fix: task list 500s when combining assignee_id=null with assignee_ids

services/api/internal/repository/postgres/task_repository.go

@@ -464,7 +464,7 @@ func applyTaskFilter(b *queryBuilder, filter taskdom.TaskFilter) {
 			b.args = append(b.args, id.String())
 			b.idx++
 		}
-		b.whereClauses = append(b.whereClauses, "(assignee_id IS NULL OR assignee_id IN ("+strings.Join(placeholders, ",")+")")
+		b.whereClauses = append(b.whereClauses, "(assignee_id IS NULL OR assignee_id IN ("+strings.Join(placeholders, ",")+"))")
 	case filter.AssigneeNull:
 		b.whereClauses = append(b.whereClauses, "assignee_id IS NULL")
 	case len(filter.AssigneeIDs) > 0:

services/api/internal/repository/postgres/task_repository_test.go

@@ -0,0 +1,66 @@
+package postgres
+
+import (
+	"context"
+	"testing"
+
+	taskdom "github.com/Paca-AI/api/internal/domain/task"
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+	_ "github.com/mattn/go-sqlite3"
+)
+
+// openTaskRepoTestDB sets up an in-memory SQLite DB with a minimal tasks
+// table for exercising applyTaskFilter via CountTasks.
+func openTaskRepoTestDB(t *testing.T) *sqlx.DB {
+	t.Helper()
+	db, err := sqlx.Open("sqlite3", ":memory:")
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	t.Cleanup(func() { _ = db.Close() })
+
+	schema := `
+		CREATE TABLE tasks (
+			id          TEXT PRIMARY KEY,
+			project_id  TEXT NOT NULL,
+			assignee_id TEXT,
+			deleted_at  DATETIME
+		);`
+	if _, err := db.ExecContext(context.Background(), schema); err != nil {
+		t.Fatalf("create schema: %v", err)
+	}
+	return db
+}
+
+// TestTaskRepository_CountTasks_AssigneeNullWithAssigneeIDs is a regression
+// test for https://github.com/Paca-AI/paca/issues/272: combining the
+// "unassigned" filter with an "assigned to specific users" filter used to
+// produce a WHERE clause with unbalanced parentheses, which crashed with a
+// SQL syntax error instead of returning matching tasks.
+func TestTaskRepository_CountTasks_AssigneeNullWithAssigneeIDs(t *testing.T) {
+	db := openTaskRepoTestDB(t)
+	repo := NewTaskRepository(db)
+	ctx := context.Background()
+
+	projectID := uuid.New()
+	assigneeIn := uuid.New()
+	assigneeOut := uuid.New()
+
+	db.MustExec(`INSERT INTO tasks (id, project_id, assignee_id) VALUES ($1, $2, NULL)`, uuid.New().String(), projectID.String())
+	db.MustExec(`INSERT INTO tasks (id, project_id, assignee_id) VALUES ($1, $2, $3)`, uuid.New().String(), projectID.String(), assigneeIn.String())
+	db.MustExec(`INSERT INTO tasks (id, project_id, assignee_id) VALUES ($1, $2, $3)`, uuid.New().String(), projectID.String(), assigneeOut.String())
+
+	filter := taskdom.TaskFilter{
+		AssigneeNull: true,
+		AssigneeIDs:  []uuid.UUID{assigneeIn},
+	}
+
+	count, err := repo.CountTasks(ctx, projectID, filter)
+	if err != nil {
+		t.Fatalf("expected no error combining AssigneeNull with AssigneeIDs, got: %v", err)
+	}
+	if count != 2 {
+		t.Fatalf("expected 2 tasks (unassigned + matching assignee), got %d", count)
+	}
+}