@@ -0,0 +1,16 @@
+---
+'@objectstack/plugin-auth': patch
+---
+
+fix(plugin-auth): the last-administrator guard answers a read it could not complete with 503 `SERVICE_UNAVAILABLE`, the code the sibling guards on the same doors answer their read faults with
+
+Clause-②: no
+Metadata-protocol: none
+
+The last-administrator guard fails closed: when a read its verdict depends on throws, it refuses the write. That refusal answered 403 `PERMISSION_DENIED`, the same code as a refusal on the merits, so a caller could not tell "you may not" from "try again" by the code. The organization slug guard (`auth-manager.ts`) and the organization delete admission already answer their read faults with 503 `SERVICE_UNAVAILABLE`.
+
+- **Every half of the guard, one shape.** The shared fail-closed envelope and the Default Organization slug-capture rule now throw the same read fault: `code: 'SERVICE_UNAVAILABLE'`, `status: 503`, the read's own failure on `cause`. The write is still refused and nothing is written. A refusal on the merits, and an enumeration that completed but returned more rows than the guard can judge, keep 403 `PERMISSION_DENIED` unchanged.
+- **The data API** answers the read fault 503 `SERVICE_UNAVAILABLE` with the server-fault message, as it does every declared 5xx.
+- **better-auth's endpoints** (`organization/update`, the admin ban and remove-user endpoints, and every other endpoint that writes through the adapter) answer it 503 `SERVICE_UNAVAILABLE`. The adapter now maps any engine error that declares `status: 503` and `code: 'SERVICE_UNAVAILABLE'` to that answer; before, such an error reached better-auth's router as an unhandled 500 with an empty body. The driver's message is not relayed.
+- **`organization/delete`** of the Default Organization answers the guard's read fault 503 `SERVICE_UNAVAILABLE`, the code the admission's own read faults answer with. A refusal on the merits there stays 403 `PERMISSION_DENIED`.
+- **For a client:** branch on the code. `SERVICE_UNAVAILABLE` means retry once the data store is reachable; `PERMISSION_DENIED` means the write itself is refused.
@@ -289,9 +289,26 @@ export async function assertOrganizationDeletable(this: Pick<ActorGuardHost, 'ge
     try {
       await this.organizationDeleteStandingJudge(organizationId);
     } catch (err) {
-      if ((err as { code?: unknown } | null)?.code !== 'PERMISSION_DENIED') throw err;
+      const code = (err as { code?: unknown } | null)?.code;
       const { APIError } = await import('better-auth/api');
-      throw new APIError('FORBIDDEN', { message: (err as Error).message, code: 'PERMISSION_DENIED' });
+      if (code === 'PERMISSION_DENIED') {
+        throw new APIError('FORBIDDEN', { message: (err as Error).message, code: 'PERMISSION_DENIED' });
+      }
+      // The guard's fail-closed answer to a lookup it could not complete (503,
+      // `last-admin-guard.ts` `readFault`): this admission's own read-fault
+      // code, and never the driver's message, which names the failed read.
+      if (code === ORGANIZATION_DELETE_READ_FAULT_CODE && (err as { status?: unknown }).status === 503) {
+        const fault = new APIError('SERVICE_UNAVAILABLE', {
+          code: ORGANIZATION_DELETE_READ_FAULT_CODE,
+          message:
+            'The organization delete could not verify the remaining administrators, so whether this ' +
+            'organization may be deleted was never decided, and nothing was deleted. This is a server-side ' +
+            'outage, not a refusal of the delete: retry once the data store is reachable.',
+        });
+        (fault as { cause?: unknown }).cause = err;
+        throw fault;
+      }
+      throw err;
     }
   }
 
 
@@ -20,7 +20,7 @@
  * engine over better-sqlite3 `:memory:`, so the engine dispatches the hooks.
  */
 
-import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import { ObjectQL } from '@objectstack/objectql';
 import { SqlDriver } from '@objectstack/driver-sql';
 import { ADMIN_FULL_ACCESS } from '@objectstack/spec/identity';
@@ -266,6 +266,21 @@ describe('[#24948] the same question about deleting the Default Organization, as
     expect((await members(engine)).map((m) => m.id)).toEqual(['mem_owner']);
   });
 
+  it('a read the question cannot complete refuses with the read-fault code, once: an inner envelope\'s answer passes the outer one unchanged', async () => {
+    const engine = await promotedSingle();
+    const real = engine.find.bind(engine);
+    vi.spyOn(engine, 'find').mockImplementation(async (object: string, ...rest: any[]) => {
+      if (object === 'sys_user_permission_set') throw new Error('sys_user_permission_set is unreadable');
+      return (real as any)(object, ...rest);
+    });
+
+    const err = (await guard.judgeOrganizationDelete('org_default').catch((e: unknown) => e)) as Error & { code?: unknown; status?: unknown };
+    vi.restoreAllMocks();
+    expect(err).toMatchObject({ code: 'SERVICE_UNAVAILABLE', status: 503 });
+    expect(err.message.match(/SERVICE_UNAVAILABLE:/g)).toHaveLength(1);
+    expect(err.message).toContain('sys_user_permission_set is unreadable');
+  });
+
   it('CONTROL: with another administrator standing elsewhere, the delete is admitted', async () => {
     const engine = await promotedSingle({ secondOrganization: true, grantOwner: 'org_b' });
     await engine.insert('sys_organization', { id: 'org_c', name: 'C', slug: 'c' }, SYSTEM);
 
@@ -590,7 +590,9 @@ describe('[#5892] the guard fails CLOSED — an unverifiable population refuses
     // because the guard could not prove it.
     await expect(ban(engine, 'usr_admin')).rejects.toThrow(/could not be verified/i);
     await expect(ban(engine, 'usr_admin')).rejects.toThrow(/sys_member is unreadable/);
-    await expect(ban(engine, 'usr_admin')).rejects.toMatchObject({ code: 'PERMISSION_DENIED' });
+    // [ADR-0112] Refused, but no verdict: the read-fault code, not the 403 a
+    // refusal on the merits answers.
+    await expect(ban(engine, 'usr_admin')).rejects.toMatchObject({ code: 'SERVICE_UNAVAILABLE', status: 503 });
     expect(await bannedFlag(engine, 'usr_admin')).toBeFalsy();
   });
 
@@ -601,6 +603,9 @@ describe('[#5892] the guard fails CLOSED — an unverifiable population refuses
     await seedUser(engine, 'usr_admin', { role: 'admin' });
 
     await expect(ban(engine, 'usr_admin')).rejects.toThrow(/more than 1 rows/);
+    // A read that COMPLETED and proved too large is a refusal with a remedy,
+    // not a read fault: it keeps the 403.
+    await expect(ban(engine, 'usr_admin')).rejects.toMatchObject({ code: 'PERMISSION_DENIED', status: 403 });
   });
 
   it('an environment with no administrator at all is not blocked (nothing to protect)', async () => {
@@ -935,7 +940,8 @@ describe('[#5941] the delete guard fails CLOSED too', () => {
     await expect(removeUser(engine, 'usr_admin')).rejects.toThrow(/could not be verified/i);
     await expect(removeUser(engine, 'usr_admin')).rejects.toThrow(/sys_member is unreadable/);
     await expect(removeUser(engine, 'usr_admin')).rejects.toMatchObject({
-      code: 'PERMISSION_DENIED',
+      code: 'SERVICE_UNAVAILABLE',
+      status: 503,
     });
     expect(await userExists(engine, 'usr_admin')).toBe(true);
   });
@@ -1557,7 +1563,7 @@ describe('[#5978] the standing halves fail CLOSED', () => {
 
     await expect(
       engine.update('sys_member', { id: 'mem_usr_admin', role: 'member' }, SYSTEM),
-    ).rejects.toMatchObject({ code: 'PERMISSION_DENIED', object: 'sys_member' });
+    ).rejects.toMatchObject({ code: 'SERVICE_UNAVAILABLE', status: 503, object: 'sys_member' });
     expect(await memberRole(engine, 'mem_usr_admin')).toBe('admin');
   });
 
 
@@ -314,6 +314,7 @@
  * reach the identity tables reach this guard.
  */
 
+import type { StandardErrorCode } from '@objectstack/spec/api';
 import type { BaseEngineOptions, EngineQueryOptions } from '@objectstack/spec/data';
 import {
   ADMIN_FULL_ACCESS,
@@ -636,6 +637,44 @@ function isRefusal(err: unknown): boolean {
   return (err as { code?: string } | null)?.code === 'PERMISSION_DENIED';
 }
 
+/**
+ * [ADR-0112] The code every half of this guard answers a read it could not
+ * complete with: the standard catalog's member for a store that cannot answer
+ * right now, the code the organization slug guard's read fault
+ * (`auth-manager.ts`, `slugGuardReadFaultApiError`) and the organization delete
+ * admission's read fault answer with too. One code for the fault across every
+ * guard on a door, so a caller tells "you may not" (403 `PERMISSION_DENIED`)
+ * from "try again" by the code alone.
+ */
+const LAST_ADMIN_GUARD_READ_FAULT_CODE: StandardErrorCode = 'SERVICE_UNAVAILABLE';
+
+/**
+ * The fail-closed answer to a read the guard could not complete: 503
+ * {@link LAST_ADMIN_GUARD_READ_FAULT_CODE}. Still a refusal of the write — the
+ * hook throws, so nothing is written — but no verdict about it, so it does not
+ * wear {@link refuse}'s 403. The read's own failure rides `cause`. `object`
+ * names the table the caller was writing, as {@link refuse}'s does.
+ */
+function readFault(message: string, object: string, cause: unknown): Error {
+  const err = new Error(`${LAST_ADMIN_GUARD_READ_FAULT_CODE}: ${message}`) as Error & {
+    code?: string;
+    status?: number;
+    object?: string;
+    cause?: unknown;
+  };
+  err.code = LAST_ADMIN_GUARD_READ_FAULT_CODE;
+  err.status = 503;
+  err.object = object;
+  err.cause = cause;
+  return err;
+}
+
+/** Marker so a nested fail-closed wrapper re-throws an inner read fault unchanged. */
+function isReadFault(err: unknown): boolean {
+  const e = err as { code?: unknown; status?: unknown } | null;
+  return e?.code === LAST_ADMIN_GUARD_READ_FAULT_CODE && e.status === 503;
+}
+
 const BREAK_GLASS_CITATION =
   'break-glass invariant, ADR-0135 D5.2 — an environment must always keep at least one ' +
   'administrator who can sign in';
@@ -945,7 +984,7 @@ function touchesAny(data: Record<string, unknown>, keys: readonly string[]): boo
  * slug is an engine update: better-auth's `organization/update` (its adapter
  * writes through the engine, and the refusal reaches the caller as 403
  * `PERMISSION_DENIED`), a seed, a plugin, the data API. A read that throws
- * propagates to the hook, which fails closed.
+ * propagates to the hook, which fails closed with 503 `SERVICE_UNAVAILABLE`.
  */
 async function refuseDefaultOrganizationSlugCapture(
   engine: Pick<LastAdminGuardEngine, 'find'>,
@@ -1573,24 +1612,30 @@ export function registerLastAdminGuard(
    * The fail-CLOSED envelope every half runs inside. A lookup that throws is
    * not "probably fine": the guard could not prove another administrator
    * survives, and the cost of guessing wrong is a permanently locked-out
-   * environment. A deliberate refusal from inside passes through unchanged.
+   * environment. A deliberate refusal from inside passes through unchanged,
+   * and so does a read fault an inner envelope already answered.
+   *
+   * The write is refused either way; only the code tells the two apart: a
+   * verdict is 403 `PERMISSION_DENIED` ({@link refuse}), a lookup that could
+   * not complete is 503 `SERVICE_UNAVAILABLE` ({@link readFault}).
    */
   const failClosed = async (op: GuardedOp, judge: () => Promise<void>): Promise<void> => {
     const words = OP_WORDS[op];
     try {
       await judge();
     } catch (err) {
-      if (isRefusal(err)) throw err;
+      if (isRefusal(err) || isReadFault(err)) throw err;
       const reason = (err as Error)?.message ?? String(err);
       logger?.warn(
         `[LastAdminGuard] administrator lookup failed — ${words.noun} refused: ${reason}`,
       );
-      throw refuse(
+      throw readFault(
         `Refusing this ${words.noun}: the remaining administrators could not be verified ` +
           `(${reason}). This guard fails closed — a ${words.noun} is only permitted when at ` +
           `least one other unbanned administrator is provably left (${BREAK_GLASS_CITATION}). ` +
           'Retry once the identity tables are readable again.',
         words.table,
+        err,
       );
     }
   };
@@ -1920,7 +1965,8 @@ export function registerLastAdminGuard(
    * ({@link refuseDefaultOrganizationSlugCapture}). A payload naming another
    * slug, or none, costs no reads. Fails closed like every half: the rows a
    * predicate write addresses, or the Default Organization itself, that cannot
-   * be read refuse the write.
+   * be read refuse the write, with the same 503 read-fault code
+   * ({@link readFault}) as {@link failClosed}.
    */
   const guardOrganizationDefaultSlug = async (rawCtx: unknown): Promise<void> => {
     const ctx = ctxOf(rawCtx);
@@ -1931,14 +1977,15 @@ export function registerLastAdminGuard(
       const ids = await resolveTargetIds('organization-update', SystemObjectName.ORGANIZATION, ctx.input?.id, ctx.input?.options, data);
       await refuseDefaultOrganizationSlugCapture(engine, ids, data.slug);
     } catch (err) {
-      if (isRefusal(err)) throw err;
+      if (isRefusal(err) || isReadFault(err)) throw err;
       const reason = (err as Error)?.message ?? String(err);
-      throw refuse(
+      throw readFault(
         `Refusing this organization update: the organizations it addresses, or the Default Organization, ` +
           `could not be read (${reason}), so whether the slug '${DEFAULT_ORGANIZATION_SLUG}' would make ` +
           'another organization the Default Organization is unknown (ADR-0131 D5). Retry once ' +
           `'${SystemObjectName.ORGANIZATION}' is readable again.`,
         SystemObjectName.ORGANIZATION,
+        err,
       );
     }
   };
 
@@ -546,6 +546,27 @@ function isEnginePolicyRefusal(err: unknown): err is { code?: string; message?:
   return (err as { code?: unknown }).code === 'PERMISSION_DENIED';
 }
 
+/**
+ * A declared READ FAULT — an engine-level guard that could not complete a read
+ * its verdict depends on, and so refused the write without reaching one
+ * (`SERVICE_UNAVAILABLE`, 503, ADR-0112). The concrete case is the
+ * last-administrator guard's fail-closed envelope (`last-admin-guard.ts`,
+ * `readFault`): without this arm its refusal reached better-auth's router as an
+ * unhandled fault, a 500 with an empty body.
+ *
+ * Read by the declared `code` AND `status`, both, the way `mapDataError`'s
+ * declared-5xx relay reads them for the REST data routes, so the two
+ * transports answer the same engine error with the same status and code. The
+ * engine's prose is not relayed, as that relay withholds a declared fault's:
+ * the guard's message names the driver's own failure, which is for the server
+ * log, not the caller.
+ */
+function isEngineReadFault(err: unknown): boolean {
+  if (!err || typeof err !== 'object') return false;
+  const e = err as { code?: unknown; status?: unknown };
+  return e.code === 'SERVICE_UNAVAILABLE' && e.status === 503;
+}
+
 /**
  * [#7724] A REFERENTIAL refusal — the engine's `cascadeDeleteRelations` found
  * dependent rows it may neither cascade nor null, so it vetoed the delete
@@ -607,9 +628,9 @@ function wireDependents(raw: unknown): Array<{ object: string; count?: number }>
 
 /**
  * Re-throw `err` as a better-auth `APIError` when it is an ObjectQL validation
- * failure (400), an engine policy refusal (403) or a referential delete
- * restriction (409); otherwise re-throw it verbatim. Always throws — the return
- * type is `never`.
+ * failure (400), an engine policy refusal (403), a referential delete
+ * restriction (409) or a declared engine read fault (503); otherwise re-throw
+ * it verbatim. Always throws — the return type is `never`.
  */
 async function rethrowAsBetterAuthError(err: unknown): Promise<never> {
   if (isObjectQLValidationError(err)) {
@@ -634,6 +655,17 @@ async function rethrowAsBetterAuthError(err: unknown): Promise<never> {
       code: 'PERMISSION_DENIED',
     });
   }
+  if (isEngineReadFault(err)) {
+    const { APIError } = await import('better-auth/api');
+    const apiErr = new APIError('SERVICE_UNAVAILABLE', {
+      message:
+        'This request could not be completed: the data store could not answer a read it depends on. ' +
+        'This is a server-side outage, not a refusal of the request: retry once the data store is reachable.',
+      code: 'SERVICE_UNAVAILABLE',
+    });
+    (apiErr as { cause?: unknown }).cause = err;
+    throw apiErr;
+  }
   if (isReferentialDeleteRestriction(err)) {
     const { APIError } = await import('better-auth/api');
     throw new APIError('CONFLICT', {
@@ -655,9 +687,9 @@ async function rethrowAsBetterAuthError(err: unknown): Promise<never> {
 
 /**
  * Wrap every function-valued method of a better-auth adapter so an ObjectQL
- * `ValidationError` (400), an engine policy refusal (403) or a referential
- * delete restriction (409, #7724) thrown from the underlying engine surfaces as
- * a 4xx `APIError` instead of an opaque 500. Non-function properties pass
+ * `ValidationError` (400), an engine policy refusal (403), a referential
+ * delete restriction (409, #7724) or a declared engine read fault (503) thrown
+ * from the underlying engine surfaces as an `APIError` instead of an opaque 500. Non-function properties pass
  * through untouched, and every error that carries none of those signatures is
  * re-thrown verbatim.
  */