diff --git a/.project/designs/app/src/pages/WorldGen/Lab.tsx b/.project/designs/app/src/pages/WorldGen/Lab.tsx
index 86c99b6a..e2baeb89 100644
--- a/.project/designs/app/src/pages/WorldGen/Lab.tsx
+++ b/.project/designs/app/src/pages/WorldGen/Lab.tsx
@@ -275,8 +275,8 @@ function renderCanvas(ctx: CanvasRenderingContext2D, grid: GridCell[][], st: Lab
       if (st.substrateOn && wasmGrid !== null) {
         const raw = wasmGrid.tileSubstrateJson(col, row);
         if (raw) {
-          const tile = JSON.parse(raw) as { substrate_id: string };
-          const [r, g, b] = substrateRgba(tile.substrate_id);
+          const tile = JSON.parse(raw) as { id: string };
+          const [r, g, b] = substrateRgba(tile.id);
           baseColor = [r, g, b];
         }
       }
diff --git a/.project/designs/app/src/pages/WorldGen/Substrate.tsx b/.project/designs/app/src/pages/WorldGen/Substrate.tsx
index e5ea521f..b88665a1 100644
--- a/.project/designs/app/src/pages/WorldGen/Substrate.tsx
+++ b/.project/designs/app/src/pages/WorldGen/Substrate.tsx
@@ -30,6 +30,9 @@ const SUBSTRATE_LEGEND: { id: string; label: string; color: string }[] = [
   { id: "lava",       label: "lava",       color: "rgba(220,80,30,0.95)" },
 ];
 
+interface TileSubstrate { id: string }
+interface TileFloraCover { id: string; biome_label: string }
+
 type TileInfo = { substrate_id: string; flora_cover_id: string; biome_label_id: string } | null;
 
 function SubstrateCanvas({ grid }: { grid: import("../../../../../../.local/build/wasm/magic_civ_physics").WasmGrid }): React.ReactElement {
@@ -54,7 +57,7 @@ function SubstrateCanvas({ grid }: { grid: import("../../../../../../.local/buil
       for (let col = 0; col < w; col++) {
         const raw = grid.tileSubstrateJson(col, row);
         if (!raw) continue;
-        const tile = JSON.parse(raw) as NonNullable<TileInfo>;
+        const tile = JSON.parse(raw) as TileSubstrate;
         const { x: cx, y: cy } = hexToPixel(col, row, size);
         const corners = hexCorners(cx, cy, size);
 
@@ -62,7 +65,7 @@ function SubstrateCanvas({ grid }: { grid: import("../../../../../../.local/buil
         ctx.moveTo(corners[0][0], corners[0][1]);
         for (let i = 1; i < 6; i++) ctx.lineTo(corners[i][0], corners[i][1]);
         ctx.closePath();
-        ctx.fillStyle = SUBSTRATE_COLORS[tile.substrate_id] ?? "rgba(180,180,180,0.7)";
+        ctx.fillStyle = SUBSTRATE_COLORS[tile.id] ?? "rgba(180,180,180,0.7)";
         ctx.fill();
         ctx.strokeStyle = "rgba(0,0,0,0.18)";
         ctx.lineWidth = 0.5;
@@ -86,8 +89,16 @@ function SubstrateCanvas({ grid }: { grid: import("../../../../../../.local/buil
     const c = Math.max(0, Math.min(grid.width - 1, col));
     const row = Math.round((py - (c % 2 === 1 ? h_step / 2 : 0) - h_step / 2) / h_step);
     const r = Math.max(0, Math.min(grid.height - 1, row));
-    const raw = grid.tileSubstrateJson(c, r);
-    setHoveredTile(raw ? JSON.parse(raw) as NonNullable<TileInfo> : null);
+    const rawSub = grid.tileSubstrateJson(c, r);
+    const rawFlora = grid.tileFloraCoverJson(c, r);
+    if (!rawSub) { setHoveredTile(null); return; }
+    const sub = JSON.parse(rawSub) as TileSubstrate;
+    const flora = rawFlora ? JSON.parse(rawFlora) as TileFloraCover : null;
+    setHoveredTile({
+      substrate_id: sub.id,
+      flora_cover_id: flora?.id ?? "",
+      biome_label_id: flora?.biome_label ?? "",
+    });
   }
 
   return (
diff --git a/src/simulator/crates/mc-mapgen/src/lib.rs b/src/simulator/crates/mc-mapgen/src/lib.rs
index 2fdb1ccb..f0e8c819 100644
--- a/src/simulator/crates/mc-mapgen/src/lib.rs
+++ b/src/simulator/crates/mc-mapgen/src/lib.rs
@@ -951,6 +951,7 @@ fn to_grid_state(gen_tiles: &HashMap<(i32, i32), GenTile>, w: i32, h: i32) -> Gr
                 tile.moisture = gt.moisture;
                 tile.elevation = gt.elevation;
                 tile.biome_id = gt.biome_id.clone();
+                tile.substrate_id = biome_to_substrate(&gt.biome_id).to_string();
                 tile.wind_direction = gt.wind_direction;
                 tile.wind_speed = gt.wind_speed;
                 tile.quality = gt.quality;
diff --git a/src/simulator/crates/mc-mapgen/tests/substrate_migration.rs b/src/simulator/crates/mc-mapgen/tests/substrate_migration.rs
index a6bee0d0..6209eed0 100644
--- a/src/simulator/crates/mc-mapgen/tests/substrate_migration.rs
+++ b/src/simulator/crates/mc-mapgen/tests/substrate_migration.rs
@@ -90,3 +90,21 @@ fn substrate_migration_coarse_distribution() {
         "{unpopulated_sub} tiles have empty substrate_id after generate()"
     );
 }
+
+#[test]
+fn substrate_id_populated_after_generate() {
+    let gen = MapGenerator::new("{}");
+    let grid = gen.generate(42, "tiny");
+    let substrate_empty = grid.tiles.iter().filter(|t| t.substrate_id.is_empty()).count();
+    assert_eq!(substrate_empty, 0, "{substrate_empty} tiles have empty substrate_id after generate");
+}
+
+#[test]
+fn substrate_id_deterministic() {
+    let gen = MapGenerator::new("{}");
+    let g1 = gen.generate(42, "tiny");
+    let g2 = gen.generate(42, "tiny");
+    for (i, (t1, t2)) in g1.tiles.iter().zip(g2.tiles.iter()).enumerate() {
+        assert_eq!(t1.substrate_id, t2.substrate_id, "substrate_id differs at tile {i}");
+    }
+}