diff --git a/backend/app/api/routes/compile.py b/backend/app/api/routes/compile.py
index ba54ea5a..7440f449 100644
--- a/backend/app/api/routes/compile.py
+++ b/backend/app/api/routes/compile.py
@@ -216,6 +216,13 @@ def _resolve_files(request: CompileRequest) -> list[dict[str, str]]:
     )
 
 
+def _bare_lib_names(names) -> set[str] | None:
+    """Strip a trailing @version (or @wokwi:hash) from each manifest name and
+    drop empties. None if nothing remains. See _resolve_compile_scope (P2.1h)."""
+    out = {n.split("@", 1)[0].strip() for n in names if n and n.split("@", 1)[0].strip()}
+    return out or None
+
+
 async def _resolve_compile_scope(
     request: CompileRequest, requester_id: str | None
 ) -> tuple[set[str] | None, str | None]:
@@ -248,13 +255,19 @@ async def _resolve_compile_scope(
     # owner-bytes gate; P2.2-sec).
     gated_owner = await resolve_compile_owner(request.project_id, requester_id)
 
+    # P2.1h: strip a trailing @version from each manifest name. norm_name fuses
+    # the version digits into the name otherwise ("ArduinoJson@6.21.5" ->
+    # "arduinojson6215"), which misses the cache entry ("arduinojson") and forces
+    # a global-dir scan-all. The per-board manifest (boards_json[].libraries,
+    # which the client sends in request.libraries) is the path that still carries
+    # @version. We don't version-pin today, so the bare name is what resolves.
     allowed_libraries: set[str] | None = None
     if request.libraries:
-        allowed_libraries = set(request.libraries)
+        allowed_libraries = _bare_lib_names(request.libraries)
     elif gated_owner is not None:
         project_libs = await get_project_libraries(request.project_id)
         if project_libs:
-            allowed_libraries = set(project_libs)
+            allowed_libraries = _bare_lib_names(project_libs)
 
     owner_id = gated_owner if gated_owner is not None else requester_id
     return allowed_libraries, owner_id
diff --git a/backend/app/services/arduino_cli.py b/backend/app/services/arduino_cli.py
index 8f94d900..e7d3b4cc 100644
--- a/backend/app/services/arduino_cli.py
+++ b/backend/app/services/arduino_cli.py
@@ -359,6 +359,17 @@ class ArduinoCLIService:
                 compile_env = dict(os.environ)
                 if scope_dir is not None:
                     compile_env["ARDUINO_DIRECTORIES_USER"] = str(scope_dir.parent)
+                else:
+                    # P2.1h: NO manifest -> point the default sketchbook at the
+                    # content-addressed cache (VELXIO_FALLBACK_SKETCHBOOK, whose
+                    # libraries/ is the cache root) instead of the shared global
+                    # volume, so a from-scratch / no-manifest compile (and the
+                    # scan-all retry, which re-enters here unscoped) resolves user
+                    # libraries from the cache. Unset (OSS self-host) -> arduino-
+                    # cli's default sketchbook (legacy global volume).
+                    _fb = os.environ.get("VELXIO_FALLBACK_SKETCHBOOK")
+                    if _fb:
+                        compile_env["ARDUINO_DIRECTORIES_USER"] = _fb
 
                 # Run compilation using subprocess.run in a thread (Windows compatible)
                 # ESP32 lcgamboa emulator requires DIO flash mode and
diff --git a/backend/app/services/espidf_compiler.py b/backend/app/services/espidf_compiler.py
index 3be72ace..c116f052 100644
--- a/backend/app/services/espidf_compiler.py
+++ b/backend/app/services/espidf_compiler.py
@@ -562,8 +562,19 @@ class ESPIDFCompiler:
         return '\n'.join(lines) + '\n'
 
     def _find_arduino_libraries_dir(self) -> Path | None:
-        """Find the Arduino global user-libraries directory (installed via arduino-cli)."""
-        candidates = [
+        """Find the Arduino global user-libraries directory (installed via arduino-cli).
+
+        P2.1h: when the pro overlay sets VELXIO_FALLBACK_LIBRARIES_DIR (the
+        content-addressed cache root, itself a valid libraries dir whose children
+        are library folders), prefer it — so the no-manifest scan + the scan-all
+        retry resolve from the cache instead of the shared global volume, letting
+        the global volume be retired. Unset (OSS self-host) -> legacy global.
+        """
+        candidates: list[Path] = []
+        _fb = os.environ.get('VELXIO_FALLBACK_LIBRARIES_DIR')
+        if _fb:
+            candidates.append(Path(_fb))
+        candidates += [
             Path.home() / 'Arduino' / 'libraries',
             Path.home() / 'Documents' / 'Arduino' / 'libraries',
             Path('/root/Arduino/libraries'),              # Docker / CI as root