[MISC] Optimize prompt studio list endpoint with lightweight serializer (#1901)

* [PERF] Optimize prompt studio list endpoint with lightweight serializer

Reduce list endpoint queries from O(tools × (4 + prompts)) to ~1 by:
- New CustomToolListSerializer skips profile lookups, prompt fetching, coverage calculation
- Added get_serializer_class() to route list action to lightweight serializer
- Optimized get_queryset() with select_related("created_by") + Subquery annotation for prompt_count

Tested: 4.6x faster (0.057s vs 0.263s with 9 tools). No regressions on detail/retrieve/create/update/delete.
Consumers verified: ListOfTools.jsx (OSS), LinkProjectModal.jsx (Cloud lookups).

* Address review comments: warn on N+1 fallback, use Count("id")

- Add logger.warning when _prompt_count annotation is missing to surface
  misuse instead of silently degrading to per-instance queries
- Use Count("id") instead of Count("*") to match codebase convention

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Kirtiman Mishra <110175055+kirtimanmishrazipstack@users.noreply.github.com>

Author: 3 people

backend/prompt_studio/prompt_studio_core_v2/serializers.py

@@ -34,6 +34,47 @@
     from file_management.constants import FileInformationKey as FileKey
 
 
+class CustomToolListSerializer(serializers.ModelSerializer):
+    """Lightweight serializer for the list endpoint.
+
+    Avoids the O(tools x prompts) queries that CustomToolSerializer.to_representation
+    causes by skipping profile lookups, prompt fetching, and coverage calculation.
+    """
+
+    created_by_email = serializers.SerializerMethodField()
+    prompt_count = serializers.SerializerMethodField()
+
+    class Meta:
+        model = CustomTool
+        fields = [
+            "tool_id",
+            "tool_name",
+            "description",
+            "author",
+            "created_by",
+            "created_at",
+            "modified_at",
+            "shared_to_org",
+            "icon",
+            "created_by_email",
+            "prompt_count",
+        ]
+
+    def get_created_by_email(self, instance):
+        return instance.created_by.email if instance.created_by else ""
+
+    def get_prompt_count(self, instance):
+        if hasattr(instance, "_prompt_count"):
+            return instance._prompt_count or 0
+        # Fallback triggers a per-instance query if annotation is missing
+        logger.warning(
+            "CustomToolListSerializer used without _prompt_count annotation "
+            "for tool %s — falling back to per-instance query",
+            instance.tool_id,
+        )
+        return instance.mapped_prompt.count()
+
+
 class CustomToolSerializer(IntegrityErrorMixin, AuditSerializer):
     shared_users = serializers.PrimaryKeyRelatedField(
         queryset=User.objects.filter(is_service_account=False),

backend/prompt_studio/prompt_studio_core_v2/views.py

@@ -12,7 +12,7 @@
 from celery import signature
 from celery.result import AsyncResult
 from django.db import IntegrityError
-from django.db.models import QuerySet
+from django.db.models import Count, OuterRef, QuerySet, Subquery
 from django.http import HttpRequest, HttpResponse
 from file_management.constants import FileInformationKey as FileKey
 from file_management.exceptions import FileNotFound
@@ -77,6 +77,7 @@
 
 from .models import CustomTool
 from .serializers import (
+    CustomToolListSerializer,
     CustomToolSerializer,
     FileInfoIdeSerializer,
     FileUploadIdeSerializer,
@@ -95,14 +96,32 @@ class PromptStudioCoreView(viewsets.ModelViewSet):
 
     serializer_class = CustomToolSerializer
 
+    def get_serializer_class(self):
+        if self.action == "list":
+            return CustomToolListSerializer
+        return CustomToolSerializer
+
     def get_permissions(self) -> list[Any]:
         if self.action == "destroy":
             return [IsOwner()]
 
         return [IsOwnerOrSharedUserOrSharedToOrg()]
 
     def get_queryset(self) -> QuerySet | None:
-        return CustomTool.objects.for_user(self.request.user)
+        qs = CustomTool.objects.for_user(self.request.user)
+        if self.action == "list":
+            # Subquery avoids conflict with distinct("tool_id") from for_user()
+            prompt_count_sq = (
+                ToolStudioPrompt.objects.filter(tool_id=OuterRef("pk"))
+                .order_by()
+                .values("tool_id")
+                .annotate(cnt=Count("id"))
+                .values("cnt")
+            )
+            qs = qs.select_related("created_by").annotate(
+                _prompt_count=Subquery(prompt_count_sq)
+            )
+        return qs
 
     def get_object(self):
         """Override get_object to trigger lazy migration when accessing tools."""