@@ -0,0 +1,19 @@
+---
+'@objectstack/rest': patch
+---
+
+fix(rest)!: a bracketed `id` key on the app list (`?id[]=NAME`, `?id[0]=NAME`) is refused 400 naming the key instead of being ignored and answered with the whole list
+
+Clause-②: no (narrowing) — `GET /meta/app` / `GET /meta/apps` with any query key that opens `id[` (on `RestServer` and on the runtime dispatcher, which share the list chain) is refused with `400 VALIDATION_ERROR`; it answered 200 with the caller's whole visible list before. Nothing is widened: no export, answer member, error code or status is added.
+Metadata-protocol: none
+
+<!-- adr-0087: not-required (no-migration-prescription) No key of any schema is added, removed, renamed or re-shaped, and no export or type changes: a list request that spells its one-app filter as an array key is refused instead of served unfiltered. There is nothing for a migration to rewrite and no tombstone. -->
+
+**BREAKING** accept-set narrowing on the app list read (`@objectstack/rest`).
+
+- **Before.** `?id[]=setup` and `?id[0]=setup` (the qs-style array spellings) reached the list chain under the key as spelled, which the one-app filter never read, so a caller who asked for one app got every app it may see.
+- **Now.** The list chain refuses a key that opens `id[` after the per-caller visibility gate: `400 VALIDATION_ERROR`, message `Query parameter 'id[]' names no app; the 'id' filter takes exactly one app name, spelled ?id=NAME. …` (the message names the key as sent, in the frame the empty `?id=` refusal uses), and no list in the body. A bracketed key beside a plain `?id=` is refused too.
+- **Unchanged.** `?id=NAME` narrows to that app (or an empty list when it names none); no `id` lists every app the caller may see; an empty `?id=` and a repeated `id` keep their 400s; a bracketed `id` key on any type but `app` is still ignored, as `?id=` is there.
+- **Who sees it.** An API client that serialises arrays qs-style. `@objectstack/client` and the console send no `id` on this read.
+
+**The remedy.** FROM `?id[]=NAME` (or `?id[0]=NAME`) TO `?id=NAME`; leave `id` out to list every app.
@@ -164,4 +164,51 @@ describe('[#26572] the v18 /meta read doors, over HTTP on a booted showcase', ()
       expect((await sortability(admin)).budget?.sortable).toBe(true);
     });
   });
+
+  // A client that serialises arrays qs-style sends `?id[]=NAME` or
+  // `?id[0]=NAME`. The adapter hands the key through as spelled (`id[]`), so
+  // the app list's one-app filter never read it and the caller's whole list
+  // came back for a request that asked for one app. It is refused like the
+  // empty `?id=`: 400 VALIDATION_ERROR naming the key, no list.
+  describe('GET /meta/app with a bracketed `id` key is refused, never answered with the whole list', () => {
+    const appNames = (body: any): string[] => ((Array.isArray(body) ? body : body?.items) ?? []).map((a: any) => a?.name);
+
+    for (const spelling of ['id[]', 'id[0]']) {
+      it(`?${spelling}=setup: 400 VALIDATION_ERROR naming the key, for the admin and for a member`, async () => {
+        for (const [who, token] of [['admin', admin], ['member', liaison]] as const) {
+          const res = await stack.apiAs(token, 'GET', `/meta/app?${spelling}=setup`);
+          const body: any = await res.json();
+          expect({ who, status: res.status, code: codeOf(body) }, JSON.stringify(body).slice(0, 400))
+            .toEqual({ who, status: 400, code: 'VALIDATION_ERROR' });
+          expect(messageOf(body)).toMatch(new RegExp(`^Query parameter '${spelling.replace(/[[\]]/g, '\\$&')}' names no app`));
+          expect(appNames(body)).toEqual([]);
+        }
+      });
+    }
+
+    it('CONTROLS — ?id=NAME is one app, ?id= and a repeated id are 400, no id is the pruned list', async () => {
+      const one = await stack.apiAs(admin, 'GET', '/meta/app?id=showcase_app');
+      expect(one.status).toBe(200);
+      expect(appNames(await one.json())).toEqual(['showcase_app']);
+      // The filter narrows: `setup` names no app on this stack, so `?id=setup`
+      // is an empty list where `?id[]=setup` answered the whole one.
+      const none = await stack.apiAs(admin, 'GET', '/meta/app?id=setup');
+      expect(none.status).toBe(200);
+      expect(appNames(await none.json())).toEqual([]);
+      for (const query of ['id=', 'id=showcase_app&id=showcase_app']) {
+        const res = await stack.apiAs(admin, 'GET', `/meta/app?${query}`);
+        expect({ query, status: res.status, code: codeOf(await res.json()) })
+          .toEqual({ query, status: 400, code: 'VALIDATION_ERROR' });
+      }
+      const adminList = await stack.apiAs(admin, 'GET', '/meta/app');
+      const memberList = await stack.apiAs(liaison, 'GET', '/meta/app');
+      expect(adminList.status).toBe(200);
+      expect(memberList.status).toBe(200);
+      const adminApps = appNames(await adminList.json());
+      const memberApps = appNames(await memberList.json());
+      expect(adminApps, JSON.stringify({ adminApps, memberApps })).toContain('showcase_app');
+      expect(memberApps, JSON.stringify({ adminApps, memberApps })).toContain('showcase_app');
+      expect(memberApps, 'a member never sees setup').not.toContain('setup');
+    });
+  });
 });
@@ -338,6 +338,42 @@ describe('#7566 — `GET /meta/app?id=` narrows the list instead of being droppe
         }
     });
 
+    // A client that serialises arrays qs-style sends `?id[]=crm` or
+    // `?id[0]=crm`. The adapters hand the key through as spelled, so the
+    // filter, which reads `query.id`, never saw it and the caller who asked for
+    // one app got every app it may see. Refused with the empty `?id=`'s
+    // answer, naming the key as sent; the dispatcher's twin is pinned in
+    // `@objectstack/runtime`'s list parity census.
+    it.each(['id[]', 'id[0]', 'id[name]'])('a BRACKETED `%s` key is refused with 400 naming it — never the whole list', async (key) => {
+        for (const perms of [['studio.access'], ['manage_users'], []]) {
+            const { rest } = setup(perms);
+            const res = await getList(rest, 'app', { [key]: 'crm' });
+
+            expect(res.statusCode, `caller holding [${perms}]`).toBe(400);
+            expect(res.body?.code).toBe('VALIDATION_ERROR');
+            expect(String(res.body?.error).startsWith(`Query parameter '${key}' names no app`)).toBe(true);
+            expect(res.body?.items).toBeUndefined();
+            expect(Array.isArray(res.body)).toBe(false);
+        }
+        // The plural spelling refuses identically, and so does a bracketed key
+        // beside a plain one: two spellings of one filter.
+        for (const [type, query] of [['apps', { [key]: 'crm' }], ['app', { id: 'crm', [key]: 'account' }]] as const) {
+            const res = await getList(setup(['manage_users']).rest, type, query);
+            expect(res.statusCode, `${type} ${JSON.stringify(query)}`).toBe(400);
+            expect(res.body?.code).toBe('VALIDATION_ERROR');
+        }
+    });
+
+    it('a bracketed `id` key on another list type is not read, as no other `id` spelling is (control)', async () => {
+        const { rest, protocol } = setup(['manage_users']);
+        protocol.getMetaItems = vi.fn(async ({ type: t }: any) =>
+            String(t ?? '').startsWith('view') ? [{ name: 'all_leads', viewKind: 'list', object: 'lead' }] : []);
+        const res = await getList(rest, 'view', { 'id[]': 'nope' });
+
+        expect(res.statusCode).toBe(200);
+        expect(namesFrom(res.body)).toEqual(['all_leads']);
+    });
+
     it('a WHITESPACE id keeps its answer — a value no app is named, so an empty list (#28548 control)', async () => {
         const { rest } = setup(['manage_users']);
         const res = await getList(rest, 'app', { id: ' ' });
 
@@ -3418,9 +3418,46 @@ export type MetaListAnswer =
  * the dispatcher's `errorFromThrown`), as {@link refuseUnknownMetaListType} does.
  */
 function refuseEmptyListFilter(param: 'id' | 'object', names: 'app' | 'object', lists: 'app' | 'view'): never {
+    throwListFilterRefusal(param, param, names, lists, 'got ""', `an empty ${names} name`);
+}
+
+/**
+ * The app list's answer to a BRACKETED spelling of its one-app filter —
+ * `?id[]=NAME`, `?id[0]=NAME`, any key that opens `id[`. A client that
+ * serialises arrays qs-style sends it; every adapter hands the key through as
+ * spelled (`readQuery` in `plugin-hono-server`, `readDispatchQuery` in
+ * `@objectstack/hono`), so step 3 of {@link createMetaListAnswer}, which reads
+ * `query.id`, never saw it, and the caller who asked for one app was answered
+ * every app it may see. Refused with {@link refuseEmptyListFilter}'s answer —
+ * one sentence frame, `400 VALIDATION_ERROR` — naming the key as sent: the
+ * filter takes exactly one name, spelled `?id=NAME`. A bracketed key beside a
+ * plain `?id=` is refused too: the request carries two spellings of one filter.
+ *
+ * Only the app list reads `id`, so only it judges the bracketed spelling; on
+ * every other type the key is not read, as no other `id` spelling is.
+ */
+function refuseBracketedAppIdFilter(query: Readonly<Record<string, unknown>> | undefined): void {
+    if (!query || typeof query !== 'object') return;
+    const key = Object.keys(query).find((k) => k.startsWith('id['));
+    if (key === undefined) return;
+    throwListFilterRefusal(
+        'id', key, 'app', 'app',
+        "the 'id' filter takes exactly one app name, spelled ?id=NAME", "a bracketed 'id' key",
+    );
+}
+
+/** The one sentence frame of the list chain's narrowing-filter refusals. */
+function throwListFilterRefusal(
+    param: 'id' | 'object',
+    key: string,
+    names: 'app' | 'object',
+    lists: 'app' | 'view',
+    got: string,
+    notAFilter: string,
+): never {
     const err: any = new Error(
-        `Query parameter '${param}' names no ${names}; got "". The list was not served: an empty ${names} `
-        + `name is not a filter. Name one ${names}, or leave the parameter out to list every ${lists} you can see.`,
+        `Query parameter '${key}' names no ${names}; ${got}. The list was not served: ${notAFilter} `
+        + `is not a filter. Name one ${names}, or leave the parameter out to list every ${lists} you can see.`,
     );
     err.status = 400;
     err.code = 'VALIDATION_ERROR';
@@ -3516,8 +3553,9 @@ async function describeMetaPositionList(sources: MetaListAnswerSources, list: an
  *     items, before the locale collapse (`_packageId` scopes membership).
  *  3. `app` — `?id=<app>` narrows to the app of that `name` (#7566); absent
  *     means no filter, and [#28548] an EMPTY value is refused (`400
- *     VALIDATION_ERROR`), never read as absent; no match is an EMPTY list,
- *     never a 404.
+ *     VALIDATION_ERROR`), never read as absent, as is a bracketed key
+ *     (`?id[]=`, `?id[0]=`), never read as no filter; no match is an EMPTY
+ *     list, never a 404.
  *  4. `view` — `?object=<object>` keeps the independent views bound to that
  *     object, sorted by `order` then `name` (the switcher); [#28548] an EMPTY
  *     value is refused (`400 VALIDATION_ERROR`), never read as absent.
@@ -3575,7 +3613,9 @@ export function createMetaListAnswer(
         }
 
         // 3. [#7566] `?id=<app>`. [#28548] An EMPTY one is refused, never
-        // read as absent (see {@link refuseEmptyListFilter}).
+        // read as absent (see {@link refuseEmptyListFilter}), and so is a
+        // bracketed spelling of the key (see {@link refuseBracketedAppIdFilter}).
+        if (metaType === 'app') refuseBracketedAppIdFilter(query);
         const appIdFilter = metaType === 'app' ? query?.id : undefined;
         if (appIdFilter === '') refuseEmptyListFilter('id', 'app', 'app');
         if (typeof appIdFilter === 'string') {
 
@@ -525,6 +525,10 @@ const PARAM_PROBES: ReadonlyArray<{ label: string; param?: string; query: Record
     // [#28548] An EMPTY app name is refused (400 VALIDATION_ERROR) on both
     // transports, never read as absent.
     { label: '?id= (empty)', param: 'id', query: { id: '' } },
+    // …and so is a BRACKETED `id` key (a qs-style array spelling), never
+    // read as no filter.
+    { label: '?id[]=crm', param: 'id', query: { 'id[]': 'crm' } },
+    { label: '?id[0]=crm', param: 'id', query: { 'id[0]': 'crm' } },
     { label: '?object=lead', param: 'object', query: { object: 'lead' } },
     { label: '?object=nope', param: 'object', query: { object: 'nope' } },
     // [#28548] …and so is an EMPTY view object.
@@ -687,6 +691,8 @@ describe('[#20320] row A: the reference moves — each probe changes RestServer\
         expect((await read('/meta/app', { id: 'nope' })).items).toEqual([]);
         const blank = await read('/meta/app', { id: '' });
         expect({ status: blank.status, code: blank.code, items: blank.items }).toEqual({ status: 400, code: 'VALIDATION_ERROR', items: undefined });
+        const bracketed = await read('/meta/app', { 'id[]': 'crm' });
+        expect({ status: bracketed.status, code: bracketed.code, items: bracketed.items }).toEqual({ status: 400, code: 'VALIDATION_ERROR', items: undefined });
         expect(names((await read('/meta/view', { object: 'lead' })).items)).toEqual(['lead_mine', 'lead_all']);
         expect((await read('/meta/views', { object: 'nope' })).items).toEqual([]);
         const blankObject = await read('/meta/views', { object: '' });