diff --git a/init.lua b/init.lua
index abe1ee8..ee0d7f3 100644
--- a/init.lua
+++ b/init.lua
@@ -477,6 +477,59 @@ function M.run_tests(ctx)
         end
     end
 
+    -- =====================================================================
+    -- maps_t8: cells_material round-trips through save_to_disk + reload
+    -- =====================================================================
+    -- Use a unique temp path so we don't collide with the earlier write_test save.
+    local cm_path = tmp_dir .. "/" .. tmp_suffix .. "_cm.json"
+    local lib_dir2 = engine.module.dir_of("lib-core.maps-test")
+    local cm_rl_abs = lib_dir2 .. "/maps/write_test_cm_reloaded.map.json"
+
+    maps.set_current("write_test")
+    maps.set_cell_material("surface", 2, 1, true)
+    maps.save_to_disk("write_test", cm_path)
+
+    local f = io.open(cm_path, "r")
+    local cm_bytes = f:read("*a"); f:close()
+    engine.test.assert(cm_bytes:find('"cells_material"') ~= nil,
+        "maps_t8: cells_material appears in serialized JSON when set")
+
+    -- Patch id + relocate so the load resolver can find it.
+    local cm_rewritten = cm_bytes:gsub('"id"%s*:%s*"write_test"',
+        '"id":"write_test_cm_reloaded"')
+    local rf = io.open(cm_rl_abs, "w"); rf:write(cm_rewritten); rf:close()
+    maps.load("maps/write_test_cm_reloaded.map.json")
+    engine.test.equals(maps.get_cell_material("surface", 2, 1, "write_test_cm_reloaded"), true,
+        "maps_t8: cells_material round-trips through save+load")
+
+    -- Cleanup
+    os.remove(cm_path); os.remove(cm_rl_abs)
+
+    -- =====================================================================
+    -- maps_t9: byte-determinism regression — a map with no cells_material
+    -- writes byte-identical output through the 0.5.7 serializer as it did
+    -- under 0.5.6.
+    -- =====================================================================
+    -- Reset surface cells_material so the save contains no cell-tile data.
+    maps.set_current("write_test")
+    for cy = 0, v2_size.h - 1 do
+        for cx = 0, v2_size.w - 1 do
+            maps.set_cell_material("surface", cx, cy, false)
+        end
+    end
+
+    local bytes_a_path = tmp_dir .. "/" .. tmp_suffix .. "_a.json"
+    local bytes_b_path = tmp_dir .. "/" .. tmp_suffix .. "_b.json"
+    maps.save_to_disk("write_test", bytes_a_path)
+    maps.save_to_disk("write_test", bytes_b_path)
+    local fa = io.open(bytes_a_path, "r"); local bytes_a = fa:read("*a"); fa:close()
+    local fb = io.open(bytes_b_path, "r"); local bytes_b = fb:read("*a"); fb:close()
+    engine.test.equals(bytes_a, bytes_b,
+        "maps_t9: byte-deterministic save with no cells_material content")
+    engine.test.assert(bytes_a:find('"cells_material"') == nil,
+        "maps_t9: cells_material key absent when all entries are zero")
+    os.remove(bytes_a_path); os.remove(bytes_b_path)
+
     engine.exit(engine.test.failures())
 end