Zielkette, Stufe 1: jede Aufgabe kann sagen, wofuer sie da ist (#714)

- TaskItem.ParentId, in der Datei als parent, in der DB als ParentId. Die
  Elternaufgabe ist der Zweck ("wofuer"), blocked_by die Voraussetzung
  ("wann").
- GoalChain: Eltern bis zu acht Stufen, Zyklen brechen den Gang ab und
  werden beim Setzen abgelehnt. Oben steht das Ziel der Einheit - der
  obersten Aufgabe, die zu einer Einheit gehoert (ein Ziel, das ein Mensch
  auf der obersten Ebene setzt, gehoert keiner), sonst der naechsten Einheit
  darueber mit Ziel.
- Der Lauf einer Aufgabe bekommt den Absatz "Why this task exists" und die
  Bitte, zu melden, wenn die Aufgabe ihrem Zweck nicht mehr dient.
- Taskboard-Tool: parent bei task_create/task_update, geprueft auf Existenz,
  Sichtbarkeit (#684) und Zyklen; task_list zeigt "Serves".
- WebUI: Feld im Aufgabendialog, Anzeige auf der Karte; die Verwaltung
  prueft wie das Tool.

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

src/ClawdDotNet.App/AppHost.cs

@@ -741,7 +741,8 @@ public sealed class AppHost : IAsyncDisposable
             dauerfreigaben: dauerfreigaben);
 
         var dispatcher = new EngineTaskDispatcher(
-            engine, () => instance.Agents, instance.InstanceId, tools, stateStore, loggerFactory);
+            engine, () => instance.Agents, instance.InstanceId, tools, stateStore, loggerFactory,
+            tasks: taskRepository);
 
         var headroom = engine.BudgetGuard is { } guard
             ? new BudgetHeadroom(guard, () => instance.Agents, () => engine.InstanceBudget)

src/ClawdDotNet.App/Services/TaskAdministration.cs

@@ -20,7 +20,8 @@ public sealed record TaskInput(
     string? WhenValue = null,
     string? TimeZone = null,
     string Acceptance = "",
-    int Priority = 3);
+    int Priority = 3,
+    string ParentId = "");
 
 /// <summary>
 /// Das Taskboard fuer die Oberflaechen (#594): anlegen, aendern, pausieren, sofort ausfuehren,
@@ -85,6 +86,8 @@ public sealed class TaskAdministration(
 
         if (Pruefe(input, out var assignee, out var when) is { } fehler)
             return AdminResult.Failure(fehler);
+        if (await CheckParentAsync(null, input.ParentId, ct) is { } parentProblem)
+            return AdminResult.Failure(parentProblem);
 
         var created = await board.CreateAsync(new TaskItem
         {
@@ -95,7 +98,8 @@ public sealed class TaskAdministration(
             Assignee = assignee,
             When = when,
             Status = TaskItemStatus.Todo,
-            Type = TaskItemType.Work
+            Type = TaskItemType.Work,
+            ParentId = input.ParentId.Trim()
         }, ct);
 
         return AdminResult.Success(MitHinweis($"Aufgabe '{created.Title}' angelegt ({created.Id}).", created));
@@ -108,6 +112,8 @@ public sealed class TaskAdministration(
 
         if (Pruefe(input, out var assignee, out var when) is { } fehler)
             return AdminResult.Failure(fehler);
+        if (await CheckParentAsync(id, input.ParentId, ct) is { } parentProblem)
+            return AdminResult.Failure(parentProblem);
 
         var updated = await board.UpdateAsync(id, current => current with
         {
@@ -116,7 +122,8 @@ public sealed class TaskAdministration(
             Acceptance = input.Acceptance.Trim(),
             Priority = Math.Clamp(input.Priority, 1, 5),
             Assignee = assignee,
-            When = when
+            When = when,
+            ParentId = input.ParentId.Trim()
         }, ct);
 
         return updated is null
@@ -124,6 +131,19 @@ public sealed class TaskAdministration(
             : AdminResult.Success(MitHinweis($"Aufgabe '{updated.Title}' gespeichert.", updated));
     }
 
+    /// <summary>#714: The parent must exist and must not lie below the task itself.</summary>
+    private async Task<string?> CheckParentAsync(string? taskId, string parentId, CancellationToken ct)
+    {
+        parentId = parentId.Trim();
+        if (parentId.Length == 0 || board is null)
+            return null;
+        if (await board.GetAsync(parentId, ct) is null)
+            return $"There is no task '{parentId}' to serve as parent.";
+        if (taskId is not null && await GoalChain.WouldCycleAsync(taskId, parentId, board.GetAsync, ct))
+            return $"'{parentId}' lies below '{taskId}' (or is the task itself) — a task cannot serve itself.";
+        return null;
+    }
+
     /// <summary>Pausieren heisst <c>backlog</c>, Aktivieren <c>todo</c> - dieselben Zustaende, die der Scanner kennt.</summary>
     public async Task<AdminResult> SetEnabledAsync(string id, bool enabled, CancellationToken ct)
     {

src/ClawdDotNet.Core/Storage/SqliteStorage.cs

@@ -380,6 +380,9 @@ public sealed class SqliteStorage
         // #684: Who created a task — decides its board when nobody in particular is assigned.
         SpalteErgaenzen(connection, "Tasks", "CreatedBy", "TEXT NOT NULL DEFAULT ''");
 
+        // #714: The task a task serves — the goal chain.
+        SpalteErgaenzen(connection, "Tasks", "ParentId", "TEXT NOT NULL DEFAULT ''");
+
         // Findet offene Aufrufe beim Start, ohne die ganze Tabelle zu lesen.
         using var index = connection.CreateCommand();
         index.CommandText = """

src/ClawdDotNet.Core/Tasks/EngineTaskDispatcher.cs

@@ -30,6 +30,7 @@ public sealed class EngineTaskDispatcher : ITaskDispatcher
     private readonly IStateStore _stateStore;
     private readonly ILogger _logger;
     private readonly TimeProvider _clock;
+    private readonly ITaskRepository? _tasks;
 
     public EngineTaskDispatcher(
         AgentEngine engine,
@@ -38,8 +39,10 @@ public sealed class EngineTaskDispatcher : ITaskDispatcher
         ToolRegistry toolRegistry,
         IStateStore stateStore,
         ILoggerFactory loggerFactory,
-        TimeProvider? clock = null)
+        TimeProvider? clock = null,
+        ITaskRepository? tasks = null)
     {
+        _tasks = tasks;
         _engine = engine;
         _agents = agents;
         _instanceId = instanceId;
@@ -56,7 +59,7 @@ public sealed class EngineTaskDispatcher : ITaskDispatcher
             return await DispatchToolJobAsync(task, ct);
 
         var agents = _agents();
-        var message = BuildMessage(task);
+        var message = BuildMessage(task, await WhyAsync(task, ct));
         var kind = TaskAssignee.KindOf(task.Assignee);
 
         AgentRunResult result;
@@ -158,7 +161,27 @@ public sealed class EngineTaskDispatcher : ITaskDispatcher
         return agents.Count == 1 ? agents[0] : null;
     }
 
-    internal string BuildMessage(TaskItem task)
+    /// <summary>
+    /// #714: The goal chain of the task — its parents up to the goal of the unit. A lookup
+    /// that fails costs the paragraph, never the run.
+    /// </summary>
+    private async Task<string?> WhyAsync(TaskItem task, CancellationToken ct)
+    {
+        if (_tasks is null)
+            return null;
+        try
+        {
+            var parents = await GoalChain.ParentsAsync(task, _tasks.GetAsync, ct);
+            return GoalChain.Describe(task, parents, _engine.OrganisationProvider?.Invoke());
+        }
+        catch (Exception ex) when (ex is not OperationCanceledException)
+        {
+            _logger.LogWarning(ex, "Goal chain of task {TaskId} could not be read; the run goes ahead without it", task.Id);
+            return null;
+        }
+    }
+
+    internal string BuildMessage(TaskItem task, string? why = null)
     {
         var sb = new StringBuilder();
         sb.AppendLine($"[Aufgabe {task.Id}] {task.Title}");
@@ -168,6 +191,12 @@ public sealed class EngineTaskDispatcher : ITaskDispatcher
         // und ein Modell ohne diese Zeile raet aus seinem Trainingsstand.
         sb.AppendLine(Zeitangabe(task, _clock.GetUtcNow()));
 
+        if (!string.IsNullOrWhiteSpace(why))
+        {
+            sb.AppendLine();
+            sb.AppendLine(why.Trim());
+        }
+
         if (!string.IsNullOrWhiteSpace(task.Body))
         {
             sb.AppendLine();

src/ClawdDotNet.Core/Tasks/GoalChain.cs

@@ -0,0 +1,104 @@
+using System.Text;
+using ClawdDotNet.Core.Organisation;
+
+namespace ClawdDotNet.Core.Tasks;
+
+/// <summary>
+/// #714: Why a task exists — its parent, that one's parent, up to the goal of the unit.
+/// Taken from Paperclip (Organisation-Konzept, 9a): an agent that only sees its own order
+/// cannot weigh it. "Write a product text" is a different job when it serves "launch the
+/// spring collection by Friday" than when it serves "clean up old listings".
+///
+/// <para><b>Purpose, not precondition.</b> <see cref="TaskItem.BlockedBy"/> says when a task
+/// may run; the parent says what it is for. Both can point to the same task, and mean
+/// different things.</para>
+///
+/// <para>Walked at run time, never stored as text: a parent that is renamed or finished
+/// shows up as it is now. The walk stops at a cycle and after <see cref="MaxDepth"/> steps,
+/// so a broken chain cannot hang a run or flood its prompt.</para>
+/// </summary>
+public static class GoalChain
+{
+    public const int MaxDepth = 8;
+
+    /// <summary>The parents of a task, nearest first. Stops at a missing parent or a cycle.</summary>
+    public static async Task<IReadOnlyList<TaskItem>> ParentsAsync(
+        TaskItem task, Func<string, CancellationToken, Task<TaskItem?>> lookup, CancellationToken ct)
+    {
+        var parents = new List<TaskItem>();
+        var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { task.Id };
+        var next = task.ParentId;
+
+        while (!string.IsNullOrWhiteSpace(next) && parents.Count < MaxDepth && seen.Add(next))
+        {
+            if (await lookup(next, ct) is not { } parent)
+                break;
+            parents.Add(parent);
+            next = parent.ParentId;
+        }
+        return parents;
+    }
+
+    /// <summary>
+    /// Whether making <paramref name="parentId"/> the parent of <paramref name="taskId"/>
+    /// would close a circle — the parent is the task itself or lies below it.
+    /// </summary>
+    public static async Task<bool> WouldCycleAsync(
+        string taskId, string parentId, Func<string, CancellationToken, Task<TaskItem?>> lookup, CancellationToken ct)
+    {
+        if (string.Equals(taskId, parentId, StringComparison.OrdinalIgnoreCase))
+            return true;
+
+        var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        var next = parentId;
+        while (!string.IsNullOrWhiteSpace(next) && seen.Add(next))
+        {
+            if (string.Equals(next, taskId, StringComparison.OrdinalIgnoreCase))
+                return true;
+            next = (await lookup(next, ct))?.ParentId;
+        }
+        return false;
+    }
+
+    /// <summary>
+    /// The goal above the chain: that of the unit of the topmost task that belongs to one —
+    /// a goal a human set on the top level belongs to no unit, so the next one down counts —
+    /// or of the nearest unit above it that has a goal. <c>null</c> without units or without
+    /// a goal.
+    /// </summary>
+    public static (string UnitName, string Goal)? UnitGoal(IReadOnlyList<TaskItem> fromTop, OrgChart? org)
+    {
+        if (org is not { HasUnits: true })
+            return null;
+        var unitId = fromTop.Select(t => TaskBoard.UnitOf(t, org)).FirstOrDefault(u => u is not null);
+        if (org.Unit(unitId) is not { } unit)
+            return null;
+
+        foreach (var candidate in new[] { unit }.Concat(org.Ancestors(unit.Id)))
+        {
+            if (!string.IsNullOrWhiteSpace(candidate.Goal))
+                return (candidate.Name, candidate.Goal.Trim());
+        }
+        return null;
+    }
+
+    /// <summary>
+    /// The paragraph that goes into the run: "you do this because … because … because".
+    /// <c>null</c> when there is nothing above the task — then the run looks as before.
+    /// </summary>
+    public static string? Describe(TaskItem task, IReadOnlyList<TaskItem> parents, OrgChart? org)
+    {
+        var goal = UnitGoal([.. parents.Reverse(), task], org);
+        if (parents.Count == 0 && goal is null)
+            return null;
+
+        var sb = new StringBuilder();
+        sb.AppendLine("Why this task exists — it serves, from nearest to farthest:");
+        foreach (var parent in parents)
+            sb.AppendLine($"- {parent.Id} \"{parent.Title}\" ({TaskText.Of(parent.Status)})");
+        if (goal is { } g)
+            sb.AppendLine($"- the goal of the unit \"{g.UnitName}\": {g.Goal}");
+        sb.Append("Weigh your work against this. If the task no longer serves it — the parent is done or canceled, or the task contradicts the goal — say so in your result instead of working blindly.");
+        return sb.ToString();
+    }
+}

src/ClawdDotNet.Core/Tasks/SqliteTaskRepository.cs

@@ -34,11 +34,11 @@ public sealed class SqliteTaskRepository : ITaskRepository
                     INSERT INTO Tasks
                         (Id, Title, Status, Type, Priority, Assignee, WhenKind, WhenValue, WhenTz,
                          RequireApproval, Acceptance, BlockedBy, OnlyWhenMarketOpen, Body, FileName,
-                         ToolName, JobTypeId, LastOccurrence, ClaimToken, ClaimedAt, CreatedAt, UpdatedAt, CreatedBy)
+                         ToolName, JobTypeId, LastOccurrence, ClaimToken, ClaimedAt, CreatedAt, UpdatedAt, CreatedBy, ParentId)
                     VALUES
                         (@id, @title, @status, @type, @priority, @assignee, @whenKind, @whenValue, @whenTz,
                          @requireApproval, @acceptance, @blockedBy, @market, @body, @fileName,
-                         @toolName, @jobType, NULL, NULL, NULL, @createdAt, @updatedAt, @createdBy);
+                         @toolName, @jobType, NULL, NULL, NULL, @createdAt, @updatedAt, @createdBy, @parentId);
                     """;
                 BindDefinition(insert, task);
                 insert.Parameters.AddWithValue("@status", TaskText.Of(task.Status));
@@ -61,6 +61,7 @@ public sealed class SqliteTaskRepository : ITaskRepository
                            BlockedBy = @blockedBy, OnlyWhenMarketOpen = @market, Body = @body,
                            FileName = @fileName, ToolName = @toolName, JobTypeId = @jobType,
                            CreatedBy = CASE WHEN @createdBy = '' THEN CreatedBy ELSE @createdBy END,
+                           ParentId = @parentId,
                            UpdatedAt = @updatedAt
                      WHERE Id = @id
                     """;
@@ -332,7 +333,7 @@ public sealed class SqliteTaskRepository : ITaskRepository
         "Id, Title, Status, Type, Priority, Assignee, WhenKind, WhenValue, WhenTz, " +
         "RequireApproval, Acceptance, BlockedBy, OnlyWhenMarketOpen, Body, FileName, " +
         "ToolName, JobTypeId, LastOccurrence, ClaimToken, ClaimedAt, CreatedAt, UpdatedAt, "
-        + "BudgetMissCount, LastBudgetMissAt, DeferredUntil, CatchUpPending, CreatedBy";
+        + "BudgetMissCount, LastBudgetMissAt, DeferredUntil, CatchUpPending, CreatedBy, ParentId";
 
     private static void BindDefinition(SqliteCommand cmd, TaskItem task)
     {
@@ -354,6 +355,7 @@ public sealed class SqliteTaskRepository : ITaskRepository
         cmd.Parameters.AddWithValue("@jobType", task.JobTypeId);
         // #684: a file without it (written by hand, or older) keeps what the database knows.
         cmd.Parameters.AddWithValue("@createdBy", task.CreatedBy ?? "");
+        cmd.Parameters.AddWithValue("@parentId", task.ParentId ?? "");
     }
 
     private static async Task<TaskItem?> ReadAsync(SqliteConnection conn, string id, CancellationToken ct)
@@ -405,7 +407,8 @@ public sealed class SqliteTaskRepository : ITaskRepository
             LastBudgetMissAt = r.IsDBNull(23) ? null : Parse(r.GetString(23)),
             DeferredUntil = r.IsDBNull(24) ? null : Parse(r.GetString(24)),
             CatchUpPending = r.GetInt32(25) != 0,
-            CreatedBy = r.GetString(26)
+            CreatedBy = r.GetString(26),
+            ParentId = r.GetString(27)
         };
     }
 

src/ClawdDotNet.Core/Tasks/TaskFrontmatter.cs

@@ -153,6 +153,7 @@ public static class TaskFrontmatter
             ToolName = scalars.GetValueOrDefault("tool_name", ""),
             JobTypeId = scalars.GetValueOrDefault("job_type", ""),
             CreatedBy = scalars.GetValueOrDefault("created_by", ""),
+            ParentId = scalars.GetValueOrDefault("parent", ""),
             Body = body
         };
         return true;
@@ -172,6 +173,8 @@ public static class TaskFrontmatter
         sb.Append($"assignee: {Scalar(task.Assignee)}\n");
         if (!string.IsNullOrWhiteSpace(task.CreatedBy))
             sb.Append($"created_by: {Scalar(task.CreatedBy)}\n");
+        if (!string.IsNullOrWhiteSpace(task.ParentId))
+            sb.Append($"parent: {Scalar(task.ParentId)}\n");
 
         if (task.When is { } w)
         {

src/ClawdDotNet.Core/Tasks/TaskModels.cs

@@ -235,6 +235,13 @@ public sealed record TaskItem
     /// </summary>
     public string CreatedBy { get; init; } = "";
 
+    /// <summary>
+    /// #714: The task this one serves — its purpose ("what for"), where <see cref="BlockedBy"/>
+    /// is its precondition ("when"). Empty for a task at the top; above the top stands the
+    /// goal of the unit. See <see cref="GoalChain"/>.
+    /// </summary>
+    public string ParentId { get; init; } = "";
+
     /// <summary>A one-time task that was refused for lack of budget and carried over.</summary>
     public bool IsCarriedOver => !IsRecurring && BudgetMissCount > 0;
 

src/ClawdDotNet.Manager.Web/wwwroot/i18n/de.json

@@ -349,6 +349,8 @@
   "tasks.board": "Brett",
   "tasks.boardAll": "Alle Bretter",
   "tasks.boardTopLevel": "Oberste Ebene",
+  "tasks.parent": "Dient Aufgabe (Eltern)",
+  "tasks.serves": "dient {id}",
 
   "services.title": "Dienste",
   "services.newService": "Neuer Dienst",

src/ClawdDotNet.Manager.Web/wwwroot/i18n/en.json

@@ -349,6 +349,8 @@
   "tasks.board": "Board",
   "tasks.boardAll": "All boards",
   "tasks.boardTopLevel": "Top level",
+  "tasks.parent": "Serves task (parent)",
+  "tasks.serves": "serves {id}",
 
   "services.title": "Services",
   "services.newService": "New Service",

src/ClawdDotNet.Manager.Web/wwwroot/views/tasks.js

@@ -189,7 +189,7 @@ function erstelleAufgabenKarte(task) {
       erstellePlakette(holeStatusText(task.status), STATUS_ART[task.status] || "neutral"),
       erstelleElement("span", {
         klasse: "gedaempft text-klein",
-        text: `${task.id} · ${task.assignee || "—"}${task.boardPath ? ` · ▤ ${task.boardPath}` : ""}`
+        text: `${task.id} · ${task.assignee || "—"}${task.boardPath ? ` · ▤ ${task.boardPath}` : ""}${task.parentId ? ` · ↑ ${t("tasks.serves", { id: task.parentId })}` : ""}`
       })
     )
   );
@@ -343,6 +343,9 @@ async function oeffneAufgabenDialog(task) {
   const abnahme = erstelleElement("textarea", { rows: "2", placeholder: "..." });
   abnahme.value = task?.acceptance || "";
 
+  // #714: The task this one serves — the run is told the chain up to the unit's goal.
+  const eltern = erstelleElement("input", { type: "text", value: task?.parentId || "", placeholder: "t-…" });
+
   const hinweis = erstelleElement("p", { klasse: "gedaempft text-klein" });
 
   const aktualisiere = () => {
@@ -369,6 +372,7 @@ async function oeffneAufgabenDialog(task) {
     erstelleElement("label", { text: "Value:" }), terminWert,
     erstelleElement("label", { text: "Timezone:" }), zeitzone,
     erstelleElement("label", { text: "Acceptance Criteria:" }), abnahme,
+    erstelleElement("label", { text: t("tasks.parent") + ":" }), eltern,
     erstelleElement("span"), hinweis
   );
 
@@ -399,7 +403,8 @@ async function oeffneAufgabenDialog(task) {
       whenValue: terminArt.value ? terminWert.value.trim() : null,
       timeZone: terminArt.value ? zeitzone.value.trim() : null,
       acceptance: abnahme.value.trim(),
-      priority: task?.priority ?? 3
+      priority: task?.priority ?? 3,
+      parentId: eltern.value.trim()
     };
 
     try {

src/ClawdDotNet.Tools.Taskboard/TaskboardTool.cs

@@ -41,6 +41,10 @@ public sealed class TaskboardTool : IAgentTool
         - '@<agentId>'→ the named agent with their context
         - '@human'    → waits for a human (default if unclear)
 
+        Purpose: set 'parent' to the task this one serves (task_create, task_update).
+        The run of a task is told the chain up to the goal of the unit — so split work
+        into sub-tasks under the task they serve instead of loose tasks.
+
         Schedules ('when' field): one-time ('at'), recurring ('every') or 'cron'.
         The time zone 'tz' is required (e.g. 'Europe/Berlin').
 
@@ -107,6 +111,10 @@ public sealed class TaskboardTool : IAgentTool
               "type": "string",
               "description": "Acceptance criteria against which the result is verified."
             },
+            "parent": {
+              "type": "string",
+              "description": "ID of the task this one serves (its purpose — unlike blocked_by, which is a precondition). Empty string removes it (task_update)."
+            },
             "blocked_by": {
               "type": "array",
               "items": { "type": "string" },
@@ -250,6 +258,20 @@ public sealed class TaskboardTool : IAgentTool
         return TaskSimilarity.FindOpen(visible is null ? alle : alle.Where(visible).ToList(), title, exceptId);
     }
 
+    /// <summary>
+    /// #714: A parent must exist, be on a board the agent sees, and must not lie below the
+    /// task itself — a circle would make the chain endless.
+    /// </summary>
+    private static async Task<string?> CheckParentAsync(
+        string? taskId, string parentId, TaskboardService board, Func<TaskItem, bool>? visible, CancellationToken ct)
+    {
+        if (!await CanSeeAsync(board, parentId, visible, ct) || await board.GetAsync(parentId, ct) is null)
+            return $"No task found with ID '{parentId}' to serve as parent.";
+        if (taskId is not null && await GoalChain.WouldCycleAsync(taskId, parentId, board.GetAsync, ct))
+            return $"'{parentId}' lies below '{taskId}' (or is the task itself) — a task cannot serve itself.";
+        return null;
+    }
+
     /// <summary>
     /// #684: A task on a board the agent does not see is answered like one that does not
     /// exist — the answer must not tell that it is there.
@@ -267,6 +289,10 @@ public sealed class TaskboardTool : IAgentTool
         if (!TryReadWhen(input, out var when, out var whenError))
             return ToolResult.Fail(whenError!);
 
+        var parent = GetString(input, "parent")?.Trim() ?? "";
+        if (parent.Length > 0 && await CheckParentAsync(null, parent, board, visible, ct) is { } parentProblem)
+            return ToolResult.Fail(parentProblem);
+
         // #674: The same finding a second time is not a second task. It is recorded on the
         // existing one — so that later one can see how often and for how long.
         if (GetBool(input, "force") != true)
@@ -302,7 +328,8 @@ public sealed class TaskboardTool : IAgentTool
             BlockedBy = GetStringList(input, "blocked_by"),
             OnlyWhenMarketOpen = GetBool(input, "onlyWhenMarketOpen") ?? false,
             // #684: decides the board when the task is for a human or a fresh run.
-            CreatedBy = author
+            CreatedBy = author,
+            ParentId = parent
         };
 
         var created = await board.CreateAsync(definition, ct);
@@ -343,6 +370,10 @@ public sealed class TaskboardTool : IAgentTool
             return ToolResult.Fail(whenError!);
         var hasWhen = input.TryGetProperty("when", out _);
 
+        var parent = GetString(input, "parent")?.Trim();
+        if (parent is { Length: > 0 } && await CheckParentAsync(id.Trim(), parent, board, visible, ct) is { } parentProblem)
+            return ToolResult.Fail(parentProblem);
+
         var result = await board.UpdateAsync(id.Trim(), current => current with
         {
             Title = GetString(input, "title")?.Trim() ?? current.Title,
@@ -355,7 +386,8 @@ public sealed class TaskboardTool : IAgentTool
             RequireApproval = GetBool(input, "require_approval") ?? current.RequireApproval,
             Acceptance = GetString(input, "acceptance")?.Trim() ?? current.Acceptance,
             BlockedBy = input.TryGetProperty("blocked_by", out _) ? GetStringList(input, "blocked_by") : current.BlockedBy,
-            OnlyWhenMarketOpen = GetBool(input, "onlyWhenMarketOpen") ?? current.OnlyWhenMarketOpen
+            OnlyWhenMarketOpen = GetBool(input, "onlyWhenMarketOpen") ?? current.OnlyWhenMarketOpen,
+            ParentId = parent ?? current.ParentId
         }, ct);
 
         if (result is null)
@@ -407,6 +439,8 @@ public sealed class TaskboardTool : IAgentTool
             block.Append($"{task.Id} [{TaskText.Of(task.Status)}] {task.Title}");
             block.AppendLine();
             block.AppendLine($"  Assignee: {task.Assignee} | Priority: {task.Priority}{FormatWhen(task.When)}");
+            if (!string.IsNullOrWhiteSpace(task.ParentId))
+                block.AppendLine($"  Serves: {task.ParentId}");
             if (task.BlockedBy.Count > 0)
                 block.AppendLine($"  Waiting for: {string.Join(", ", task.BlockedBy)}");
 

src/ClawdDotNet.Web/Contracts/Dtos.cs

@@ -119,6 +119,8 @@ public sealed record TaskDto(
     string? Hinweis = null,
     TaskBudgetMissDto? BudgetMiss = null,
     string? CreatedBy = null,
+    // #714: the task this one serves.
+    string? ParentId = null,
     // #684: the unit whose board the task is on, computed from the organisation; null on the top level.
     string? Board = null,
     string? BoardPath = null);
@@ -189,7 +191,8 @@ public sealed record TaskInputDto(
     string? WhenValue = null,
     string? TimeZone = null,
     string Acceptance = "",
-    int Priority = 3);
+    int Priority = 3,
+    string? ParentId = null);
 
 /// <summary>Die Kostenprognose einer Instanz (#595).</summary>
 public sealed record ForecastDto(

src/ClawdDotNet.Web/Contracts/Mapping.cs

@@ -108,7 +108,8 @@ internal static class Mapping
             input.WhenValue,
             input.TimeZone,
             input.Acceptance ?? "",
-            input.Priority);
+            input.Priority,
+            input.ParentId ?? "");
 
     public static ForecastDto ToDto(this InstanzPrognose prognose)
         => new(
@@ -154,7 +155,8 @@ internal static class Mapping
                 ? new TaskBudgetMissDto(task.BudgetMissCount, task.LastBudgetMissAt, task.DeferredUntil,
                     task.CatchUpPending, task.IsCarriedOver)
                 : null,
-            CreatedBy: string.IsNullOrWhiteSpace(task.CreatedBy) ? null : task.CreatedBy);
+            CreatedBy: string.IsNullOrWhiteSpace(task.CreatedBy) ? null : task.CreatedBy,
+            ParentId: string.IsNullOrWhiteSpace(task.ParentId) ? null : task.ParentId);
 
     /// <summary>#684: The board of a task, for a list the human sees whole.</summary>
     public static TaskDto WithBoard(this TaskDto dto, TaskItem task, Core.Organisation.OrgChart org)