diff --git a/backend/app/api/routes/compile.py b/backend/app/api/routes/compile.py
index 7f236e8d..04243bb8 100644
--- a/backend/app/api/routes/compile.py
+++ b/backend/app/api/routes/compile.py
@@ -205,6 +205,7 @@ async def _run_compile(
     request: CompileRequest,
     files: list[dict[str, str]],
     progress_callback: Any = None,
+    requester_id: str | None = None,
 ) -> CompileResponse:
     """Do the actual compile (ESP-IDF for esp32:*, arduino-cli otherwise).
 
@@ -234,10 +235,13 @@ async def _run_compile(
             project_libs = await get_project_libraries(request.project_id)
             if project_libs:
                 allowed_libraries = set(project_libs)
-        # P2.2 — the OWNER of the project (not the requester) owns any per-user
-        # custom libraries the manifest references, so resolve the owner so the
-        # scope materializer can find them. None for unsaved/anon compiles.
+        # P2.2 — resolve whose per-user custom libraries the manifest may
+        # reference: the project OWNER for a saved project (so a shared/embed
+        # compile finds that owner's libs), else the REQUESTER for an unsaved
+        # compile (the libs they just uploaded are their own). None for anon.
         owner_id = await get_project_owner(request.project_id)
+        if owner_id is None:
+            owner_id = requester_id
         result = await espidf_compiler.compile(
             files, request.board_fqbn,
             progress_callback=progress_callback,
@@ -368,6 +372,7 @@ async def _compile_job(
                 COMPILE_JOBS[job_id]["state"] = "running"
                 response = await _run_compile(
                     request, files, progress_callback=on_progress_line,
+                    requester_id=user_id,
                 )
         COMPILE_JOBS[job_id] = {
             "state": "done",
@@ -440,7 +445,7 @@ async def compile_sketch(
     files = _resolve_files(request)
     started = time.monotonic()
     try:
-        response = await _run_compile(request, files)
+        response = await _run_compile(request, files, requester_id=user_id)
     except Exception as e:
         await record_compile(
             user_id=user_id,
diff --git a/frontend/src/components/simulator/LibraryManagerModal.tsx b/frontend/src/components/simulator/LibraryManagerModal.tsx
index 606bc84b..d94719e4 100644
--- a/frontend/src/components/simulator/LibraryManagerModal.tsx
+++ b/frontend/src/components/simulator/LibraryManagerModal.tsx
@@ -256,6 +256,22 @@ export const LibraryManagerModal: React.FC<LibraryManagerModalProps> = ({ isOpen
     return () => window.removeEventListener('velxio-open-library-manager', toProject);
   }, []);
 
+  // P2.2 — a custom .zip upload lands in the user's PER-USER store (not the
+  // shared dir), so auto-declare it on the active board (velxio.json) and show
+  // the Project tab; the compile then resolves it via the owner per-user path.
+  useEffect(() => {
+    const onUploaded = (e: Event) => {
+      const name = (e as CustomEvent).detail?.library;
+      if (name) {
+        addToManifest(name);
+        setActiveTab('project');
+      }
+      fetchInstalled();
+    };
+    window.addEventListener('velxio-custom-library-installed', onUploaded);
+    return () => window.removeEventListener('velxio-custom-library-installed', onUploaded);
+  }, [addToManifest, fetchInstalled]);
+
   // Keep the raw velxio.json draft in sync with the manifest while not editing.
   useEffect(() => {
     setJsonDraft(JSON.stringify({ libraries: manifestLibs ?? [] }, null, 2));