxGround should not provide runtime physics collision"
+	)
 	_check(
 		configured_texture_count == 6,
 		(
@@ -215,6 +221,9 @@ func _run() -> void:
 				not path.is_empty(),
 				"Jajce navigation path should exist from %s to %s" % [origin, destination]
 			)
+			_check_path_tracks_terrain(
+				terrain, path, "Jajce navigation path should track Terrain3D height"
+			)
 
 	var adapter := ActiveWorldAdapter.new()
 	adapter.pantry_storage = pantry
@@ -304,3 +313,20 @@ func _wait_for_navigation_map(navigation_map: RID) -> void:
 func _check(condition: bool, message: String) -> void:
 	if not condition:
 		failures.append(message)
+
+
+func _check_path_tracks_terrain(terrain: Terrain3D, path: PackedVector3Array, message: String) -> void:
+	if path.is_empty():
+		return
+
+	for point in path:
+		var terrain_height: float = terrain.data.get_height(point)
+		if is_nan(terrain_height):
+			failures.append("%s: missing terrain height at %s" % [message, point])
+			return
+		if absf(point.y - terrain_height) > 0.85:
+			failures.append(
+				"%s: path point %s is too far from terrain height %.2f"
+				% [message, point, terrain_height]
+			)
+			return