diff --git a/init.lua b/init.lua
index 4774566..12d67e0 100644
--- a/init.lua
+++ b/init.lua
@@ -305,6 +305,19 @@ local actions = (function()
         state.mark_dirty()
     end
 
+    -- 0.3.1: Cell-Tile mode paint actions. Write the per-cell material
+    -- flag via lib-core.maps; the renderer's 47-blob branch picks the
+    -- slot.
+    function M.paint_cell_material_at(cell_x, cell_y)
+        maps.set_cell_material(state.get_active_layer(), cell_x, cell_y, true)
+        state.mark_dirty()
+    end
+
+    function M.erase_cell_material_at(cell_x, cell_y)
+        maps.set_cell_material(state.get_active_layer(), cell_x, cell_y, false)
+        state.mark_dirty()
+    end
+
     -- =====================================================================
     -- Hotkey-driven actions
     -- =====================================================================
@@ -1586,6 +1599,21 @@ local function self_test()
     engine.test.equals(state.get_mode(), "direct", "t12: cycle cell -> direct")
     state.cycle_mode()
     engine.test.equals(state.get_mode(), "auto-tile", "t12: cycle direct -> auto")
+
+    -- t10: paint_cell_material_at writes the flag readable by
+    -- maps.get_cell_material; cell-tile mode dispatches LMB to it.
+    state.set_mode("cell-tile")
+    local layer = state.get_active_layer()
+    -- Clear first to avoid pollution from earlier tests
+    if maps.get_cell_material(layer, 5, 5) then
+        maps.set_cell_material(layer, 5, 5, false)
+    end
+    actions.paint_cell_material_at(5, 5)
+    engine.test.equals(maps.get_cell_material(layer, 5, 5), true,
+        "t10: paint_cell_material_at sets the flag")
+    actions.erase_cell_material_at(5, 5)
+    engine.test.equals(maps.get_cell_material(layer, 5, 5), false,
+        "t10: erase_cell_material_at clears the flag")
 end
 
 -- =====================================================================
@@ -1723,6 +1751,15 @@ function update(ctx, dt)
                 if snap then
                     actions.paint_vertex_at(snap.vx, snap.vy, dp == "paint")
                 end
+            elseif state.get_mode() == "cell-tile" then
+                local cell = state.get_mouse_cell()
+                if cell then
+                    if dp == "paint" then
+                        actions.paint_cell_material_at(cell.x, cell.y)
+                    else
+                        actions.erase_cell_material_at(cell.x, cell.y)
+                    end
+                end
             else
                 local cell = state.get_mouse_cell()
                 if cell then