Q47: task parents cannot form a cycle

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/main/services/organization-service.ts

@@ -492,10 +492,7 @@ export class OrganizationService {
   updateTask(input: UpdateTaskInput): void {
     const now = new Date().toISOString();
     if (input.assigneeAgentId) this.verifyOptionalOrgEntity("agents", input.assigneeAgentId, input.orgId, "Assignee agent");
-    if (input.parentTaskId) {
-      const parent = this.db.prepare("select id from tasks where id = ? and org_id = ?").get(input.parentTaskId, input.orgId);
-      if (!parent) throw new Error("Parent task was not found.");
-    }
+    if (input.parentTaskId) this.assertParentAllowed(input.orgId, input.taskId, input.parentTaskId, input.title.trim());
     const result = this.db.prepare(`
       update tasks set title = ?, description = ?, assignee_agent_id = ?, risk = ?, priority = ?, parent_task_id = ?, updated_at = ?
       where id = ? and org_id = ?
@@ -545,6 +542,20 @@ export class OrganizationService {
     });
   }
 
+  private assertParentAllowed(orgId: string, taskId: string, parentTaskId: string, title: string): void {
+    if (parentTaskId === taskId) throw new Error(`"${title}" cannot be its own parent.`);
+    const parent = this.db.prepare("select title from tasks where id = ? and org_id = ?").get(parentTaskId, orgId) as { title: string } | undefined;
+    if (!parent) throw new Error("Parent task was not found.");
+    const loop = this.db.prepare(`
+      with recursive ancestors(id) as (
+        select parent_task_id from tasks where id = ?
+        union select t.parent_task_id from tasks t join ancestors a on t.id = a.id where t.parent_task_id is not null
+      )
+      select 1 from ancestors where id = ? limit 1
+    `).get(parentTaskId, taskId);
+    if (loop) throw new Error(`"${title}" cannot sit under "${parent.title}": "${parent.title}" is already beneath "${title}".`);
+  }
+
   private assertNoOpenSubtasks(orgId: string, taskId: string): void {
     const open = this.db.prepare("select title from tasks where org_id = ? and parent_task_id = ? and status <> 'done' order by created_at")
       .all(orgId, taskId) as Array<{ title: string }>;

src/renderer/src/pages/work/TaskDetailDialog.tsx

@@ -13,7 +13,7 @@ import type {
 } from "../../../../shared/domain";
 import { TASK_STATUS } from "../../../../shared/domain";
 import { auditEventLabel, formatRelative } from "../types";
-import { nextStatus, prettyRunState, prettyStatus, providerFor, runBlocker, runsForTask, runTone } from "./task-helpers";
+import { descendantIds, nextStatus, prettyRunState, prettyStatus, providerFor, runBlocker, runsForTask, runTone } from "./task-helpers";
 import type { SnapshotActions } from "../../app/useSnapshot";
 import { t } from "../../i18n/i18n";
 
@@ -59,6 +59,7 @@ export function TaskDetailDialog({
 }) {
   const [tab, setTab] = useState<Tab>("overview");
   const [editing, setEditing] = useState(false);
+  const [editError, setEditError] = useState<string | null>(null);
 
   if (!task) return null;
   const assignee = agents.find((a) => a.id === task.assigneeAgentId) ?? null;
@@ -76,6 +77,7 @@ export function TaskDetailDialog({
     event.preventDefault();
     const form = new FormData(event.currentTarget);
     const val = (key: string) => String(form.get(key) ?? "").trim();
+    setEditError(null);
     void actions.run(() => window.agentCompany.updateTask({
       orgId,
       taskId: task.id,
@@ -85,7 +87,7 @@ export function TaskDetailDialog({
       risk: (val("risk") || "low") as "low" | "medium" | "high",
       priority: (val("priority") || "normal") as "low" | "normal" | "high",
       parentTaskId: val("parentTaskId") || null,
-    })).then((r) => { if (r) setEditing(false); });
+    })).then((r) => { if (r) setEditing(false); }, (error: unknown) => setEditError(error instanceof Error ? error.message : String(error)));
   };
 
   const deleteTask = () => {
@@ -99,7 +101,7 @@ export function TaskDetailDialog({
         <div className="row" style={{ alignItems: "flex-start", gap: 10, marginBottom: 8 }}>
           {task.priority === "high" && <span style={{ width: 3, height: 18, background: "var(--red)", borderRadius: 2, marginTop: 2, flexShrink: 0 }} />}
           <h2 className="modal-title" style={{ margin: 0, flex: 1 }}>{task.title}</h2>
-          <IconButton icon="edit" label={t("Edit")} onClick={() => setEditing(true)} />
+          <IconButton icon="edit" label={t("Edit")} onClick={() => { setEditError(null); setEditing(true); }} />
           <IconButton icon="x" label={t("Close")} onClick={onClose} />
         </div>
         <div className="row" style={{ flexWrap: "wrap", gap: 6, marginBottom: 12 }}>
@@ -127,7 +129,7 @@ export function TaskDetailDialog({
           {advanceTo && <Button tone="primary" onClick={() => onAdvance(task)}><Icon name="check" size={16} /> {prettyStatus(advanceTo)}</Button>}
         </div>
 
-        {editing && <EditOverlay task={task} agents={agents} allTasks={allTasks} onSubmit={submitEdit} onCancel={() => setEditing(false)} />}
+        {editing && <EditOverlay task={task} agents={agents} allTasks={allTasks} error={editError} onSubmit={submitEdit} onCancel={() => { setEditing(false); setEditError(null); }} />}
       </div>
     </div>
   );
@@ -221,11 +223,12 @@ function ActivityTab({ audit, approvals }: { audit: AuditEventRecord[]; approval
   );
 }
 
-function EditOverlay({ task, agents, allTasks, onSubmit, onCancel }: {
-  task: TaskRecord; agents: AgentRecord[]; allTasks: TaskRecord[];
+function EditOverlay({ task, agents, allTasks, error, onSubmit, onCancel }: {
+  task: TaskRecord; agents: AgentRecord[]; allTasks: TaskRecord[]; error: string | null;
   onSubmit: (e: FormEvent<HTMLFormElement>) => void; onCancel: () => void;
 }) {
-  const otherTasks = allTasks.filter((t) => t.id !== task.id && t.orgId === task.orgId);
+  const beneath = descendantIds(task.id, allTasks);
+  const otherTasks = allTasks.filter((t) => t.id !== task.id && t.orgId === task.orgId && !beneath.has(t.id));
   return (
     <div style={{ position: "absolute", inset: 0, background: "var(--bg)", borderRadius: 14, padding: 24, overflow: "auto", zIndex: 2 }}>
       <form onSubmit={onSubmit}>
@@ -256,6 +259,7 @@ function EditOverlay({ task, agents, allTasks, onSubmit, onCancel }: {
             {otherTasks.map((t) => <option key={t.id} value={t.id}>{t.title}</option>)}
           </select>
         </Field>
+        {error && <div className="form-error" role="alert">{error}</div>}
         <div className="modal-actions">
           <Button onClick={onCancel}>{t("Cancel")}</Button>
           <Button tone="primary" type="submit">{t("Save")}</Button>

src/renderer/src/pages/work/task-helpers.ts

@@ -62,3 +62,19 @@ export function latestTaskRun(taskId: string, taskRuns: TaskRunRecord[]): TaskRu
 export function runsForTask(taskId: string, taskRuns: TaskRunRecord[]): TaskRunRecord[] {
   return taskRuns.filter((run) => run.taskId === taskId);
 }
+
+/** Every task beneath this one; none of them may become its parent. */
+export function descendantIds(taskId: string, tasks: TaskRecord[]): Set<string> {
+  const found = new Set<string>();
+  const frontier = [taskId];
+  while (frontier.length > 0) {
+    const current = frontier.pop();
+    for (const task of tasks) {
+      if (task.parentTaskId === current && !found.has(task.id)) {
+        found.add(task.id);
+        frontier.push(task.id);
+      }
+    }
+  }
+  return found;
+}