diff --git a/backend/app/api/routes/libraries.py b/backend/app/api/routes/libraries.py
index bc62cd98..b1352651 100644
--- a/backend/app/api/routes/libraries.py
+++ b/backend/app/api/routes/libraries.py
@@ -1,6 +1,7 @@
-from fastapi import APIRouter, HTTPException, Query
+from fastapi import APIRouter, Depends, HTTPException, Query
 from pydantic import BaseModel
 from app.api.routes.compile import arduino_cli
+from app.core.hooks import get_current_user_id, warm_library
 
 router = APIRouter()
 
@@ -36,11 +37,30 @@ async def search_libraries(q: str = Query(..., description="Search query for lib
         raise HTTPException(status_code=500, detail=str(e))
 
 @router.post("/install", response_model=InstallResponse)
-async def install_library(request: InstallLibraryRequest):
+async def install_library(
+    request: InstallLibraryRequest,
+    requester_id: str | None = Depends(get_current_user_id),
+):
     """
     Install a specific Arduino library by name.
+
+    On velxio.dev this WARMS the shared content-addressed cache (the global
+    mutable libraries volume is being retired) rather than mutating that volume;
+    the overlay also enforces the anonymous policy. With no overlay (OSS
+    self-host) it falls back to the legacy arduino-cli global install.
     """
     try:
+        # P2.1 — prefer warming the content-addressed cache (no global write).
+        warmed = await warm_library(request.name, request.version, requester_id)
+        if warmed is not None:
+            return InstallResponse(
+                success=bool(warmed.get("success")),
+                error=warmed.get("error"),
+                stdout=warmed.get("stdout"),
+                fallback=warmed.get("fallback"),
+                requested_version=warmed.get("requested_version"),
+            )
+        # OSS / no overlay: legacy global install (self-host parity).
         spec = f"{request.name}@{request.version}" if request.version else request.name
         result = await arduino_cli.install_library(spec)
         if not result["success"]:
diff --git a/backend/app/core/hooks.py b/backend/app/core/hooks.py
index 41c6d8b1..8a0a3d0d 100644
--- a/backend/app/core/hooks.py
+++ b/backend/app/core/hooks.py
@@ -202,6 +202,41 @@ async def get_project_owner(project_id: Optional[str]) -> Optional[str]:
         return None
 
 
+# ── warm_library ──────────────────────────────────────────────────────────────
+# "Install" an index library by WARMING the shared content-addressed cache
+# (install into a throwaway sketchbook -> publish to the cache) instead of
+# mutating the single shared global libraries volume — so the global dir stops
+# growing and can be retired. `requester_id` enforces the anon policy (an
+# anonymous user may only use libraries already referenced by an example/project,
+# i.e. already cached; warming a fresh uncached lib requires sign-in). Returns a
+# result dict ({success, error?, ...}) or None when no overlay is loaded -> the
+# OSS route falls back to its legacy arduino-cli global install (self-host parity).
+
+WarmLibraryHook = Callable[..., Awaitable[Optional[dict]]]
+
+_warm_library_hook: Optional[WarmLibraryHook] = None
+
+
+def register_warm_library(hook: WarmLibraryHook) -> None:
+    """Install the cache-warm library installer. Called by overlays in register_pro."""
+    global _warm_library_hook
+    _warm_library_hook = hook
+
+
+async def warm_library(
+    name: str, version: Optional[str] = None, requester_id: Optional[str] = None
+) -> Optional[dict]:
+    """Warm an index library into the shared cache. None -> no overlay (the OSS
+    route does its legacy global install). Never raises."""
+    if _warm_library_hook is None:
+        return None
+    try:
+        return await _warm_library_hook(name=name, version=version, requester_id=requester_id)
+    except Exception:
+        logger.exception("warm_library hook failed")
+        return {"success": False, "error": "Library install failed."}
+
+
 # ── lifespan startup ──────────────────────────────────────────────────────────
 # Overlays that need to run async setup during FastAPI lifespan (DB init,
 # table creation, legacy column migrations, etc.) register a coroutine here.