diff --git a/frontend/src/components/editor/EditorToolbar.tsx b/frontend/src/components/editor/EditorToolbar.tsx
index f7a0c888..b69aecd0 100644
--- a/frontend/src/components/editor/EditorToolbar.tsx
+++ b/frontend/src/components/editor/EditorToolbar.tsx
@@ -529,6 +529,26 @@ export const EditorToolbar = ({
     blog('info', `Starting compilation for ${boardLabel} (${fqbn})...`);
 
     try {
+      // Reconcile the two "active group" pointers before reading sources.
+      // If the editor drifted to another BOARD's group (dangling pointer
+      // after a board delete/add, a restore race), Monaco edits — and the
+      // agent's write_file calls — land in a group this compile silently
+      // ignores, shipping a stale same-named sketch.ino instead of the code
+      // on screen. Re-point the editor at the group being compiled. Chip
+      // program groups are legitimate cross-edits and stay untouched.
+      const edGroup = useEditorStore.getState().activeGroupId;
+      if (
+        activeBoard?.activeFileGroupId &&
+        edGroup !== activeBoard.activeFileGroupId &&
+        !edGroup.startsWith('group-chip-')
+      ) {
+        blog(
+          'warning',
+          `Editor file group (${edGroup}) diverged from the compiled board group ` +
+            `(${activeBoard.activeFileGroupId}) — switching the editor to the compiled group.`,
+        );
+        useEditorStore.getState().setActiveGroup(activeBoard.activeFileGroupId);
+      }
       const groupFiles = activeBoard?.activeFileGroupId
         ? useEditorStore.getState().getGroupFiles(activeBoard.activeFileGroupId)
         : files;
diff --git a/frontend/src/store/useEditorStore.ts b/frontend/src/store/useEditorStore.ts
index 9013ca1b..6b69fca5 100644
--- a/frontend/src/store/useEditorStore.ts
+++ b/frontend/src/store/useEditorStore.ts
@@ -517,11 +517,19 @@ export const useEditorStore = create<EditorState>((set, get) => ({
       const { [groupId]: _a, ...restActive } = s.activeGroupFileId;
       const { [groupId]: _o, ...restOpen } = s.openGroupFileIds;
       const { [groupId]: _f, ...restFolders } = s.folderGroups;
+      // Never leave activeGroupId dangling at the deleted group: Monaco (and
+      // every agent write_file) keeps targeting it, while compile reads the
+      // board's group — two same-named sketch.ino files silently diverge and
+      // the build ships the WRONG one (the "leftover template Blink compiled
+      // instead of my code" class). Re-point at any surviving group.
+      const nextActive =
+        s.activeGroupId === groupId ? Object.keys(rest)[0] ?? '' : s.activeGroupId;
       return {
         fileGroups: rest,
         activeGroupFileId: restActive,
         openGroupFileIds: restOpen,
         folderGroups: restFolders,
+        activeGroupId: nextActive,
       };
     });
   },
diff --git a/frontend/src/utils/projectPayload.ts b/frontend/src/utils/projectPayload.ts
index 5715640a..2e158d41 100644
--- a/frontend/src/utils/projectPayload.ts
+++ b/frontend/src/utils/projectPayload.ts
@@ -38,7 +38,12 @@ function serialisableBoard(b: BoardInstance) {
     activeFileGroupId: b.activeFileGroupId,
     languageMode: b.languageMode,
     serialBaudRate: b.serialBaudRate,
-    compiledProgram: b.compiledProgram,
+    // compiledProgram is deliberately NOT persisted. The firmware blob was
+    // 99.99% of a typical ESP32 save payload (~1.04 MB base64 for a bare
+    // board), pushing every real project against the request-size ceiling,
+    // and a restored binary is stale the moment the code differs from what
+    // was compiled. Sources are saved; the binary is recompiled on demand.
+    // (Loaders still accept compiledProgram from old rows.)
     // ESP32 board options + uploaded SPIFFS files. Optional, only present
     // after the user has opened Board Options... at least once. Both ride
     // inside boards_json so there's no DB migration.