diff --git a/backend/app/api/routes/compile.py b/backend/app/api/routes/compile.py
index e652d669..20c5bf7b 100644
--- a/backend/app/api/routes/compile.py
+++ b/backend/app/api/routes/compile.py
@@ -8,7 +8,7 @@ from typing import Any
 from fastapi import APIRouter, Depends, HTTPException, Request
 from pydantic import BaseModel
 
-from app.core.hooks import get_current_user_id, record_compile
+from app.core.hooks import get_current_user_id, get_project_libraries, record_compile
 from app.services.arduino_cli import ArduinoCLIService
 from app.services.espidf_compiler import espidf_compiler
 
@@ -215,12 +215,23 @@ async def _run_compile(
             [f.model_dump() for f in request.spiffs_files]
             if request.spiffs_files else None
         )
+        # Library manifest = ESP-IDF resolution SCOPE. Prefer the manifest SAVED
+        # with the project (authoritative, read server-side from the project
+        # record) so a reloaded project always scopes to its own libraries
+        # regardless of what the client sends; fall back to the manifest the
+        # client sent (unsaved examples). None/empty → legacy scan-all.
+        allowed_libraries = None
+        project_libs = await get_project_libraries(request.project_id)
+        if project_libs:
+            allowed_libraries = set(project_libs)
+        elif request.libraries:
+            allowed_libraries = set(request.libraries)
         result = await espidf_compiler.compile(
             files, request.board_fqbn,
             progress_callback=progress_callback,
             board_options=request.board_options,
             spiffs_files=spiffs_dicts,
-            allowed_libraries=set(request.libraries) if request.libraries is not None else None,
+            allowed_libraries=allowed_libraries,
         )
         return CompileResponse(
             success=result["success"],
diff --git a/backend/app/core/hooks.py b/backend/app/core/hooks.py
index 0034537f..d7b6b68a 100644
--- a/backend/app/core/hooks.py
+++ b/backend/app/core/hooks.py
@@ -103,6 +103,36 @@ async def get_current_user_id(request: Request) -> Optional[str]:  # FastAPI dep
         return None
 
 
+# ── get_project_libraries ─────────────────────────────────────────────────────
+# Returns the declared library manifest (list of library names) SAVED with a
+# project. The ESP-IDF compiler uses it as the resolution SCOPE so a project
+# only merges its own declared libraries — never another user's, or another
+# project's, stray install in the shared library dir. Sourced authoritatively
+# from the project record (not the client), so it is robust regardless of what
+# the frontend sends. Returns None for an unknown project, an empty manifest,
+# or when no overlay is loaded (→ legacy scan-all).
+
+GetProjectLibrariesHook = Callable[[str], Awaitable[Optional[list[str]]]]
+
+_get_project_libraries_hook: Optional[GetProjectLibrariesHook] = None
+
+
+def register_get_project_libraries(hook: GetProjectLibrariesHook) -> None:
+    """Install the project-manifest resolver. Called by overlays in register_pro."""
+    global _get_project_libraries_hook
+    _get_project_libraries_hook = hook
+
+
+async def get_project_libraries(project_id: Optional[str]) -> Optional[list[str]]:
+    if _get_project_libraries_hook is None or not project_id:
+        return None
+    try:
+        return await _get_project_libraries_hook(project_id)
+    except Exception:
+        logger.exception("get_project_libraries hook failed (treating as no manifest)")
+        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.