diff --git a/frontend/src/__tests__/multi-board-integration.test.ts b/frontend/src/__tests__/multi-board-integration.test.ts
index 8f420305..ccbb93d4 100644
--- a/frontend/src/__tests__/multi-board-integration.test.ts
+++ b/frontend/src/__tests__/multi-board-integration.test.ts
@@ -301,6 +301,30 @@ describe('useSimulatorStore — multi-board', () => {
     expect(boards.find((b) => b.id === id)).toBeUndefined();
   });
 
+  it('delete-all then addBoard re-points the editor file group at the new board', () => {
+    // Regression: deleting the default Uno and adding a different board (e.g. a
+    // Pico) via the canvas picker left the editor editing the deleted board's
+    // file group while compile read the NEW board's (default) group — so code
+    // typed into the editor was silently dropped and the board ran its default
+    // sketch. addBoard/removeBoard must keep the editor's active group in sync
+    // with the active board.
+    const sim = useSimulatorStore.getState();
+    sim.boards.map((b) => b.id).forEach((bid) => sim.removeBoard(bid));
+    const picoId = useSimulatorStore.getState().addBoard('raspberry-pi-pico', 0, 0);
+
+    expect(useSimulatorStore.getState().activeBoardId).toBe(picoId);
+    const board = useSimulatorStore.getState().boards.find((b) => b.id === picoId)!;
+    expect(board.activeFileGroupId).toBe(`group-${picoId}`);
+    // The editor's active group must follow the active board.
+    expect(useEditorStore.getState().activeGroupId).toBe(board.activeFileGroupId);
+
+    // And editing the active file lands in the group that compile reads.
+    const editor = useEditorStore.getState();
+    editor.setFileContent(editor.activeFileId, '// PICO SKETCH MARKER');
+    const groupFiles = useEditorStore.getState().getGroupFiles(board.activeFileGroupId);
+    expect(groupFiles.some((f) => f.content.includes('PICO SKETCH MARKER'))).toBe(true);
+  });
+
   it('setActiveBoardId switches legacy flat fields', () => {
     const { addBoard, setActiveBoardId } = useSimulatorStore.getState();
     addBoard('arduino-mega', 0, 0);
diff --git a/frontend/src/store/useSimulatorStore.ts b/frontend/src/store/useSimulatorStore.ts
index 99b781f0..a60b26a4 100644
--- a/frontend/src/store/useSimulatorStore.ts
+++ b/frontend/src/store/useSimulatorStore.ts
@@ -1252,6 +1252,16 @@ export const useSimulatorStore = create<SimulatorState>((set, get) => {
       });
       // Create the editor file group for this board
       useEditorStore.getState().createFileGroup(`group-${id}`);
+      // If this board is now the active one (it's the first board, or the
+      // previously-active board was removed), point the editor at its file
+      // group too. The canvas board picker calls addBoard directly WITHOUT
+      // setActiveBoardId (which is the only other place that syncs the editor
+      // group), so without this the editor keeps editing the previous/deleted
+      // board's group while compile reads THIS board's group — the code you
+      // type is silently dropped and the board runs its default sketch.
+      if (get().activeBoardId === id) {
+        useEditorStore.getState().setActiveGroup(`group-${id}`);
+      }
       // Init VFS for Raspberry Pi 3 boards
       if (isPiBoardKind(boardKind)) {
         useVfsStore.getState().initBoardVfs(id);
@@ -1313,6 +1323,15 @@ export const useSimulatorStore = create<SimulatorState>((set, get) => {
       if (board) {
         useEditorStore.getState().deleteFileGroup(board.activeFileGroupId);
       }
+      // The removed board may have been the active one; activeBoardId was just
+      // reassigned (above) to a remaining board, or null. Re-point the editor's
+      // active file group at whatever board is active now, so the editor never
+      // keeps showing/editing the deleted board's group.
+      const newActiveId = get().activeBoardId;
+      if (newActiveId) {
+        const nb = get().boards.find((b) => b.id === newActiveId);
+        if (nb) useEditorStore.getState().setActiveGroup(nb.activeFileGroupId);
+      }
       // ── Interconnect: drop board and rebuild routes ──────────────────
       icUnbindBoard(boardId);
       icUpdateWires(get().wires);