diff --git a/server/src/perception/observer/tests.rs b/server/src/perception/observer/tests.rs
index f2b1d3024..d564eac2f 100644
--- a/server/src/perception/observer/tests.rs
+++ b/server/src/perception/observer/tests.rs
@@ -2466,3 +2466,116 @@ fn blocked_entities_sorted_ascending() {
     assert!(snapshot.blocked_entities.contains(&npc_b_sid.0));
     assert!(snapshot.blocked_entities.contains(&npc_c_sid.0));
 }
+
+// -----------------------------------------------------------------------
+// Zone enrichment tests (#523, D-077)
+// -----------------------------------------------------------------------
+
+#[test]
+fn zone_map_enriches_visible_tiles_with_zone_id() {
+    // Tile inside a zone should get zone_id = Some(zone_id)
+    use crate::simulation::zone::ZoneMap;
+
+    let mut world = setup_world(32, 32);
+    let mut zone_map = ZoneMap::default();
+    // Zone 42 covers (14..18, 14..18) — includes the observer's tile at (16,16)
+    zone_map.set_rect(14, 14, 4, 4, 0, 42);
+    world.insert_resource(zone_map);
+
+    world.spawn((
+        PlayerCharacter,
+        TilePosition::new(16, 16, 0),
+        Facing::default(),
+        KnowledgeGraph::new(),
+        NearbyInteractionBuffer::default(),
+        MonologueBuffer::default(),
+    ));
+
+    run_observer_pipeline(&mut world);
+
+    let buffer = world.resource::<SnapshotBuffer>();
+    let snapshot = buffer.snapshot.as_ref().unwrap();
+
+    // Observer's tile (16, 16) is inside zone 42
+    let observer_tile = snapshot
+        .visible_tiles
+        .iter()
+        .find(|t| t.x == 16 && t.y == 16 && t.z == 0)
+        .expect("observer tile should be visible");
+    assert_eq!(
+        observer_tile.zone_id,
+        Some(42),
+        "tile inside zone should have zone_id"
+    );
+}
+
+#[test]
+fn zone_map_tiles_outside_zone_get_none() {
+    // Tile outside any zone should get zone_id = None
+    use crate::simulation::zone::ZoneMap;
+
+    let mut world = setup_world(32, 32);
+    let mut zone_map = ZoneMap::default();
+    // Zone only covers (0..2, 0..2) — far from observer at (16,16)
+    zone_map.set_rect(0, 0, 2, 2, 0, 7);
+    world.insert_resource(zone_map);
+
+    world.spawn((
+        PlayerCharacter,
+        TilePosition::new(16, 16, 0),
+        Facing::default(),
+        KnowledgeGraph::new(),
+        NearbyInteractionBuffer::default(),
+        MonologueBuffer::default(),
+    ));
+
+    run_observer_pipeline(&mut world);
+
+    let buffer = world.resource::<SnapshotBuffer>();
+    let snapshot = buffer.snapshot.as_ref().unwrap();
+
+    // Observer's tile (16, 16) is outside any zone
+    let observer_tile = snapshot
+        .visible_tiles
+        .iter()
+        .find(|t| t.x == 16 && t.y == 16 && t.z == 0)
+        .expect("observer tile should be visible");
+    assert_eq!(
+        observer_tile.zone_id, None,
+        "tile outside any zone should have zone_id = None"
+    );
+}
+
+#[test]
+fn no_zone_map_resource_tiles_have_no_zone_id() {
+    // When ZoneMap resource is absent, all tiles should have zone_id = None
+    let mut world = setup_world(32, 32);
+    // Do NOT insert ZoneMap resource
+
+    world.spawn((
+        PlayerCharacter,
+        TilePosition::new(16, 16, 0),
+        Facing::default(),
+        KnowledgeGraph::new(),
+        NearbyInteractionBuffer::default(),
+        MonologueBuffer::default(),
+    ));
+
+    run_observer_pipeline(&mut world);
+
+    let buffer = world.resource::<SnapshotBuffer>();
+    let snapshot = buffer.snapshot.as_ref().unwrap();
+
+    assert!(
+        !snapshot.visible_tiles.is_empty(),
+        "should have visible tiles"
+    );
+    // All tiles should have zone_id = None when no ZoneMap exists
+    for tile in &snapshot.visible_tiles {
+        assert_eq!(
+            tile.zone_id, None,
+            "tile ({},{}) should have zone_id = None without ZoneMap resource",
+            tile.x, tile.y
+        );
+    }
+}
diff --git a/server/src/simulation/zone.rs b/server/src/simulation/zone.rs
index baccf096a..ebefd79ef 100644
--- a/server/src/simulation/zone.rs
+++ b/server/src/simulation/zone.rs
@@ -18,6 +18,9 @@ use bevy_ecs::prelude::*;
 ///
 /// Zone IDs are opaque u16 values — the client maintains its own
 /// `zone_id → zone_name / temperature_tint / ambient_layer` mapping.
+///
+/// Production population path is TBD — currently populated only by
+/// Gauntlet room builders via `set_rect` / `set`.
 #[derive(Resource, Debug, Default)]
 pub struct ZoneMap {
     zones: BTreeMap<(i32, i32, i32), u16>,
diff --git a/server/src/test_world/rooms/decay_observatory.rs b/server/src/test_world/rooms/decay_observatory.rs
index 7daef4412..152a88e50 100644
--- a/server/src/test_world/rooms/decay_observatory.rs
+++ b/server/src/test_world/rooms/decay_observatory.rs
@@ -1,8 +1,9 @@
 //! Decay Observatory — Room 13 (24x14)
 //!
-//! Tests D-041 (knowledge graph decay). Observer starts with Direct confidence
-//! on an NPC in LOS, then LOS is broken. Tick advances verify confidence
-//! degrades: Direct → KnowsDetails → KnowsOf → Suspects → Stale (D-041).
+//! Tests D-041 (knowledge graph decay). Observer sees an NPC (Direct), then
+//! LOS is broken (drops to KnowsDetails). Tick advances verify confidence
+//! degrades: KnowsDetails → KnowsOf → Suspects (floor). Separate test
+//! confirms Stale state triggers when age exceeds stale_after threshold.
 //!
 //! Decay runs once per game-minute (every 10 ticks, D-031). Confidence floor
 //! is Suspects — decay never removes an entity from the knowledge graph.