@@ -0,0 +1,13 @@
+---
+'@objectstack/spec': minor
+---
+
+`ISecurityService` declares an optional, feature-detected member that answers the existing ADR-0055 master-detail write check: `checkControlledByParentWrite(object, recordId, context)`.
+
+Clause-②: yes (widening)
+
+- **What it answers.** It answers what the master-detail write check decides for an update of one record by the caller. A by-id write to a `controlled_by_parent` record requires edit access to its master, and the write path checks each master up the chain: the object-level `update` grant, the master's write row-level security, and record sharing. The answer equals what a by-id update of the same record gets from that check, the ADR-0090 D10 delegator leg included. It adds no verdict: it exposes the one the write path already reaches, so a gate on a record's attachments or comments can judge the record as its own update is judged.
+- **What it resolves with.** `ControlledByParentWriteOutcome` is a discriminated union. `allow` means the check does not refuse. `deny` names the leg that refused (`ControlledByParentWriteDenialLeg`: `object_permission`, `row_level_security`, `record_sharing` or `master_chain`). `not_applicable` means the object is not `controlled_by_parent`. `unresolvable` names why the check reached no verdict (`ControlledByParentWriteUnresolvedReason`: a missing `master_detail` relation, a record that does not exist, or an empty master reference). A store fault is not an outcome: the member rejects with the engine's own error, so the fault keeps its declared status (a datasource outage is `503`).
+- **Optional, and absence is typed.** A security service without the member still satisfies the contract, and an unguarded call does not compile. Absence means no master check is composed for a consumer to ask, which is not a policy that admits.
+
+Nothing an author writes changes, and no runtime behaviour changes. `@objectstack/plugin-security` does not serve the member yet. An implementation typed as `ISecurityService` that serves this name must serve it under the declared signature.
@@ -68,6 +68,7 @@ const DECLARED_MEMBERS = {
   resolveWriteScope: 'required',
   describeDelegationNarrowing: 'optional',
   checkAuthoredRowWrite: 'optional',
+  checkControlledByParentWrite: 'optional',
   explain: 'required',
   describeDelegableScope: 'required',
   listAudienceBindingSuggestions: 'required',
 
@@ -64,6 +64,9 @@
     "ConnectorSourcePullRequest (interface)",
     "ConnectorSourcePullResult (interface)",
     "ConnectorSourcePullSummary (interface)",
+    "ControlledByParentWriteDenialLeg (type)",
+    "ControlledByParentWriteOutcome (type)",
+    "ControlledByParentWriteUnresolvedReason (type)",
     "CoreServiceContract (type)",
     "CoreServiceContracts (interface)",
     "CounterIncrOptions (interface)",
 
@@ -64,6 +64,9 @@
     "ConnectorSourcePullRequest": "src/contracts/automation-service.ts#ConnectorSourcePullRequest (interface)",
     "ConnectorSourcePullResult": "src/contracts/automation-service.ts#ConnectorSourcePullResult (interface)",
     "ConnectorSourcePullSummary": "src/contracts/automation-service.ts#ConnectorSourcePullSummary (interface)",
+    "ControlledByParentWriteDenialLeg": "src/contracts/security-service.ts#ControlledByParentWriteDenialLeg (type)",
+    "ControlledByParentWriteOutcome": "src/contracts/security-service.ts#ControlledByParentWriteOutcome (type)",
+    "ControlledByParentWriteUnresolvedReason": "src/contracts/security-service.ts#ControlledByParentWriteUnresolvedReason (type)",
     "CoreServiceContract": "src/contracts/core-service-contracts.ts#CoreServiceContract (type)",
     "CoreServiceContracts": "src/contracts/core-service-contracts.ts#CoreServiceContracts (interface)",
     "CounterIncrOptions": "src/contracts/cluster-service.ts#CounterIncrOptions (interface)",
 
@@ -6,6 +6,9 @@ import type {
   AuthoredRowWriteVerdict,
   AuthoredRowWriteOperation,
   OwnershipFloorAlternate,
+  ControlledByParentWriteOutcome,
+  ControlledByParentWriteDenialLeg,
+  ControlledByParentWriteUnresolvedReason,
 } from './security-service';
 
 /**
@@ -642,4 +645,134 @@ describe('Security Service Contract', () => {
     };
     expect(notOneLimb.operation).toBe('all');
   });
+
+  it('checkControlledByParentWrite is OPTIONAL — absence is typed, and it means no master check, not allow', async () => {
+    // A security service without the member still satisfies the contract, and
+    // a consumer cannot reach the master check without handling the absent
+    // case first.
+    const withoutIt: ISecurityService = makeService();
+    expect(typeof withoutIt.checkControlledByParentWrite).toBe('undefined');
+
+    // The unguarded call does not compile. Never invoked: its only job is to
+    // make the COMPILER prove the point.
+    const mustNotCompileWithoutAGuard = () =>
+      // @ts-expect-error possibly undefined — a consumer must feature-detect first
+      withoutIt.checkControlledByParentWrite('order_line', 'r1', { userId: 'u1' });
+    expect(typeof mustNotCompileWithoutAGuard).toBe('function');
+
+    // The shape a gate writes. Absence proceeds, because the parent's own
+    // update meets no master check there either, and it is reported as its own
+    // state: nothing measured the master, so it is never relabelled `allow`.
+    const gate = async (svc: ISecurityService) => {
+      if (typeof svc.checkControlledByParentWrite !== 'function') return 'no-master-check';
+      const answer = await svc.checkControlledByParentWrite('order_line', 'r1', { userId: 'u1' });
+      return answer.outcome === 'allow' || answer.outcome === 'not_applicable' ? 'proceed' : 'refuse';
+    };
+    await expect(gate(withoutIt)).resolves.toBe('no-master-check');
+
+    const seen: unknown[] = [];
+    const withIt = makeService({
+      checkControlledByParentWrite: async (object, recordId, context) => {
+        seen.push([object, recordId, context]);
+        return { outcome: 'deny', leg: 'record_sharing' };
+      },
+    });
+    await expect(gate(withIt)).resolves.toBe('refuse');
+    expect(seen).toEqual([['order_line', 'r1', { userId: 'u1' }]]);
+    await expect(gate(makeService({ checkControlledByParentWrite: async () => ({ outcome: 'allow' }) })))
+      .resolves.toBe('proceed');
+  });
+
+  it('ControlledByParentWriteOutcome: deny names its leg, unresolvable its reason, and neither arm can omit it (compile-time)', () => {
+    const everyOutcome: ControlledByParentWriteOutcome[] = [
+      { outcome: 'allow' },
+      { outcome: 'deny', leg: 'object_permission' },
+      { outcome: 'not_applicable' },
+      { outcome: 'unresolvable', reason: 'record_not_found' },
+    ];
+    expect(everyOutcome).toHaveLength(4);
+
+    // A refusal with no leg, and a non-verdict with no reason, are not
+    // outcomes: the union holds each field to its arm.
+    // @ts-expect-error a `deny` must name the leg that refused
+    const denyWithoutLeg: ControlledByParentWriteOutcome = { outcome: 'deny' };
+    // @ts-expect-error an `unresolvable` must name why no verdict was reached
+    const unresolvableWithoutReason: ControlledByParentWriteOutcome = { outcome: 'unresolvable' };
+    expect([denyWithoutLeg, unresolvableWithoutReason]).toHaveLength(2);
+
+    // `abstain` is the sharing verdict's state, not this check's: the object
+    // either has no master check (`not_applicable`) or the check answers.
+    // @ts-expect-error `abstain` is not an outcome of the master-detail check
+    const notAnOutcome: ControlledByParentWriteOutcome = { outcome: 'abstain' };
+    expect(notAnOutcome.outcome).toBe('abstain');
+
+    // Both vocabularies are closed, so a consumer branches on a value and
+    // never parses the refusal's prose.
+    const everyLeg: ControlledByParentWriteDenialLeg[] = [
+      'object_permission',
+      'row_level_security',
+      'record_sharing',
+      'master_chain',
+    ];
+    const everyReason: ControlledByParentWriteUnresolvedReason[] = [
+      'master_detail_relation_missing',
+      'record_not_found',
+      'master_reference_missing',
+    ];
+    expect(everyLeg).toHaveLength(4);
+    expect(everyReason).toHaveLength(3);
+    // A store fault is a REJECTION with its own status, never a reason here.
+    // @ts-expect-error a store fault rejects; it is not a reason the check gives
+    const faultIsNotAReason: ControlledByParentWriteUnresolvedReason = 'store_fault';
+    expect(faultIsNotAReason).toBe('store_fault');
+
+    // An exhaustive consumer: every arm is handled, and a fifth arm added to
+    // the union would stop this switch compiling at the `never` line.
+    const decide = (answer: ControlledByParentWriteOutcome): 'proceed' | 'refuse' => {
+      switch (answer.outcome) {
+        case 'allow':
+        case 'not_applicable':
+          return 'proceed';
+        case 'deny':
+        case 'unresolvable':
+          return 'refuse';
+        default: {
+          const unhandled: never = answer;
+          return unhandled;
+        }
+      }
+    };
+    expect(everyOutcome.map(decide)).toEqual(['proceed', 'refuse', 'proceed', 'refuse']);
+  });
+
+  it('checkControlledByParentWrite: a store fault REJECTS with its own status, and a gate propagates it', async () => {
+    // The check lets a read fault propagate rather than report it as an
+    // access answer, so the member rejects with the engine's own error. A gate
+    // that propagates the rejection keeps the declared 503; one that caught it
+    // into `deny` would answer an outage as a refusal, and one that caught it
+    // into `allow` would fail open.
+    const outage = Object.assign(new Error('datasource unavailable'), {
+      code: 'ERR_DATASOURCE_UNAVAILABLE',
+      status: 503,
+    });
+    const failing = makeService({
+      checkControlledByParentWrite: async () => {
+        throw outage;
+      },
+    });
+    const gate = async (svc: ISecurityService) => {
+      if (typeof svc.checkControlledByParentWrite !== 'function') return 'no-master-check';
+      const answer = await svc.checkControlledByParentWrite('order_line', 'r1', { userId: 'u1' });
+      return answer.outcome === 'allow' || answer.outcome === 'not_applicable' ? 'proceed' : 'refuse';
+    };
+    await expect(gate(failing)).rejects.toMatchObject({ code: 'ERR_DATASOURCE_UNAVAILABLE', status: 503 });
+
+    // A non-verdict is an outcome, and the gate refuses on it: a record whose
+    // access IS derived from a master and whose check reached no verdict is
+    // not a record nothing refuses.
+    const missingRow = makeService({
+      checkControlledByParentWrite: async () => ({ outcome: 'unresolvable', reason: 'record_not_found' }),
+    });
+    await expect(gate(missingRow)).resolves.toBe('refuse');
+  });
 });