diff --git a/backend/app/api/routes/compile.py b/backend/app/api/routes/compile.py
index 996a5dab..7f236e8d 100644
--- a/backend/app/api/routes/compile.py
+++ b/backend/app/api/routes/compile.py
@@ -8,7 +8,12 @@ from typing import Any
 from fastapi import APIRouter, Depends, HTTPException, Request
 from pydantic import BaseModel
 
-from app.core.hooks import get_current_user_id, get_project_libraries, record_compile
+from app.core.hooks import (
+    get_current_user_id,
+    get_project_libraries,
+    get_project_owner,
+    record_compile,
+)
 from app.services.arduino_cli import ArduinoCLIService
 from app.services.espidf_compiler import espidf_compiler
 
@@ -229,12 +234,17 @@ 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.
+        owner_id = await get_project_owner(request.project_id)
         result = await espidf_compiler.compile(
             files, request.board_fqbn,
             progress_callback=progress_callback,
             board_options=request.board_options,
             spiffs_files=spiffs_dicts,
             allowed_libraries=allowed_libraries,
+            owner_id=owner_id,
         )
         return CompileResponse(
             success=result["success"],
diff --git a/backend/app/core/hooks.py b/backend/app/core/hooks.py
index 64aee787..41c6d8b1 100644
--- a/backend/app/core/hooks.py
+++ b/backend/app/core/hooks.py
@@ -145,7 +145,11 @@ async def get_project_libraries(project_id: Optional[str]) -> Optional[list[str]
 # empty -> the compiler uses its single default libraries dir (OSS self-host
 # parity / scan-all). SYNC — pure filesystem (symlink creation).
 
-MaterializeLibraryScopeHook = Callable[[set], Optional[tuple]]
+# `owner_id` is the project OWNER's id (NOT the requester's): a shared / embed /
+# anonymous compile of someone else's project must resolve THAT user's custom
+# (per-user-store) libraries. The overlay treats it as an opaque key; the OSS
+# compiler only threads it through. None for unsaved/anon-no-project compiles.
+MaterializeLibraryScopeHook = Callable[[set, Optional[str]], Optional[tuple]]
 
 _materialize_library_scope_hook: Optional[MaterializeLibraryScopeHook] = None
 
@@ -156,19 +160,48 @@ def register_materialize_library_scope(hook: MaterializeLibraryScopeHook) -> Non
     _materialize_library_scope_hook = hook
 
 
-def materialize_library_scope(allowed_libraries: Optional[set]) -> Optional[tuple]:
+def materialize_library_scope(
+    allowed_libraries: Optional[set], owner_id: Optional[str] = None
+) -> Optional[tuple]:
     """Return (libraries_dir, content_token) for the manifest, or None to use the
     compiler's default single libraries dir. Never raises (a failing materializer
     degrades to the default dir)."""
     if _materialize_library_scope_hook is None or not allowed_libraries:
         return None
     try:
-        return _materialize_library_scope_hook(allowed_libraries)
+        return _materialize_library_scope_hook(allowed_libraries, owner_id)
     except Exception:
         logger.exception("materialize_library_scope hook failed (using default libraries dir)")
         return None
 
 
+# ── get_project_owner ─────────────────────────────────────────────────────────
+# Resolve a project's OWNER user-id from its id (the value stored as user_id on
+# the project record). Used so a compile resolves the OWNER's per-user custom
+# libraries, not the requester's. Returns None for an unknown project or when no
+# overlay is loaded.
+
+GetProjectOwnerHook = Callable[[str], Awaitable[Optional[str]]]
+
+_get_project_owner_hook: Optional[GetProjectOwnerHook] = None
+
+
+def register_get_project_owner(hook: GetProjectOwnerHook) -> None:
+    """Install the project-owner resolver. Called by overlays in register_pro."""
+    global _get_project_owner_hook
+    _get_project_owner_hook = hook
+
+
+async def get_project_owner(project_id: Optional[str]) -> Optional[str]:
+    if _get_project_owner_hook is None or not project_id:
+        return None
+    try:
+        return await _get_project_owner_hook(project_id)
+    except Exception:
+        logger.exception("get_project_owner hook failed (treating as no owner)")
+        return None
+
+
 # ── lifespan startup ──────────────────────────────────────────────────────────
 # Overlays that need to run async setup during FastAPI lifespan (DB init,
 # table creation, legacy column migrations, etc.) register a coroutine here.
diff --git a/backend/app/services/espidf_compiler.py b/backend/app/services/espidf_compiler.py
index 9b8bc5ee..3be72ace 100644
--- a/backend/app/services/espidf_compiler.py
+++ b/backend/app/services/espidf_compiler.py
@@ -1718,6 +1718,7 @@ class ESPIDFCompiler:
         board_options: dict | None = None,
         spiffs_files: list[dict] | None = None,
         allowed_libraries: set[str] | None = None,
+        owner_id: str | None = None,
     ) -> dict:
         """
         Compile Arduino sketch using ESP-IDF.
@@ -1804,7 +1805,7 @@ class ESPIDFCompiler:
             # cache update) gets its own clean build dir — and the throwaway
             # scope dir is removed after the attempt (its files were already
             # copied into the build's user_libs_all by _compile_in_dir).
-            scope = materialize_library_scope(allowed)
+            scope = materialize_library_scope(allowed, owner_id)
             scope_dir = scope[0] if scope else None
             scope_token = scope[1] if scope else ''
             # Fold the effective library set + resolved content into the build-dir