diff --git a/backend/app/api/endpoints/admin_categories.py b/backend/app/api/endpoints/admin_categories.py
index 39218bcb..36aa4abc 100644
--- a/backend/app/api/endpoints/admin_categories.py
+++ b/backend/app/api/endpoints/admin_categories.py
@@ -11,6 +11,7 @@ from app.models.category import TreeCategory
 from app.models.tree import Tree
 from app.schemas.admin import GlobalCategoryCreate, GlobalCategoryUpdate, GlobalCategoryResponse
 from app.api.deps import require_admin
+from app.core.service_account import PLATFORM_ACCOUNT_ID
 
 router = APIRouter(prefix="/admin/categories", tags=["admin-categories"])
 
@@ -22,7 +23,7 @@ async def list_global_categories(
 ):
     """List all global categories (account_id IS NULL)."""
     result = await db.execute(
-        select(TreeCategory).where(TreeCategory.account_id.is_(None)).order_by(TreeCategory.name)
+        select(TreeCategory).where(TreeCategory.account_id == PLATFORM_ACCOUNT_ID).order_by(TreeCategory.name)
     )
     categories = result.scalars().all()
 
@@ -51,18 +52,18 @@ async def create_global_category(
     """Create a global category."""
     # Check slug uniqueness for global categories
     existing = await db.execute(
-        select(TreeCategory).where(TreeCategory.slug == data.slug, TreeCategory.account_id.is_(None))
+        select(TreeCategory).where(TreeCategory.slug == data.slug, TreeCategory.account_id == PLATFORM_ACCOUNT_ID)
     )
     if existing.scalar_one_or_none():
         raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail="Global category with this slug already exists")
 
-    category = TreeCategory(name=data.name, slug=data.slug, account_id=None)
+    category = TreeCategory(name=data.name, slug=data.slug, account_id=PLATFORM_ACCOUNT_ID)
     db.add(category)
     await log_audit(db, current_user.id, "global_category.create", "category", details={"name": data.name})
     await db.commit()
     await db.refresh(category)
 
-    return GlobalCategoryResponse(id=category.id, name=category.name, slug=category.slug, account_id=None, tree_count=0)
+    return GlobalCategoryResponse(id=category.id, name=category.name, slug=category.slug, account_id=PLATFORM_ACCOUNT_ID, tree_count=0)
 
 
 @router.put("/global/{category_id}", response_model=GlobalCategoryResponse)
@@ -74,7 +75,7 @@ async def update_global_category(
 ):
     """Update a global category."""
     result = await db.execute(
-        select(TreeCategory).where(TreeCategory.id == category_id, TreeCategory.account_id.is_(None))
+        select(TreeCategory).where(TreeCategory.id == category_id, TreeCategory.account_id == PLATFORM_ACCOUNT_ID)
     )
     category = result.scalar_one_or_none()
     if not category:
@@ -86,7 +87,7 @@ async def update_global_category(
         # Check slug uniqueness
         existing = await db.execute(
             select(TreeCategory).where(
-                TreeCategory.slug == data.slug, TreeCategory.account_id.is_(None), TreeCategory.id != category_id
+                TreeCategory.slug == data.slug, TreeCategory.account_id == PLATFORM_ACCOUNT_ID, TreeCategory.id != category_id
             )
         )
         if existing.scalar_one_or_none():
@@ -103,7 +104,7 @@ async def update_global_category(
 
     return GlobalCategoryResponse(
         id=category.id, name=category.name, slug=category.slug,
-        account_id=None, tree_count=tree_count,
+        account_id=PLATFORM_ACCOUNT_ID, tree_count=tree_count,
     )
 
 
@@ -115,7 +116,7 @@ async def delete_global_category(
 ):
     """Delete (archive) a global category."""
     result = await db.execute(
-        select(TreeCategory).where(TreeCategory.id == category_id, TreeCategory.account_id.is_(None))
+        select(TreeCategory).where(TreeCategory.id == category_id, TreeCategory.account_id == PLATFORM_ACCOUNT_ID)
     )
     category = result.scalar_one_or_none()
     if not category:
diff --git a/backend/app/api/endpoints/categories.py b/backend/app/api/endpoints/categories.py
index 73505c05..e4c8b37e 100644
--- a/backend/app/api/endpoints/categories.py
+++ b/backend/app/api/endpoints/categories.py
@@ -12,6 +12,7 @@ from app.models.user import User
 from app.schemas.category import CategoryCreate, CategoryUpdate, CategoryResponse, CategoryListResponse
 from app.api.deps import get_current_active_user
 from app.core.permissions import can_manage_category, can_create_category
+from app.core.service_account import PLATFORM_ACCOUNT_ID
 
 router = APIRouter(prefix="/categories", tags=["categories"])
 
@@ -47,13 +48,13 @@ async def list_categories(
     elif current_user.account_id:
         query = query.where(
             or_(
-                TreeCategory.account_id.is_(None),  # Global
+                TreeCategory.account_id == PLATFORM_ACCOUNT_ID,  # Global
                 TreeCategory.account_id == current_user.account_id  # User's account
             )
         )
     else:
         # User has no account, only show global categories
-        query = query.where(TreeCategory.account_id.is_(None))
+        query = query.where(TreeCategory.account_id == PLATFORM_ACCOUNT_ID)
 
     query = query.order_by(TreeCategory.display_order, TreeCategory.name)
 
@@ -173,7 +174,7 @@ async def create_category(
         name=category_data.name,
         slug=slug,
         description=category_data.description,
-        account_id=category_data.account_id,
+        account_id=category_data.account_id if category_data.account_id is not None else PLATFORM_ACCOUNT_ID,
         display_order=max_order + 1,
         created_by=current_user.id
     )
diff --git a/backend/app/api/endpoints/step_categories.py b/backend/app/api/endpoints/step_categories.py
index 5d890225..d0bb3ae9 100644
--- a/backend/app/api/endpoints/step_categories.py
+++ b/backend/app/api/endpoints/step_categories.py
@@ -16,6 +16,7 @@ from app.schemas.step_category import (
 )
 from app.api.deps import get_current_active_user
 from app.core.permissions import can_manage_step_category, can_create_step_category
+from app.core.service_account import PLATFORM_ACCOUNT_ID
 
 router = APIRouter(prefix="/step-categories", tags=["step-categories"])
 
@@ -44,13 +45,13 @@ async def list_step_categories(
     elif current_user.account_id:
         query = query.where(
             or_(
-                StepCategory.account_id.is_(None),  # Global
+                StepCategory.account_id == PLATFORM_ACCOUNT_ID,  # Global
                 StepCategory.account_id == current_user.account_id  # User's account
             )
         )
     else:
         # User has no account, only show global categories
-        query = query.where(StepCategory.account_id.is_(None))
+        query = query.where(StepCategory.account_id == PLATFORM_ACCOUNT_ID)
 
     query = query.order_by(StepCategory.display_order, StepCategory.name)
 
@@ -155,7 +156,7 @@ async def create_step_category(
         name=category_data.name,
         slug=slug,
         description=category_data.description,
-        account_id=category_data.account_id,
+        account_id=category_data.account_id if category_data.account_id is not None else PLATFORM_ACCOUNT_ID,
         display_order=max_order + 1,
         created_by=current_user.id
     )
diff --git a/backend/app/api/endpoints/tags.py b/backend/app/api/endpoints/tags.py
index 334e33f8..30f01504 100644
--- a/backend/app/api/endpoints/tags.py
+++ b/backend/app/api/endpoints/tags.py
@@ -12,6 +12,7 @@ from app.models.user import User
 from app.schemas.tag import TagCreate, TagResponse, TagListResponse, TagAssignment
 from app.api.deps import get_current_active_user
 from app.core.permissions import can_manage_tree_tags, can_create_tag
+from app.core.service_account import PLATFORM_ACCOUNT_ID
 
 router = APIRouter(prefix="/tags", tags=["tags"])
 
@@ -33,13 +34,13 @@ async def list_tags(
     if include_account and current_user.account_id:
         query = query.where(
             or_(
-                TreeTag.account_id.is_(None),  # Global
+                TreeTag.account_id == PLATFORM_ACCOUNT_ID,  # Global
                 TreeTag.account_id == current_user.account_id  # User's account
             )
         )
     else:
         # Only show global tags
-        query = query.where(TreeTag.account_id.is_(None))
+        query = query.where(TreeTag.account_id == PLATFORM_ACCOUNT_ID)
 
     query = query.order_by(TreeTag.usage_count.desc(), TreeTag.name)
 
@@ -71,12 +72,12 @@ async def search_tags(
     if include_account and current_user.account_id:
         query = query.where(
             or_(
-                TreeTag.account_id.is_(None),
+                TreeTag.account_id == PLATFORM_ACCOUNT_ID,
                 TreeTag.account_id == current_user.account_id
             )
         )
     else:
-        query = query.where(TreeTag.account_id.is_(None))
+        query = query.where(TreeTag.account_id == PLATFORM_ACCOUNT_ID)
 
     query = query.order_by(TreeTag.usage_count.desc(), TreeTag.name).limit(limit)
 
@@ -147,7 +148,7 @@ async def create_tag(
     new_tag = TreeTag(
         name=tag_data.name,
         slug=slug,
-        account_id=tag_data.account_id,
+        account_id=tag_data.account_id if tag_data.account_id is not None else PLATFORM_ACCOUNT_ID,
         created_by=current_user.id
     )
     db.add(new_tag)
@@ -206,7 +207,7 @@ async def add_tags_to_tree(
         tag_query = select(TreeTag).where(
             TreeTag.slug == slug,
             or_(
-                TreeTag.account_id.is_(None),  # Global tag
+                TreeTag.account_id == PLATFORM_ACCOUNT_ID,  # Global tag
                 TreeTag.account_id == tag_account_id  # Account tag
             )
         )
@@ -340,7 +341,7 @@ async def replace_tree_tags(
         tag_query = select(TreeTag).where(
             TreeTag.slug == slug,
             or_(
-                TreeTag.account_id.is_(None),
+                TreeTag.account_id == PLATFORM_ACCOUNT_ID,
                 TreeTag.account_id == tag_account_id
             )
         )
diff --git a/backend/app/api/endpoints/trees.py b/backend/app/api/endpoints/trees.py
index 73927cf8..c24edca2 100644
--- a/backend/app/api/endpoints/trees.py
+++ b/backend/app/api/endpoints/trees.py
@@ -29,6 +29,7 @@ from app.core.subscriptions import check_tree_limit, get_account_subscription, g
 from app.core.audit import log_audit
 from app.core.config import settings
 from app.core.tree_validation import can_publish_tree
+from app.core.service_account import PLATFORM_ACCOUNT_ID
 from app.core.step_sync import sync_steps_from_tree, deactivate_synced_steps_for_tree
 from app.services.rag_service import index_tree as rag_index_tree
 
@@ -470,7 +471,7 @@ async def create_tree(
         tree_structure=tree_data.tree_structure,
         intake_form=intake_form_data,
         author_id=service_account_id if is_default else current_user.id,
-        account_id=None if is_default else current_user.account_id,
+        account_id=PLATFORM_ACCOUNT_ID if is_default else current_user.account_id,
         is_public=True if is_default else tree_data.is_public,  # Default trees are always public
         is_default=is_default,
         status=tree_data.status
diff --git a/backend/app/core/service_account.py b/backend/app/core/service_account.py
index a2175981..9d00a1d9 100644
--- a/backend/app/core/service_account.py
+++ b/backend/app/core/service_account.py
@@ -18,6 +18,10 @@ logger = logging.getLogger(__name__)
 
 SERVICE_ACCOUNT_EMAIL = "noreply@resolutionflow.com"
 SERVICE_ACCOUNT_NAME = "ResolutionFlow"
+
+# Well-known UUID for the platform account — owns all default/global content.
+# Created by migration 3a40fe11b427_create_global_content_tables.
+PLATFORM_ACCOUNT_ID = uuid.UUID("00000000-0000-0000-0000-000000000001")
 SYSTEM_ACCOUNT_NAME = "ResolutionFlow System"
 SYSTEM_ACCOUNT_DISPLAY_CODE = "RF-SYS-1"