@@ -0,0 +1,10 @@
+---
+'@objectstack/plugin-security': minor
+---
+
+`security/explain` answers an update of a `controlled_by_parent` record the way that record's own by-id update is answered: its record verdict now comes from the master-detail write check (ADR-0055), which it never asked.
+
+- **What was wrong.** On such a record the sharing service's per-record gate abstains, because the effective model of `controlled_by_parent` is `public`, and explain read the abstention as "writable". It also kept the platform ownership floor (`owner_only_writes`) on the record, which the write path hands over to the master-detail write check. So `record.visible` for an update was decided by that floor (`decidedBy: 'rls'`) for every principal who did not create the record. That was refused, on the wrong layer and with no word about the master, for a principal who may not edit the master. It was also refused for a principal who may edit the master, beside a `PATCH` that answers `200`.
+- **What it does now.** For every update of a record that exists, explain asks the security service's `checkControlledByParentWrite` with the context it is explaining (another user's, when `userId` names one). A `deny` or `unresolvable` outcome makes the record not writable, decided by the `sharing` layer, whose record attribution names the refusing leg (`object_permission`, `row_level_security`, `record_sharing` or `master_chain`) or the reason the check reached no verdict. A rejection of the check is reported fail-closed, as a dependency that throws already is. `allow` and `not_applicable` leave the report exactly as before. The update's row-level composition carries the write path's own master-gate coverage vouch, so the ownership floor is handed over to that check as the write path hands it, and never on behalf of a delegator.
+- **What is unchanged.** Every object that is not `controlled_by_parent`. Object-level reports (no `recordId`). The `allowed` field, which answers whether the principal may update the object at all. `delete` is unchanged too: explain asks no master check for it and keeps its floor.
+- **Public surface.** `ExplainEngineDeps` gains the optional `checkControlledByParentWrite` dependency. A deps bag wired without it explains an update as before, and claims nothing about a master.
@@ -248,7 +248,14 @@ async function boot(options: { faultOn?: string } = {}) {
     }
   };
   const member = (context: any, object: string, id: string) => serve(object, id, context);
-  return { store, ctx, patch, member };
+  /**
+   * [ADR-0055] `security/explain`'s record-grained update verdict through the
+   * registered service (`explainAccessForCaller`). `userId` explains another
+   * user, as the REST route does; the caller is the second argument.
+   */
+  const explainUpdate = (callerContext: any, object: string, recordId: string, userId?: string) =>
+    security.explain({ object, operation: 'update', recordId, ...(userId ? { userId } : {}) }, callerContext);
+  return { store, ctx, patch, member, explainUpdate };
 }
 
 const rep = () => ({ userId: REP, tenantId: 'org-1', positions: [], permissions: ['cbpm_rep'] });
@@ -375,3 +382,40 @@ describe('[ADR-0055] checkControlledByParentWrite rejects where the write path r
     expect(rejection).toMatchObject({ code: 'ERR_DATASOURCE_UNAVAILABLE', status: 503 });
   });
 });
+
+describe('[ADR-0055] security/explain answers an update as the PATCH does, from this same check', () => {
+  const sharingDetailOf = (d: any): string => String(d.layers.find((l: any) => l.layer === 'sharing')?.record?.detail ?? '');
+
+  it('a refused update is explained refused on the leg the PATCH names; an admitted one is explained writable', async () => {
+    const h = await boot();
+    for (const [context, object, id, leg] of [
+      [rep(), 'cbpm_contact', 'c_other', 'record_sharing'],
+      [rep(), 'cbpm_region_note', 'rn_eu', 'row_level_security'],
+      [viewer(), 'cbpm_contact', 'c_own', 'object_permission'],
+      [rep(), 'cbpm_line', 'line_above_dangling', 'master_chain'],
+    ] as const) {
+      expect(await h.patch(context, object, id), `PATCH ${object}/${id} by ${context.userId}`).not.toBe('admitted');
+      const d = await h.explainUpdate(context, object, id);
+      expect(d.record, `explain ${object}/${id} by ${context.userId}`).toEqual({ recordId: id, visible: false, decidedBy: 'sharing' });
+      expect(sharingDetailOf(d)).toContain(`refuses this update on its '${leg}' leg`);
+    }
+    for (const [object, id] of [['cbpm_contact', 'c_own'], ['cbpm_contact', 'c_shared'], ['cbpm_region_note', 'rn_us'], ['cbpm_line', 'line_ok']] as const) {
+      expect(await h.patch(rep(), object, id), `PATCH ${object}/${id}`).toBe('admitted');
+      const d = await h.explainUpdate(rep(), object, id);
+      expect(d.record, `explain ${object}/${id}`).toMatchObject({ recordId: id, visible: true });
+      expect(sharingDetailOf(d)).not.toContain('master-detail write check');
+    }
+  });
+
+  it('explaining ANOTHER user asks the check for THAT user: the system caller would be admitted, the viewer it explains is not', async () => {
+    const h = await boot();
+    // Asked with the CALLER's context, the check would admit: a system context answers `allow`.
+    expect(await h.member({ isSystem: true }, 'cbpm_contact', 'c_own')).toEqual({ outcome: 'allow' });
+    // The viewer, explained by user id, resolves to the fallback set — READ only on the master.
+    expect(await h.patch(viewer(), 'cbpm_contact', 'c_own')).not.toBe('admitted');
+    const d = await h.explainUpdate({ isSystem: true }, 'cbpm_contact', 'c_own', VIEWER);
+    expect(d.principal.userId).toBe(VIEWER);
+    expect(d.record).toEqual({ recordId: 'c_own', visible: false, decidedBy: 'sharing' });
+    expect(sharingDetailOf(d)).toContain("refuses this update on its 'object_permission' leg");
+  });
+});
@@ -0,0 +1,205 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// [ADR-0055] `security/explain`'s record-grained UPDATE verdict on a
+// `controlled_by_parent` record comes from the master-detail write check
+// (`ISecurityService.checkControlledByParentWrite`), the composition step 2.8
+// of the write path runs, and every outcome maps the way the door maps it.
+//
+// The per-record sharing gate (`canEditRecord`) cannot decide such an update:
+// plugin-sharing maps `controlled_by_parent` to `public`, so it abstains, and
+// its abstention reads as `true`. This file pins the engine's half — which
+// outcome refuses, what the report names, when the check is asked and with
+// which context — over a deps bag, so each cell isolates one outcome. The
+// served member's own parity with the PATCH is pinned in
+// `controlled-by-parent-write-member.test.ts`, and the whole REST answer beside
+// the REST PATCH in `packages/qa/dogfood/test/cbp-explain-master-write.dogfood.test.ts`.
+//
+// The mapping (the spec's own consumer rule): `allow` and `not_applicable`
+// proceed, so the report is exactly the one a deps bag without the check
+// gives; `deny` and `unresolvable` refuse, on the `sharing` layer (the
+// record's write gate), naming the leg or the reason; a rejection fails the
+// request, so it is reported fail-closed; an outcome outside the vocabulary
+// refuses too.
+import { describe, it, expect, vi } from 'vitest';
+import { PermissionSetSchema } from '@objectstack/spec/security';
+import type { ExplainDecision, ExplainOperation } from '@objectstack/spec/security';
+import type {
+  ControlledByParentWriteDenialLeg,
+  ControlledByParentWriteOutcome,
+  ControlledByParentWriteUnresolvedReason,
+} from '@objectstack/spec/contracts';
+import { PermissionEvaluator } from './permission-evaluator.js';
+import { explainAccess, type ExplainEngineDeps } from './explain-engine.js';
+
+const OBJECT = 'cbx_contract';
+const CBP_SCHEMA = { name: OBJECT, sharingModel: 'controlled_by_parent' };
+
+const EDITOR = PermissionSetSchema.parse({
+  name: 'cbx_editor',
+  objects: { [OBJECT]: { allowRead: true, allowCreate: true, allowEdit: true, allowDelete: true } },
+});
+
+/** The principal being EXPLAINED. Kept as one object so a cell can assert the check received this very context. */
+const EXPLAINED = { userId: 'u_member', tenantId: 'org1', positions: ['org_member'], permissions: ['cbx_editor'] };
+const RECORD = { id: 'r1', organization_id: 'org1', owner_id: 'u_somebody' };
+
+type Master = ExplainEngineDeps['checkControlledByParentWrite'];
+
+function deps(opts: {
+  schema?: Record<string, unknown>;
+  master?: Master;
+  canEdit?: boolean;
+  record?: Record<string, unknown> | null;
+  layer1?: Record<string, unknown> | null;
+} = {}): ExplainEngineDeps {
+  return {
+    ql: { getSchema: () => opts.schema ?? CBP_SCHEMA },
+    resolveSets: async () => [EDITOR],
+    evaluator: new PermissionEvaluator(),
+    getObjectSecurityMeta: async () => ({
+      isPrivate: false,
+      requiredPermissions: { all: [], read: [], create: [], update: [], delete: [] },
+      fieldRequiredPermissions: {},
+    }),
+    requiredCaps: () => [],
+    computeRlsFilter: async () => null,
+    getFieldMask: () => ({}),
+    getPartialMaskRules: async () => ({}),
+    baselinePermissionSets: ['member_default'],
+    computeLayeredRlsFilter: async () => ({ layer0: { organization_id: 'org1' }, layer1: opts.layer1 ?? null }),
+    fetchRecord: async () => (opts.record !== undefined ? opts.record : { ...RECORD }),
+    sharingReadFilter: async () => null,
+    listRecordShares: async () => [],
+    // plugin-sharing on a `controlled_by_parent` record: it abstains, and the abstention reads as `true`.
+    canEditRecord: async () => opts.canEdit ?? true,
+    canDeleteRecord: async () => opts.canEdit ?? true,
+    ...(opts.master ? { checkControlledByParentWrite: opts.master } : {}),
+  };
+}
+
+const answering = (outcome: ControlledByParentWriteOutcome): Master => vi.fn(async () => outcome);
+
+/** `null` asks the object-level question (no `recordId`). */
+const explainUpdate = (d: ExplainEngineDeps, recordId: string | null = 'r1') =>
+  explainAccess(d, { object: OBJECT, operation: 'update', context: EXPLAINED, ...(recordId !== null ? { recordId } : {}) });
+
+const sharingRecordOf = (d: ExplainDecision) => d.layers.find((l) => l.layer === 'sharing')?.record;
+
+const LEGS: ControlledByParentWriteDenialLeg[] = ['object_permission', 'row_level_security', 'record_sharing', 'master_chain'];
+const REASONS: Array<[ControlledByParentWriteUnresolvedReason, string]> = [
+  ['master_detail_relation_missing', '422 INVALID_METADATA'],
+  ['record_not_found', '404 RECORD_NOT_FOUND'],
+  ['master_reference_missing', '422 MISSING_REQUIRED_FIELD'],
+];
+
+describe('[ADR-0055] explain refuses an update the master-detail write check refuses', () => {
+  for (const leg of LEGS) {
+    it(`deny on '${leg}': the record is not writable, decided by the sharing layer, which names the leg`, async () => {
+      const d = await explainUpdate(deps({ master: answering({ outcome: 'deny', leg }) }));
+      expect(d.record).toEqual({ recordId: 'r1', visible: false, decidedBy: 'sharing' });
+      const sharing = sharingRecordOf(d);
+      expect(sharing?.outcome).toBe('excluded');
+      expect(sharing?.detail).toContain(`refuses this update on its '${leg}' leg`);
+      expect(sharing?.detail).toContain('403 PERMISSION_DENIED');
+    });
+  }
+
+  for (const [reason, answered] of REASONS) {
+    it(`unresolvable '${reason}': not writable (fail closed), the sharing layer names the reason and the update's answer`, async () => {
+      const d = await explainUpdate(deps({ master: answering({ outcome: 'unresolvable', reason }) }));
+      expect(d.record).toEqual({ recordId: 'r1', visible: false, decidedBy: 'sharing' });
+      const sharing = sharingRecordOf(d);
+      expect(sharing?.outcome).toBe('not_evaluated');
+      expect(sharing?.detail).toContain(`reaches no verdict ('${reason}')`);
+      expect(sharing?.detail).toContain(answered);
+    });
+  }
+
+  it('a rejection (a refused context, a store fault) fails the request, so it is reported fail-closed with no predicate', async () => {
+    const fault = Object.assign(new Error('datasource unavailable'), { code: 'ERR_DATASOURCE_UNAVAILABLE', status: 503 });
+    const master: Master = vi.fn(async () => { throw fault; });
+    const d = await explainUpdate(deps({ master }));
+    expect(d.record).toEqual({ recordId: 'r1', visible: false, decidedBy: 'sharing' });
+    const sharing = sharingRecordOf(d);
+    expect(sharing?.outcome).toBe('not_evaluated');
+    expect(sharing?.rowFilter).toBeUndefined();
+    expect(sharing?.matchesRecord).toBeUndefined();
+    expect(sharing?.detail).toContain('master-detail write check (controlled_by_parent, ADR-0055) could not be evaluated');
+  });
+
+  it('an outcome outside the vocabulary refuses (fail closed), never reads as a pass', async () => {
+    const d = await explainUpdate(deps({ master: answering({ outcome: 'maybe' } as unknown as ControlledByParentWriteOutcome) }));
+    expect(d.record).toEqual({ recordId: 'r1', visible: false, decidedBy: 'sharing' });
+    expect(sharingRecordOf(d)?.outcome).toBe('not_evaluated');
+  });
+
+  it("the record's own row-level security still decides first (step 2.7 runs above step 2.8)", async () => {
+    const d = await explainUpdate(deps({
+      master: answering({ outcome: 'deny', leg: 'record_sharing' }),
+      layer1: { id: 'not_this_record' },
+    }));
+    expect(d.record).toEqual({ recordId: 'r1', visible: false, decidedBy: 'rls' });
+  });
+});
+
+describe('[ADR-0055] a proceeding outcome changes nothing in the report', () => {
+  it('allow: the report is exactly the one a deps bag without the check gives — the master is not named', async () => {
+    const without = await explainUpdate(deps());
+    const withAllow = await explainUpdate(deps({ master: answering({ outcome: 'allow' }) }));
+    expect(withAllow).toEqual(without);
+    expect(withAllow.record).toMatchObject({ visible: true });
+    expect(sharingRecordOf(withAllow)?.detail).not.toContain('master');
+  });
+
+  it('allow does not lift a refusal the rest of the pipeline makes (the sharing gate refuses)', async () => {
+    const without = await explainUpdate(deps({ canEdit: false }));
+    const withAllow = await explainUpdate(deps({ canEdit: false, master: answering({ outcome: 'allow' }) }));
+    expect(withAllow).toEqual(without);
+    expect(withAllow.record).toMatchObject({ visible: false });
+  });
+
+  for (const [label, schema, canEdit] of [
+    ['public_read_write', { name: OBJECT, sharingModel: 'public_read_write' }, true],
+    ['private, the gate admits', { name: OBJECT, sharingModel: 'private' }, true],
+    ['private, the gate refuses', { name: OBJECT, sharingModel: 'private' }, false],
+  ] as const) {
+    it(`not_applicable on a ${label} object: byte-identical to the report without the check`, async () => {
+      const without = await explainUpdate(deps({ schema, canEdit }));
+      const withCheck = await explainUpdate(deps({ schema, canEdit, master: answering({ outcome: 'not_applicable' }) }));
+      expect(withCheck).toEqual(without);
+    });
+  }
+
+  it('absent check (a kernel without the member): the update keeps the sharing gate\'s answer and claims nothing about a master', async () => {
+    const d = await explainUpdate(deps());
+    expect(d.record).toMatchObject({ recordId: 'r1', visible: true });
+    expect(sharingRecordOf(d)?.detail).not.toContain('master');
+  });
+});
+
+describe('[ADR-0055] when the check is asked, and for whom', () => {
+  it('an update of a record that exists asks it once, for that record, with the EXPLAINED context', async () => {
+    const master = answering({ outcome: 'allow' });
+    await explainUpdate(deps({ master }));
+    expect(master).toHaveBeenCalledTimes(1);
+    expect(master).toHaveBeenCalledWith(OBJECT, 'r1', EXPLAINED);
+    expect((master as ReturnType<typeof vi.fn>).mock.calls[0][2]).toBe(EXPLAINED);
+  });
+
+  for (const operation of ['read', 'create', 'delete', 'transfer', 'export'] as ExplainOperation[]) {
+    it(`a record-grained ${operation} does not ask it`, async () => {
+      const master = answering({ outcome: 'deny', leg: 'object_permission' });
+      await explainAccess(deps({ master }), { object: OBJECT, operation, context: EXPLAINED, recordId: 'r1' });
+      expect(master).not.toHaveBeenCalled();
+    });
+  }
+
+  it('an object-level update (no recordId) does not ask it, and a record that does not exist is not asked about', async () => {
+    const master = answering({ outcome: 'deny', leg: 'object_permission' });
+    const objectLevel = await explainUpdate(deps({ master }), null);
+    expect(objectLevel.record).toBeUndefined();
+    const missing = await explainUpdate(deps({ master, record: null }), 'r_missing');
+    expect(missing.record).toEqual({ recordId: 'r_missing', visible: false });
+    expect(master).not.toHaveBeenCalled();
+  });
+});