diff --git a/backend/app/models/project.py b/backend/app/models/project.py
index 57e5acc..8fbc169 100644
--- a/backend/app/models/project.py
+++ b/backend/app/models/project.py
@@ -23,6 +23,6 @@ class Project(Base):
     updated_at: Mapped[datetime] = mapped_column(default=func.now(), onupdate=func.now())
 
     # Relationships — lazy="raise" to prevent N+1 (mirrors CalendarMember pattern)
-    tasks: Mapped[List["ProjectTask"]] = relationship(back_populates="project", cascade="all, delete-orphan", lazy="raise")
+    tasks: Mapped[List["ProjectTask"]] = relationship(back_populates="project", cascade="all, delete-orphan", passive_deletes=True, lazy="raise")
     todos: Mapped[List["Todo"]] = relationship(back_populates="project", lazy="raise")
-    members: Mapped[List["ProjectMember"]] = relationship(back_populates="project", cascade="all, delete-orphan", lazy="raise")
+    members: Mapped[List["ProjectMember"]] = relationship(back_populates="project", cascade="all, delete-orphan", passive_deletes=True, lazy="raise")
diff --git a/backend/app/models/project_task.py b/backend/app/models/project_task.py
index ad25d27..cb950e0 100644
--- a/backend/app/models/project_task.py
+++ b/backend/app/models/project_task.py
@@ -36,15 +36,18 @@ class ProjectTask(Base):
     subtasks: Mapped[List["ProjectTask"]] = sa_relationship(
         back_populates="parent_task",
         cascade="all, delete-orphan",
+        passive_deletes=True,
         lazy="raise",
     )
     comments: Mapped[List["TaskComment"]] = sa_relationship(
         back_populates="task",
         cascade="all, delete-orphan",
+        passive_deletes=True,
         lazy="raise",
     )
     assignments: Mapped[List["ProjectTaskAssignment"]] = sa_relationship(
         back_populates="task",
         cascade="all, delete-orphan",
+        passive_deletes=True,
         lazy="raise",
     )
diff --git a/frontend/src/components/projects/ProjectDetail.tsx b/frontend/src/components/projects/ProjectDetail.tsx
index 6c9dcb3..89d7022 100644
--- a/frontend/src/components/projects/ProjectDetail.tsx
+++ b/frontend/src/components/projects/ProjectDetail.tsx
@@ -201,6 +201,9 @@ export default function ProjectDetail() {
       toast.success('Task deleted');
       setSelectedTaskId(null);
     },
+    onError: () => {
+      toast.error('Failed to delete task');
+    },
   });
 
   const deleteProjectMutation = useMutation({