diff --git a/src/ui/effect.rs b/src/ui/effect.rs
index 65fa5b3..ac435e4 100644
--- a/src/ui/effect.rs
+++ b/src/ui/effect.rs
@@ -940,3 +940,610 @@ pub fn help_page_prev() -> Box<dyn Effect> {
 pub fn help_page_set(page: usize) -> Box<dyn Effect> {
     Box::new(HelpPageSet(page))
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::model::cell::{CellKey, CellValue};
+    use crate::model::Model;
+
+    fn test_app() -> App {
+        let mut m = Model::new("Test");
+        m.add_category("Type").unwrap();
+        m.add_category("Month").unwrap();
+        m.category_mut("Type").unwrap().add_item("Food");
+        m.category_mut("Type").unwrap().add_item("Clothing");
+        m.category_mut("Month").unwrap().add_item("Jan");
+        m.category_mut("Month").unwrap().add_item("Feb");
+        App::new(m, None)
+    }
+
+    // ── Model mutation effects ──────────────────────────────────────────
+
+    #[test]
+    fn add_category_effect() {
+        let mut app = test_app();
+        AddCategory("Region".to_string()).apply(&mut app);
+        assert!(app.model.category("Region").is_some());
+    }
+
+    #[test]
+    fn add_item_to_existing_category() {
+        let mut app = test_app();
+        AddItem {
+            category: "Type".to_string(),
+            item: "Electronics".to_string(),
+        }
+        .apply(&mut app);
+        let items: Vec<&str> = app
+            .model
+            .category("Type")
+            .unwrap()
+            .ordered_item_names()
+            .into_iter()
+            .collect();
+        assert!(items.contains(&"Electronics"));
+    }
+
+    #[test]
+    fn add_item_to_nonexistent_category_sets_status() {
+        let mut app = test_app();
+        AddItem {
+            category: "Nonexistent".to_string(),
+            item: "X".to_string(),
+        }
+        .apply(&mut app);
+        assert!(app.status_msg.contains("Unknown category"));
+    }
+
+    #[test]
+    fn set_cell_and_clear_cell() {
+        let mut app = test_app();
+        let key = CellKey::new(vec![
+            ("Type".into(), "Food".into()),
+            ("Month".into(), "Jan".into()),
+        ]);
+        SetCell(key.clone(), CellValue::Number(42.0)).apply(&mut app);
+        assert_eq!(app.model.get_cell(&key), Some(&CellValue::Number(42.0)));
+
+        ClearCell(key.clone()).apply(&mut app);
+        assert_eq!(app.model.get_cell(&key), None);
+    }
+
+    #[test]
+    fn add_formula_valid() {
+        let mut app = test_app();
+        AddFormula {
+            raw: "Clothing = Food * 2".to_string(),
+            target_category: "Type".to_string(),
+        }
+        .apply(&mut app);
+        assert!(!app.model.formulas().is_empty());
+    }
+
+    #[test]
+    fn add_formula_invalid_sets_error_status() {
+        let mut app = test_app();
+        AddFormula {
+            raw: "this is not valid".to_string(),
+            target_category: "Type".to_string(),
+        }
+        .apply(&mut app);
+        assert!(app.status_msg.contains("Formula error"));
+    }
+
+    #[test]
+    fn remove_formula_effect() {
+        let mut app = test_app();
+        AddFormula {
+            raw: "Clothing = Food * 2".to_string(),
+            target_category: "Type".to_string(),
+        }
+        .apply(&mut app);
+        assert!(!app.model.formulas().is_empty());
+        RemoveFormula {
+            target: "Clothing".to_string(),
+            target_category: "Type".to_string(),
+        }
+        .apply(&mut app);
+        assert!(app.model.formulas().is_empty());
+    }
+
+    // ── View effects ────────────────────────────────────────────────────
+
+    #[test]
+    fn switch_view_pushes_to_back_stack() {
+        let mut app = test_app();
+        app.model.create_view("View 2");
+        assert!(app.view_back_stack.is_empty());
+
+        SwitchView("View 2".to_string()).apply(&mut app);
+        assert_eq!(app.model.active_view.as_str(), "View 2");
+        assert_eq!(app.view_back_stack, vec!["Default".to_string()]);
+        // Forward stack should be cleared
+        assert!(app.view_forward_stack.is_empty());
+    }
+
+    #[test]
+    fn switch_view_to_same_does_not_push_stack() {
+        let mut app = test_app();
+        SwitchView("Default".to_string()).apply(&mut app);
+        assert!(app.view_back_stack.is_empty());
+    }
+
+    #[test]
+    fn view_back_and_forward() {
+        let mut app = test_app();
+        app.model.create_view("View 2");
+        SwitchView("View 2".to_string()).apply(&mut app);
+        assert_eq!(app.model.active_view.as_str(), "View 2");
+
+        // Go back
+        ViewBack.apply(&mut app);
+        assert_eq!(app.model.active_view.as_str(), "Default");
+        assert_eq!(app.view_forward_stack, vec!["View 2".to_string()]);
+        assert!(app.view_back_stack.is_empty());
+
+        // Go forward
+        ViewForward.apply(&mut app);
+        assert_eq!(app.model.active_view.as_str(), "View 2");
+        assert_eq!(app.view_back_stack, vec!["Default".to_string()]);
+        assert!(app.view_forward_stack.is_empty());
+    }
+
+    #[test]
+    fn view_back_with_empty_stack_is_noop() {
+        let mut app = test_app();
+        let before = app.model.active_view.clone();
+        ViewBack.apply(&mut app);
+        assert_eq!(app.model.active_view, before);
+    }
+
+    #[test]
+    fn create_and_delete_view() {
+        let mut app = test_app();
+        CreateView("View 2".to_string()).apply(&mut app);
+        assert!(app.model.views.contains_key("View 2"));
+
+        DeleteView("View 2".to_string()).apply(&mut app);
+        assert!(!app.model.views.contains_key("View 2"));
+    }
+
+    #[test]
+    fn set_axis_effect() {
+        let mut app = test_app();
+        SetAxis {
+            category: "Type".to_string(),
+            axis: Axis::Page,
+        }
+        .apply(&mut app);
+        assert_eq!(app.model.active_view().axis_of("Type"), Axis::Page);
+    }
+
+    #[test]
+    fn transpose_axes_effect() {
+        let mut app = test_app();
+        let row_before: Vec<String> = app.model.active_view().categories_on(Axis::Row)
+            .into_iter().map(String::from).collect();
+        let col_before: Vec<String> = app.model.active_view().categories_on(Axis::Column)
+            .into_iter().map(String::from).collect();
+        TransposeAxes.apply(&mut app);
+        let row_after: Vec<String> = app.model.active_view().categories_on(Axis::Row)
+            .into_iter().map(String::from).collect();
+        let col_after: Vec<String> = app.model.active_view().categories_on(Axis::Column)
+            .into_iter().map(String::from).collect();
+        assert_eq!(row_before, col_after);
+        assert_eq!(col_before, row_after);
+    }
+
+    // ── Navigation effects ──────────────────────────────────────────────
+
+    #[test]
+    fn set_selected_effect() {
+        let mut app = test_app();
+        SetSelected(3, 5).apply(&mut app);
+        assert_eq!(app.model.active_view().selected, (3, 5));
+    }
+
+    #[test]
+    fn set_row_and_col_offset() {
+        let mut app = test_app();
+        SetRowOffset(10).apply(&mut app);
+        SetColOffset(5).apply(&mut app);
+        assert_eq!(app.model.active_view().row_offset, 10);
+        assert_eq!(app.model.active_view().col_offset, 5);
+    }
+
+    // ── App state effects ───────────────────────────────────────────────
+
+    #[test]
+    fn change_mode_effect() {
+        let mut app = test_app();
+        assert!(ChangeMode(AppMode::Help).changes_mode());
+        ChangeMode(AppMode::Help).apply(&mut app);
+        assert_eq!(app.mode, AppMode::Help);
+    }
+
+    #[test]
+    fn set_status_effect() {
+        let mut app = test_app();
+        SetStatus("hello".to_string()).apply(&mut app);
+        assert_eq!(app.status_msg, "hello");
+    }
+
+    #[test]
+    fn mark_dirty_effect() {
+        let mut app = test_app();
+        assert!(!app.dirty);
+        MarkDirty.apply(&mut app);
+        assert!(app.dirty);
+    }
+
+    #[test]
+    fn set_yanked_effect() {
+        let mut app = test_app();
+        SetYanked(Some(CellValue::Number(42.0))).apply(&mut app);
+        assert_eq!(app.yanked, Some(CellValue::Number(42.0)));
+    }
+
+    #[test]
+    fn set_search_query_and_mode() {
+        let mut app = test_app();
+        SetSearchQuery("foo".to_string()).apply(&mut app);
+        assert_eq!(app.search_query, "foo");
+        SetSearchMode(true).apply(&mut app);
+        assert!(app.search_mode);
+        SetSearchMode(false).apply(&mut app);
+        assert!(!app.search_mode);
+    }
+
+    // ── SetBuffer special behavior ──────────────────────────────────────
+
+    #[test]
+    fn set_buffer_normal_key() {
+        let mut app = test_app();
+        SetBuffer {
+            name: "edit".to_string(),
+            value: "hello".to_string(),
+        }
+        .apply(&mut app);
+        assert_eq!(app.buffers.get("edit").unwrap(), "hello");
+    }
+
+    #[test]
+    fn set_buffer_search_writes_to_search_query() {
+        let mut app = test_app();
+        SetBuffer {
+            name: "search".to_string(),
+            value: "query".to_string(),
+        }
+        .apply(&mut app);
+        // "search" buffer is special — writes to app.search_query
+        assert_eq!(app.search_query, "query");
+    }
+
+    // ── Panel effects ───────────────────────────────────────────────────
+
+    #[test]
+    fn set_panel_open_and_cursor() {
+        let mut app = test_app();
+        SetPanelOpen {
+            panel: Panel::Formula,
+            open: true,
+        }
+        .apply(&mut app);
+        assert!(app.formula_panel_open);
+
+        SetPanelCursor {
+            panel: Panel::Formula,
+            cursor: 3,
+        }
+        .apply(&mut app);
+        assert_eq!(app.formula_cursor, 3);
+
+        SetPanelOpen {
+            panel: Panel::Category,
+            open: true,
+        }
+        .apply(&mut app);
+        assert!(app.category_panel_open);
+
+        SetPanelOpen {
+            panel: Panel::View,
+            open: true,
+        }
+        .apply(&mut app);
+        assert!(app.view_panel_open);
+    }
+
+    #[test]
+    fn set_tile_cat_idx_effect() {
+        let mut app = test_app();
+        SetTileCatIdx(2).apply(&mut app);
+        assert_eq!(app.tile_cat_idx, 2);
+    }
+
+    // ── Help page effects ───────────────────────────────────────────────
+
+    #[test]
+    fn help_page_navigation() {
+        let mut app = test_app();
+        assert_eq!(app.help_page, 0);
+        HelpPageNext.apply(&mut app);
+        assert_eq!(app.help_page, 1);
+        HelpPageNext.apply(&mut app);
+        assert_eq!(app.help_page, 2);
+        HelpPagePrev.apply(&mut app);
+        assert_eq!(app.help_page, 1);
+        HelpPageSet(0).apply(&mut app);
+        assert_eq!(app.help_page, 0);
+    }
+
+    #[test]
+    fn help_page_prev_clamps_at_zero() {
+        let mut app = test_app();
+        HelpPagePrev.apply(&mut app);
+        assert_eq!(app.help_page, 0);
+    }
+
+    // ── Drill effects ───────────────────────────────────────────────────
+
+    #[test]
+    fn start_drill_and_apply_clear_drill_with_no_edits() {
+        let mut app = test_app();
+        let key = CellKey::new(vec![
+            ("Type".into(), "Food".into()),
+            ("Month".into(), "Jan".into()),
+        ]);
+        let records = vec![(key, CellValue::Number(42.0))];
+        StartDrill(records).apply(&mut app);
+        assert!(app.drill_state.is_some());
+
+        // Apply with no pending edits — should just clear state
+        ApplyAndClearDrill.apply(&mut app);
+        assert!(app.drill_state.is_none());
+        assert!(!app.dirty); // no edits → not dirty
+    }
+
+    #[test]
+    fn apply_and_clear_drill_with_value_edit() {
+        let mut app = test_app();
+        let key = CellKey::new(vec![
+            ("Type".into(), "Food".into()),
+            ("Month".into(), "Jan".into()),
+        ]);
+        // Set original cell
+        app.model
+            .set_cell(key.clone(), CellValue::Number(42.0));
+
+        let records = vec![(key.clone(), CellValue::Number(42.0))];
+        StartDrill(records).apply(&mut app);
+
+        // Stage a pending edit: change value at record 0
+        SetDrillPendingEdit {
+            record_idx: 0,
+            col_name: "Value".to_string(),
+            new_value: "99".to_string(),
+        }
+        .apply(&mut app);
+
+        ApplyAndClearDrill.apply(&mut app);
+        assert!(app.drill_state.is_none());
+        assert!(app.dirty);
+        assert_eq!(app.model.get_cell(&key), Some(&CellValue::Number(99.0)));
+    }
+
+    #[test]
+    fn apply_and_clear_drill_with_coord_rename() {
+        let mut app = test_app();
+        let key = CellKey::new(vec![
+            ("Type".into(), "Food".into()),
+            ("Month".into(), "Jan".into()),
+        ]);
+        app.model
+            .set_cell(key.clone(), CellValue::Number(42.0));
+
+        let records = vec![(key.clone(), CellValue::Number(42.0))];
+        StartDrill(records).apply(&mut app);
+
+        // Rename "Type" coord from "Food" to "Drink"
+        SetDrillPendingEdit {
+            record_idx: 0,
+            col_name: "Type".to_string(),
+            new_value: "Drink".to_string(),
+        }
+        .apply(&mut app);
+
+        ApplyAndClearDrill.apply(&mut app);
+        assert!(app.dirty);
+        // Old cell should be gone
+        assert_eq!(app.model.get_cell(&key), None);
+        // New cell should exist
+        let new_key = CellKey::new(vec![
+            ("Type".into(), "Drink".into()),
+            ("Month".into(), "Jan".into()),
+        ]);
+        assert_eq!(
+            app.model.get_cell(&new_key),
+            Some(&CellValue::Number(42.0))
+        );
+        // "Drink" should have been added as an item
+        let items: Vec<&str> = app
+            .model
+            .category("Type")
+            .unwrap()
+            .ordered_item_names()
+            .into_iter()
+            .collect();
+        assert!(items.contains(&"Drink"));
+    }
+
+    #[test]
+    fn apply_and_clear_drill_empty_value_clears_cell() {
+        let mut app = test_app();
+        let key = CellKey::new(vec![
+            ("Type".into(), "Food".into()),
+            ("Month".into(), "Jan".into()),
+        ]);
+        app.model
+            .set_cell(key.clone(), CellValue::Number(42.0));
+
+        let records = vec![(key.clone(), CellValue::Number(42.0))];
+        StartDrill(records).apply(&mut app);
+
+        // Edit value to empty string → should clear cell
+        SetDrillPendingEdit {
+            record_idx: 0,
+            col_name: "Value".to_string(),
+            new_value: "".to_string(),
+        }
+        .apply(&mut app);
+
+        ApplyAndClearDrill.apply(&mut app);
+        assert_eq!(app.model.get_cell(&key), None);
+    }
+
+    // ── Toggle effects ──────────────────────────────────────────────────
+
+    #[test]
+    fn toggle_prune_empty_effect() {
+        let mut app = test_app();
+        let before = app.model.active_view().prune_empty;
+        TogglePruneEmpty.apply(&mut app);
+        assert_ne!(app.model.active_view().prune_empty, before);
+        TogglePruneEmpty.apply(&mut app);
+        assert_eq!(app.model.active_view().prune_empty, before);
+    }
+
+    #[test]
+    fn toggle_cat_expand_effect() {
+        let mut app = test_app();
+        assert!(!app.expanded_cats.contains("Type"));
+        ToggleCatExpand("Type".to_string()).apply(&mut app);
+        assert!(app.expanded_cats.contains("Type"));
+        ToggleCatExpand("Type".to_string()).apply(&mut app);
+        assert!(!app.expanded_cats.contains("Type"));
+    }
+
+    #[test]
+    fn remove_item_and_category() {
+        let mut app = test_app();
+        RemoveItem {
+            category: "Type".to_string(),
+            item: "Food".to_string(),
+        }
+        .apply(&mut app);
+        let items: Vec<&str> = app
+            .model
+            .category("Type")
+            .unwrap()
+            .ordered_item_names()
+            .into_iter()
+            .collect();
+        assert!(!items.contains(&"Food"));
+
+        RemoveCategory("Month".to_string()).apply(&mut app);
+        assert!(app.model.category("Month").is_none());
+    }
+
+    // ── Number format ───────────────────────────────────────────────────
+
+    #[test]
+    fn set_number_format_effect() {
+        let mut app = test_app();
+        SetNumberFormat(",.2f".to_string()).apply(&mut app);
+        assert_eq!(app.model.active_view().number_format, ",.2f");
+    }
+
+    // ── Page selection ──────────────────────────────────────────────────
+
+    #[test]
+    fn set_page_selection_effect() {
+        let mut app = test_app();
+        SetPageSelection {
+            category: "Type".to_string(),
+            item: "Food".to_string(),
+        }
+        .apply(&mut app);
+        assert_eq!(
+            app.model.active_view().page_selection("Type"),
+            Some("Food")
+        );
+    }
+
+    // ── Hide/show items ─────────────────────────────────────────────────
+
+    #[test]
+    fn hide_and_show_item_effects() {
+        let mut app = test_app();
+        HideItem {
+            category: "Type".to_string(),
+            item: "Food".to_string(),
+        }
+        .apply(&mut app);
+        assert!(app.model.active_view().is_hidden("Type", "Food"));
+
+        ShowItem {
+            category: "Type".to_string(),
+            item: "Food".to_string(),
+        }
+        .apply(&mut app);
+        assert!(!app.model.active_view().is_hidden("Type", "Food"));
+    }
+
+    // ── Toggle group ────────────────────────────────────────────────────
+
+    #[test]
+    fn toggle_group_effect() {
+        let mut app = test_app();
+        ToggleGroup {
+            category: "Type".to_string(),
+            group: "MyGroup".to_string(),
+        }
+        .apply(&mut app);
+        assert!(
+            app.model
+                .active_view()
+                .is_group_collapsed("Type", "MyGroup")
+        );
+        ToggleGroup {
+            category: "Type".to_string(),
+            group: "MyGroup".to_string(),
+        }
+        .apply(&mut app);
+        assert!(
+            !app.model
+                .active_view()
+                .is_group_collapsed("Type", "MyGroup")
+        );
+    }
+
+    // ── Cycle axis ──────────────────────────────────────────────────────
+
+    #[test]
+    fn cycle_axis_effect() {
+        let mut app = test_app();
+        let before = app.model.active_view().axis_of("Type");
+        CycleAxis("Type".to_string()).apply(&mut app);
+        let after = app.model.active_view().axis_of("Type");
+        assert_ne!(before, after);
+    }
+
+    // ── Save without file path ──────────────────────────────────────────
+
+    #[test]
+    fn save_without_file_path_shows_status() {
+        let mut app = test_app();
+        Save.apply(&mut app);
+        assert!(app.status_msg.contains("No file path"));
+    }
+
+    // ── Panel mode helper ───────────────────────────────────────────────
+
+    #[test]
+    fn panel_mode_mapping() {
+        assert_eq!(Panel::Formula.mode(), AppMode::FormulaPanel);
+        assert_eq!(Panel::Category.mode(), AppMode::CategoryPanel);
+        assert_eq!(Panel::View.mode(), AppMode::ViewPanel);
+    }
+}