diff --git a/README.md b/README.md
index dceab93..79a98d9 100644
--- a/README.md
+++ b/README.md
@@ -67,7 +67,11 @@ default-exports a function receiving the DSL:
 ```ts
 export default (ct) => {
   ct.campus({ key: "mainz", name: "Mainz", shortName: "MZ" });
-  ct.group({ key: "mainz_kids_lead", name: "Mainz · Kids Leitung", parent: "mainz_area" });
+  ct.group({ key: "mainz_area", name: "Mainz · Bereiche", groupTypeId: 2 });
+  // Hierarchy is opt-in and multi-parent: `parents` are managed group keys, each declared
+  // in this config. Omit it to leave a group's hierarchy unmanaged; edges to unmanaged
+  // groups stay invisible. (`parent:` is unrelated — an ordering hint only, not hierarchy.)
+  ct.group({ key: "mainz_kids_lead", name: "Mainz · Kids Leitung", groupTypeId: 2, parents: ["mainz_area"] });
 };
 ```
 
diff --git a/src/commands/plan.ts b/src/commands/plan.ts
index 45ef94f..1fb3e1f 100644
--- a/src/commands/plan.ts
+++ b/src/commands/plan.ts
@@ -7,6 +7,7 @@ import { loadState, resolveStatePath } from "../state/state.js";
 import { loadConfig, resolveConfigPath } from "../config/load.js";
 import { computePlan } from "../engine/plan.js";
 import { renderPlan } from "../engine/render.js";
+import { parentIdsByGroupId, applyHierarchy, type HierarchyEntry } from "../engine/hierarchy.js";
 import { mapConcurrent } from "../util/concurrency.js";
 import { info, warn, out } from "../ui.js";
 
@@ -63,7 +64,26 @@ export function planCommand(): Command {
         }
       });
 
-      const plan = computePlan(desired, state, actual, { unresolved });
+      // Group hierarchy: one bulk call, folded into each opted-in group's `parents` set-field.
+      let parentIds = new Map<number, number[]>();
+      let hierarchyOk = true;
+      const hasManagedGroups = Object.values(state.resources).some((m) => m.type === "group");
+      if (hasManagedGroups) {
+        try {
+          const raw = await client.get<HierarchyEntry[]>("/groups/hierarchies");
+          parentIds = parentIdsByGroupId(Array.isArray(raw) ? raw : []);
+        } catch (err) {
+          const message = err instanceof Error ? err.message : String(err);
+          fetchErrors.push(`group hierarchies: ${message}`);
+          warn(`Failed to fetch group hierarchies: ${message}`);
+          hierarchyOk = false;
+        }
+      }
+      // On a hierarchy-fetch failure, leave `parents` undiffed rather than folding an empty map
+      // (which would fabricate spurious "add all parents" changes). The plan is flagged INCOMPLETE below.
+      const desiredWithHierarchy = hierarchyOk ? applyHierarchy(desired, state, actual, parentIds) : desired;
+
+      const plan = computePlan(desiredWithHierarchy, state, actual, { unresolved });
       if (opts.json) {
         out(plan);
       } else {
diff --git a/src/config/context.ts b/src/config/context.ts
index ace9e59..57fd1c6 100644
--- a/src/config/context.ts
+++ b/src/config/context.ts
@@ -4,7 +4,8 @@
  *
  *   export default (ct: ConfigContext) => {
  *     ct.campus({ key: "mainz", name: "Mainz", shortName: "MZ" });
- *     ct.group({ key: "mainz_kids_lead", name: "Mainz · Kids Leitung", parent: "mainz_area" });
+ *     ct.group({ key: "mainz_area", name: "Mainz · Bereiche", groupTypeId: 2 });
+ *     ct.group({ key: "mainz_kids", name: "Mainz · Kids", groupTypeId: 2, parents: ["mainz_area"] });
  *   };
  *
  * The context is injected (no global state), so blueprints are just functions
@@ -14,7 +15,14 @@ import type { DesiredResource } from "../engine/types.js";
 
 export interface ResourceInput {
   key: string;
+  /** Ordering hint: apply this resource after `parent`. A dependency edge only — NOT managed hierarchy. */
   parent?: string;
+  /**
+   * Managed parent groups (group→group hierarchy). Opt-in: omit to leave a group's hierarchy
+   * unmanaged; `[]` means "managed with no parents". Each key must reference a group declared
+   * in the same config.
+   */
+  parents?: string[];
   dependsOn?: string[];
   [field: string]: unknown;
 }
@@ -31,15 +39,50 @@ export interface ConfigContext {
 export type ConfigModule = (ct: ConfigContext) => void | Promise<void>;
 
 function toDesired(type: string, input: ResourceInput): DesiredResource {
-  const { key, parent, dependsOn = [], ...fields } = input;
+  const { key, parent, parents, dependsOn = [], ...fields } = input;
   if (!key || typeof key !== "string") {
     throw new Error(`${type} declaration is missing a string "key".`);
   }
-  const edges = [...dependsOn];
-  if (parent) {
-    edges.push(parent);
+  // A nullish/empty `parent` is "no parent", not an opt-in to managed-empty hierarchy.
+  if (parent != null && typeof parent !== "string") {
+    throw new Error(`${type} "${key}": "parent" must be a string key.`);
+  }
+  if (parents !== undefined && (!Array.isArray(parents) || parents.some((p) => typeof p !== "string"))) {
+    throw new Error(`${type} "${key}": "parents" must be an array of string group keys.`);
+  }
+  // `parent` is an ordering hint only — a dependency edge, never a diffed/managed field
+  // (its pre-hierarchy meaning; a `parent` may point at a campus). Group hierarchy is
+  // managed opt-in via `parents`: `undefined` → unmanaged, `[]` → managed with no parents.
+  const parentKey = typeof parent === "string" && parent !== "" ? parent : undefined;
+  const parentKeys = parents !== undefined ? [...new Set(parents)] : undefined;
+  const edges = [...new Set([...dependsOn, ...(parentKey ? [parentKey] : []), ...(parentKeys ?? [])])];
+  return { type, key, fields, parent: parentKey, parents: parentKeys, dependsOn: edges };
+}
+
+/**
+ * Every managed hierarchy parent must reference a group declared in the same config.
+ * A parent that resolves to nothing (typo, unmanaged group) or to a non-group would
+ * diff forever against the managed-only actual side, so reject it up front rather than
+ * emit a plan that can never converge.
+ */
+function validateReferences(resources: DesiredResource[]): void {
+  const byKey = new Map(resources.map((r) => [r.key, r]));
+  for (const r of resources) {
+    for (const parentKey of r.parents ?? []) {
+      const target = byKey.get(parentKey);
+      if (!target) {
+        throw new Error(
+          `Group "${r.key}" declares hierarchy parent "${parentKey}", which is not declared in this config. ` +
+            `Managed parents must reference a group by its key (omit unmanaged parents entirely).`,
+        );
+      }
+      if (target.type !== "group") {
+        throw new Error(
+          `Group "${r.key}" declares hierarchy parent "${parentKey}", but "${parentKey}" is a ${target.type}, not a group.`,
+        );
+      }
+    }
   }
-  return { type, key, fields, parent, dependsOn: edges };
 }
 
 export function createContext(): { ct: ConfigContext; resources: DesiredResource[] } {
@@ -72,5 +115,6 @@ export function createContext(): { ct: ConfigContext; resources: DesiredResource
 export async function evaluateConfig(mod: ConfigModule): Promise<DesiredResource[]> {
   const { ct, resources } = createContext();
   await mod(ct);
+  validateReferences(resources);
   return resources;
 }
diff --git a/src/engine/hierarchy.ts b/src/engine/hierarchy.ts
new file mode 100644
index 0000000..68c60fa
--- /dev/null
+++ b/src/engine/hierarchy.ts
@@ -0,0 +1,97 @@
+/**
+ * Group hierarchy (parent/child) is a many-to-many DAG in ChurchTools: a group
+ * can have several parents. `GET /groups/hierarchies` returns, per group, the
+ * ids of its parents and children.
+ *
+ * We surface hierarchy in the plan as an opt-in `parents` set-field on a group,
+ * resolved to logical keys and **restricted to managed groups** — an edge to an
+ * unmanaged group is invisible (managed-guard), never diffed or proposed for
+ * removal.
+ */
+
+import type { State } from "../state/state.js";
+import type { DesiredResource } from "./types.js";
+
+export interface HierarchyEntry {
+  groupId: number;
+  parents?: number[];
+  children?: number[];
+}
+
+/** Map each group id to its parent group ids. */
+export function parentIdsByGroupId(entries: HierarchyEntry[]): Map<number, number[]> {
+  const map = new Map<number, number[]>();
+  for (const entry of entries) {
+    if (typeof entry.groupId === "number") {
+      map.set(entry.groupId, Array.isArray(entry.parents) ? entry.parents : []);
+    }
+  }
+  return map;
+}
+
+/**
+ * The managed parent keys for a group: its parent ids that are themselves under
+ * management, mapped to their logical keys and sorted (stable for set diffing).
+ */
+export function managedParentKeys(parentIds: number[], groupIdToKey: Map<number, string>): string[] {
+  const keys: string[] = [];
+  for (const id of parentIds) {
+    const key = groupIdToKey.get(id);
+    if (key !== undefined) {
+      keys.push(key);
+    }
+  }
+  return keys.sort();
+}
+
+/**
+ * Fold group hierarchy into the plan inputs as a `parents` set-field.
+ *
+ * Only groups that opted into hierarchy (`parents !== undefined` on the desired
+ * side) are touched: their `actual` record gets a `parents` set of managed
+ * parent keys (from `parentIdsByGroup`), and their desired gets a sorted
+ * `parents` field so `computePlan` diffs it generically. Groups that did not opt
+ * in are left untouched on both sides — their hierarchy is not managed.
+ *
+ * Two caveats `parents` inherits from being a synthetic field:
+ *  - It is a *pseudo-field* of logical keys, NOT a real ChurchTools group column.
+ *    Apply must route it to the per-edge endpoint (`PUT`/`DELETE
+ *    /groups/{id}/parents/{parentId}`) — never PATCH it onto the group object.
+ *  - The adopt snapshot (`managed.fields`) never carries `parents`, so a
+ *    third-party re-parent surfaces as an ordinary `update`, not under drift.
+ */
+export function applyHierarchy(
+  desired: DesiredResource[],
+  state: State,
+  actual: Map<string, Record<string, unknown>>,
+  parentIdsByGroup: Map<number, number[]>,
+): DesiredResource[] {
+  const groupIdToKey = new Map<number, string>();
+  for (const managed of Object.values(state.resources)) {
+    if (managed.type === "group") {
+      groupIdToKey.set(managed.id, managed.key);
+    }
+  }
+
+  // Only groups whose desired opted into hierarchy are managed — mirror the desired-side guard
+  // below so we never write a `parents` field onto the actual of an opted-out group.
+  const optedIn = new Set(
+    desired.filter((d) => d.type === "group" && d.parents !== undefined).map((d) => d.key),
+  );
+
+  for (const managed of Object.values(state.resources)) {
+    if (managed.type !== "group" || !optedIn.has(managed.key)) {
+      continue;
+    }
+    const a = actual.get(managed.key);
+    if (a) {
+      a.parents = managedParentKeys(parentIdsByGroup.get(managed.id) ?? [], groupIdToKey);
+    }
+  }
+
+  return desired.map((d) =>
+    d.type === "group" && d.parents !== undefined
+      ? { ...d, fields: { ...d.fields, parents: [...d.parents].sort() } }
+      : d,
+  );
+}
diff --git a/src/engine/types.ts b/src/engine/types.ts
index 66ac111..cfc1fe6 100644
--- a/src/engine/types.ts
+++ b/src/engine/types.ts
@@ -8,9 +8,11 @@ export interface DesiredResource {
   key: string;
   /** Managed fields — same shape as the Phase 2 registry snapshot, so they diff like-for-like against actual. */
   fields: Record<string, unknown>;
-  /** Parent logical key (group hierarchy). Also contributes a dependency edge. */
+  /** Ordering hint: a dependency edge only (may point at a campus). NOT managed hierarchy — see `parents`. */
   parent?: string;
-  /** Logical keys this resource must be applied after (includes `parent`). */
+  /** Managed parent group keys (hierarchy). `undefined` = hierarchy not managed for this group; `[]` = should have no managed parents. */
+  parents?: string[];
+  /** Logical keys this resource must be applied after (includes `parent`/`parents`). */
   dependsOn: string[];
 }
 
diff --git a/tests/context.test.ts b/tests/context.test.ts
index fd3375b..bf5ab10 100644
--- a/tests/context.test.ts
+++ b/tests/context.test.ts
@@ -30,6 +30,64 @@ describe("config context", () => {
     expect(resources[0]?.dependsOn).toEqual(["type_x", "lead"]);
   });
 
+  it("captures multiple parents (hierarchy is a DAG) as an opt-in set, not in fields", () => {
+    const { ct, resources } = createContext();
+    ct.group({ key: "team", name: "Team", parents: ["area_a", "area_b"] });
+    expect(resources[0]?.parents).toEqual(["area_a", "area_b"]);
+    expect(resources[0]?.dependsOn).toEqual(["area_a", "area_b"]);
+    expect(resources[0]?.fields).not.toHaveProperty("parents");
+  });
+
+  it("treats parents as opt-in: omitted → undefined, explicit [] → managed-empty", () => {
+    const { ct, resources } = createContext();
+    ct.group({ key: "no_hierarchy", name: "N" });
+    ct.group({ key: "empty_hierarchy", name: "E", parents: [] });
+    expect(resources[0]?.parents).toBeUndefined();
+    expect(resources[1]?.parents).toEqual([]);
+  });
+
+  it("de-duplicates the `parents` set", () => {
+    const { ct, resources } = createContext();
+    ct.group({ key: "team", name: "T", parents: ["lead", "other", "lead"] });
+    expect(resources[0]?.parents).toEqual(["lead", "other"]);
+  });
+
+  it("keeps `parent` out of managed hierarchy: it is an ordering edge only, never in `parents`", () => {
+    const { ct, resources } = createContext();
+    ct.group({ key: "team", name: "T", parent: "lead", parents: ["other"] });
+    expect(resources[0]?.parent).toBe("lead");
+    expect(resources[0]?.parents).toEqual(["other"]); // `lead` is NOT folded into managed parents
+    expect(resources[0]?.dependsOn).toEqual(["lead", "other"]); // both still contribute ordering edges
+  });
+
+  it("treats a falsy `parent` as absent, not as opt-in to managed-empty hierarchy", () => {
+    const { ct, resources } = createContext();
+    ct.group({ key: "g", name: "G", parent: "" });
+    expect(resources[0]?.parents).toBeUndefined(); // no hierarchy management
+    expect(resources[0]?.dependsOn).toEqual([]);
+  });
+
+  it("rejects a non-array / non-string `parents`", () => {
+    const { ct } = createContext();
+    expect(() => ct.group({ key: "g", name: "G", parents: "area" as never })).toThrow(/array of string/);
+    expect(() => ct.group({ key: "h", name: "H", parents: [1] as never })).toThrow(/array of string/);
+  });
+
+  it("rejects a hierarchy parent that is not a declared group", async () => {
+    await expect(
+      evaluateConfig((ct) => {
+        ct.campus({ key: "mz", name: "Mainz" });
+        ct.group({ key: "kids", name: "Kids", parents: ["mz"] }); // mz is a campus
+      }),
+    ).rejects.toThrow(/is a campus, not a group/);
+
+    await expect(
+      evaluateConfig((ct) => {
+        ct.group({ key: "kids", name: "Kids", parents: ["ghost"] }); // never declared
+      }),
+    ).rejects.toThrow(/not declared in this config/);
+  });
+
   it("rejects a duplicate logical key", () => {
     const { ct } = createContext();
     ct.campus({ key: "mainz", name: "A" });
diff --git a/tests/hierarchy.test.ts b/tests/hierarchy.test.ts
new file mode 100644
index 0000000..d488889
--- /dev/null
+++ b/tests/hierarchy.test.ts
@@ -0,0 +1,90 @@
+import { describe, it, expect } from "vitest";
+import { parentIdsByGroupId, managedParentKeys, applyHierarchy } from "../src/engine/hierarchy.js";
+import type { DesiredResource } from "../src/engine/types.js";
+import type { State, ManagedResource } from "../src/state/state.js";
+
+function group(key: string, id: number): ManagedResource {
+  return { type: "group", id, key, fields: { name: key }, adoptedAt: "t", updatedAt: "t" };
+}
+
+describe("parentIdsByGroupId", () => {
+  it("maps group ids to their parent id lists", () => {
+    const map = parentIdsByGroupId([
+      { groupId: 1311, parents: [8, 119, 1175], children: [] },
+      { groupId: 8, children: [1311] },
+    ]);
+    expect(map.get(1311)).toEqual([8, 119, 1175]);
+    expect(map.get(8)).toEqual([]); // no parents key → empty
+  });
+});
+
+describe("managedParentKeys", () => {
+  it("keeps only managed parents, mapped to keys and sorted", () => {
+    const idToKey = new Map([
+      [8, "team_kids"],
+      [119, "not_relevant"],
+    ]);
+    // 1175 is unmanaged → dropped; result sorted
+    expect(managedParentKeys([1175, 119, 8], idToKey)).toEqual(["not_relevant", "team_kids"]);
+  });
+});
+
+describe("applyHierarchy", () => {
+  const state: State = {
+    version: 1,
+    host: "h",
+    resources: { child: group("child", 1311), parent: group("parent", 8) },
+  };
+
+  it("injects actual managed parents and augments opted-in desired groups", () => {
+    const actual = new Map<string, Record<string, unknown>>([
+      ["child", { name: "child" }],
+      ["parent", { name: "parent" }],
+    ]);
+    const parentIds = new Map([
+      [1311, [8, 119, 1175]], // only 8 is managed
+      [8, []],
+    ]);
+    const desired: DesiredResource[] = [
+      { type: "group", key: "child", fields: { name: "child" }, parents: ["parent"], dependsOn: ["parent"] },
+      { type: "group", key: "parent", fields: { name: "parent" }, dependsOn: [] }, // no opt-in
+    ];
+
+    const out = applyHierarchy(desired, state, actual, parentIds);
+
+    expect(actual.get("child")?.parents).toEqual(["parent"]); // 119/1175 unmanaged → dropped
+    // "parent" did not opt into hierarchy → its actual is left untouched (no injected pseudo-field).
+    expect(actual.get("parent")).not.toHaveProperty("parents");
+    expect(out[0]?.fields.parents).toEqual(["parent"]);
+    expect(out[1]?.fields.parents).toBeUndefined(); // group that did not opt in is untouched
+  });
+
+  it("injects an empty actual.parents for an opted-in group whose managed parents are all gone", () => {
+    // A group that opts in (parents: []) but still has a managed parent in CT must see the removal.
+    const actual = new Map<string, Record<string, unknown>>([
+      ["child", { name: "child" }],
+      ["parent", { name: "parent" }],
+    ]);
+    const parentIds = new Map([[1311, [8]]]); // CT still links child -> parent(8, managed)
+    const desired: DesiredResource[] = [
+      { type: "group", key: "child", fields: { name: "child" }, parents: [], dependsOn: [] }, // managed-empty
+      { type: "group", key: "parent", fields: { name: "parent" }, dependsOn: [] },
+    ];
+
+    const out = applyHierarchy(desired, state, actual, parentIds);
+
+    expect(actual.get("child")?.parents).toEqual(["parent"]); // real managed parent surfaced...
+    expect(out[0]?.fields.parents).toEqual([]); // ...against a desired of none → diff proposes removal
+  });
+
+  it("leaves non-group resources untouched", () => {
+    const campusState: State = {
+      version: 1,
+      host: "h",
+      resources: { mz: { type: "campus", id: 0, key: "mz", fields: {}, adoptedAt: "t", updatedAt: "t" } },
+    };
+    const actual = new Map<string, Record<string, unknown>>([["mz", { name: "Mainz" }]]);
+    applyHierarchy([], campusState, actual, new Map());
+    expect(actual.get("mz")).not.toHaveProperty("parents");
+  });
+});