@@ -0,0 +1,22 @@
+---
+'@objectstack/metadata-protocol': minor
+'@objectstack/runtime': patch
+---
+
+fix(metadata-protocol)!: the generic data door refuses a stored-metadata filter that reads the body or a content hash through a cross-field comparand or below its depth backstop, and exports its one filter-field collector and one search narrowing for the reader-context seam (#21544)
+
+Clause-②: yes (narrowing)
+
+<!-- adr-0087: not-required (no-migration-prescription) no metadata body, authorable key, spelling, export or stored shape moves; what changes is which read-query shapes the generic data door accepts over the two stored-metadata tables, and two module functions are added to the package surface, so `objectstack migrate meta` has nothing to rewrite. The other categories are closed on facts: both packages publish (not `unpublished`); no ADR-0087 id covers a refused query shape (not `registered` / `already-registered`); and the change is runtime behaviour plus additive exports, not a declaration (not `runtime-interface-only` / `type-surface-only`). -->
+
+**BREAKING**: this narrows what the generic data door (`GET /api/v1/data/:object`, `POST /api/v1/data/:object/query` and the in-process `findData`) accepts when it reads `sys_metadata` or `sys_metadata_history`. Two filter shapes read the stored body column or a content-hash column (`checksum`, `previous_checksum`, or the history table's `change_note`) without the family's refusal ever seeing them, and both ran before this release:
+
+- a cross-field comparand naming one of those columns — `{ "name": { "$ne": { "$field": "metadata" } } }`, in `where` or in an aggregation's `filter`, under `$not` included. The SQL drivers evaluate it row by row, so row presence disclosed the column's value;
+- a filter on one of those columns nested more than 32 combinators deep, which the door's field collector stopped reading at. A body `$contains` of a stored credential answered the row and a wrong guess answered none.
+
+Both now answer the door's `400 INVALID_FIELD`, naming the column, before the query runs — the answer the same filter already gets when it names the column directly. The route: filter those tables by their scalar columns (the type, the name, the state and the like), compare scalar columns with each other, and read the bodies with a plain list, which is served projected. Every other column of the two tables, and every other object, is unchanged; a dotted key into one of those columns was, and stays, refused by the door's dotted-path rule. It ships as `minor` under the launch-window convention for accept-set narrowings.
+
+- **`@objectstack/metadata-protocol`** exports two module functions the generic data door now calls itself:
+  - `collectStoredMetadataFilterFields(object, query)` — the family's one filter-field collector: every column a read query's filters read (`where`, the engine's `filter` alias and each aggregation filter): each key's head and each cross-field `{ $field }` comparand, at any depth. `[]` outside the family.
+  - `narrowStoredMetadataSearch(object, query, schema, wireSpelling?)` — the family's one default-search narrowing: an explicit search-field list naming the body or a hash column is refused, a default search is narrowed to the searchable set without them (returned for the caller to run as `searchFields`), and a set that narrows to nothing is refused. The `StoredMetadataSearchSchema` type it reads is exported beside it.
+- **`@objectstack/runtime`**: the stored-metadata reader-context seam (`ctx.api.object(...)` for action and hook bodies, a handler's `ctx.api`, and `ctx.engine.find`) calls those two functions instead of its own copy of the narrowing and `@objectstack/plugin-security`'s condition walk, so the seam and the door answer every family filter and search identically. A `count` through the seam now runs the query the guard returns. The seam's accept set is unchanged: every shape it refused before it still refuses, now through the door's collector.
@@ -220,6 +220,13 @@ export {
   storedMetadataHashEvaluateRefusal,
   storedMetadataSearchRefusal,
 } from './metadata-redaction.js';
+// [#21544] …and what those refusals are FED: the door's one default-search
+// narrowing and its one filter-field collector, module functions the door
+// itself calls. Exported so the reader-context seam calls the same two
+// functions rather than re-stating either — a second control flow over the
+// same columns is where the two doors would drift.
+export { collectStoredMetadataFilterFields, narrowStoredMetadataSearch } from './protocol.js';
+export type { StoredMetadataSearchSchema } from './protocol.js';
 
 export type { MetadataHostEngine } from './host-engine.js';
 
 
@@ -742,8 +742,9 @@ export function storedMetadataBodyGroupingRefusal(object: string, groupBy: unkno
  * the grouping refusal above take. Same family, shape and code: `INVALID_FIELD`
  * / 400, naming the field, the object and the offending `param`.
  *
- * `filterFields` is the set of head field names the caller's `where` names
- * (`collectFilterFieldKeys`), and `sortFields` the fields its `orderBy` names.
+ * `filterFields` is the set of columns the caller's filters read
+ * (`collectStoredMetadataFilterFields`, `protocol.ts`: each key's head and each
+ * cross-field comparand's), and `sortFields` the fields its `orderBy` names.
  * Filter is judged before sort — a query that does both reads "the filter was
  * not run" first. `undefined` when neither names the body column.
  */
 
@@ -0,0 +1,234 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#21544] The stored-metadata family's ONE filter-field collector and ONE
+ * default-search narrowing — the two module functions the generic data door
+ * calls and exports, so the in-process reader-context seam
+ * (`@objectstack/runtime`) calls the same two (the door / seam parity table
+ * lives beside the seam, `stored-metadata-reader-seam.test.ts`).
+ *
+ * The door's collector used to be the ingress key collector, and the
+ * measurement on this card found two shapes that READ a family column at the
+ * generic data door without the family's refusal ever seeing it, on both
+ * `sys_metadata` and `sys_metadata_history`, over HTTP and in-process alike:
+ *
+ *  - a cross-field comparand (`{ name: { $ne: { $field: 'metadata' } } }`),
+ *    which the SQL drivers evaluate row by row — the comparison partitioned
+ *    the rows by the stored body's value;
+ *  - a direct predicate nested below the key collector's 32-level depth
+ *    backstop, which ran as written: a body `$contains` of the stored
+ *    credential answered the row and a wrong guess answered none, on
+ *    driver-sqlite-wasm and driver-memory — a credential oracle.
+ *
+ * Both are refused now, before the engine is asked, in the family's envelope.
+ */
+
+import { describe, expect, it, vi } from 'vitest';
+import { SysMetadataHistoryObject, SysMetadataObject } from '@objectstack/metadata-core';
+import {
+    collectStoredMetadataFilterFields,
+    narrowStoredMetadataSearch,
+    ObjectStackProtocolImplementation,
+} from './protocol.js';
+
+/** `n` nested `$and` levels above `leaf`: the leaf sits at depth `n`. */
+const deep = (n: number, leaf: Record<string, unknown>): Record<string, unknown> =>
+    (n === 0 ? leaf : { $and: [deep(n - 1, leaf)] });
+
+const sorted = (names: readonly string[]) => [...names].sort();
+
+describe('[#21544] collectStoredMetadataFilterFields — every column a read query\'s filters read', () => {
+    const cases: Array<[string, unknown, string[]]> = [
+        ['a key', { where: { type: 'view' } }, ['type']],
+        ['a dotted key reads its head', { where: { 'metadata.config': 'x' } }, ['metadata']],
+        ['a cross-field comparand', { where: { name: { $eq: { $field: 'metadata' } } } }, ['metadata', 'name']],
+        ['a reference as the implicit-equality comparand', { where: { name: { $field: 'checksum' } } }, ['checksum', 'name']],
+        ['a reference in a list', { where: { name: { $in: [{ $field: 'checksum' }] } } }, ['checksum', 'name']],
+        [
+            'a reference in an addDays offset',
+            { where: { created_at: { $lte: { $field: 'created_at', addDays: { $field: 'previous_checksum' } } } } },
+            ['created_at', 'previous_checksum'],
+        ],
+        ['a dotted reference reads its head', { where: { name: { $ne: { $field: 'metadata.x' } } } }, ['metadata', 'name']],
+        [
+            'under $and / $or / $not',
+            { where: { $or: [{ $not: { name: { $lt: { $field: 'change_note' } } } }, { $and: [{ type: 'x' }] }] } },
+            ['change_note', 'name', 'type'],
+        ],
+        ['under an unrecognised $ key', { where: { $nor: [{ metadata: 'x' }] } }, ['metadata']],
+        [
+            'a nested-relation condition: its keys are another object\'s, a reference beneath it is read',
+            { where: { organization_id: { metadata: 'x', name: { $eq: { $field: 'checksum' } } } } },
+            ['checksum', 'organization_id'],
+        ],
+        ['at any depth: a key 40 levels down', { where: deep(40, { metadata: { $contains: 'z' } }) }, ['metadata']],
+        ['at any depth: a reference 40 levels down', { where: deep(40, { name: { $ne: { $field: 'checksum' } } }) }, ['checksum', 'name']],
+        ['a FilterArray is lowered first', { where: [['metadata', 'contains', 'z']] }, ['metadata']],
+        ['the engine\'s `filter` alias', { filter: { checksum: 'x' } }, ['checksum']],
+        [
+            'each aggregation filter',
+            { aggregations: [{ function: 'count', alias: 'n', filter: { name: { $ne: { $field: 'metadata' } } } }] },
+            ['metadata', 'name'],
+        ],
+        [
+            'not `having`: its names are the aggregated row\'s, an alias spelled like a family column included',
+            { groupBy: ['type'], aggregations: [{ function: 'count', alias: 'metadata' }], having: { metadata: { $gt: 0 } } },
+            [],
+        ],
+    ];
+    for (const [label, query, expected] of cases) {
+        it(label, () => {
+            expect(sorted(collectStoredMetadataFilterFields('sys_metadata', query))).toEqual(expected);
+            expect(sorted(collectStoredMetadataFilterFields('sys_metadata_history', query))).toEqual(expected);
+        });
+    }
+
+    it('[] for an object outside the family, and for a query that is not a record', () => {
+        expect(collectStoredMetadataFilterFields('file_blob', { where: { metadata: 'x' } })).toEqual([]);
+        expect(collectStoredMetadataFilterFields('sys_metadata', null)).toEqual([]);
+        expect(collectStoredMetadataFilterFields('sys_metadata', [{ metadata: 'x' }])).toEqual([]);
+    });
+
+    it('terminates on a self-referential live object, and reads a node shared as a comparand and a condition both ways', () => {
+        const cyclic: Record<string, unknown> = { type: 'x' };
+        cyclic.$and = [cyclic];
+        expect(collectStoredMetadataFilterFields('sys_metadata', { where: cyclic })).toEqual(['type']);
+        const shared = { metadata: 'x' };
+        expect(sorted(collectStoredMetadataFilterFields('sys_metadata', { where: { name: { $eq: shared }, $and: [shared] } })))
+            .toEqual(['metadata', 'name']);
+    });
+});
+
+describe('[#21544] narrowStoredMetadataSearch — the one default-search narrowing', () => {
+    function refusalOf(run: () => unknown): any {
+        try {
+            run();
+        } catch (e) {
+            return e;
+        }
+        throw new Error('expected a refusal, but the search was not refused');
+    }
+
+    it('an explicit list naming the body or a hash column is refused, under the slot the caller used', () => {
+        for (const [query, wire, param, field] of [
+            [{ search: 'z', searchFields: ['name', 'metadata'] }, {}, 'searchFields', 'metadata'],
+            [{ search: 'z', searchFields: 'name, checksum' }, {}, 'searchFields', 'checksum'],
+            [{ search: { query: 'z', fields: ['previous_checksum'] } }, {}, 'search', 'previous_checksum'],
+            [{ search: 'z', searchFields: 'change_note' }, { searchFields: '$searchFields' }, '$searchFields', 'change_note'],
+        ] as const) {
+            const err = refusalOf(() => narrowStoredMetadataSearch('sys_metadata_history', query, SysMetadataHistoryObject, wire));
+            expect(err).toMatchObject({ code: 'INVALID_FIELD', status: 400, param, field, object: 'sys_metadata_history' });
+        }
+    });
+
+    it('an explicit list that names neither runs as it is', () => {
+        expect(narrowStoredMetadataSearch('sys_metadata', { search: 'z', searchFields: ['name'] }, SysMetadataObject)).toBeUndefined();
+    });
+
+    it('a default search is narrowed to the searchable set without the body and hash columns', () => {
+        for (const [object, schema, unscannable] of [
+            ['sys_metadata', SysMetadataObject, ['metadata', 'checksum']],
+            ['sys_metadata_history', SysMetadataHistoryObject, ['metadata', 'checksum', 'previous_checksum', 'change_note']],
+        ] as const) {
+            const narrowed = narrowStoredMetadataSearch(object, { search: 'z' }, schema);
+            expect(narrowed).toContain('name');
+            for (const column of unscannable) expect(narrowed).not.toContain(column);
+        }
+    });
+
+    it('a default search whose set narrows to nothing is refused, never handed on empty', () => {
+        const onlyFamily = { ...SysMetadataObject, searchableFields: ['metadata', 'checksum'] };
+        const err = refusalOf(() => narrowStoredMetadataSearch('sys_metadata', { search: 'z' }, onlyFamily, { search: '$search' }));
+        expect(err).toMatchObject({ code: 'INVALID_FIELD', status: 400, param: '$search', field: 'metadata' });
+    });
+
+    it('runs as it is outside the family, without a search, and without a readable field map', () => {
+        expect(narrowStoredMetadataSearch('file_blob', { search: 'z', searchFields: ['metadata'] }, SysMetadataObject)).toBeUndefined();
+        expect(narrowStoredMetadataSearch('sys_metadata', { where: { type: 'view' } }, SysMetadataObject)).toBeUndefined();
+        for (const schema of [undefined, {}, { fields: [] }, { fields: {} }]) {
+            expect(narrowStoredMetadataSearch('sys_metadata', { search: 'z' }, schema)).toBeUndefined();
+        }
+    });
+});
+
+describe('[#21544] data door — a filter that reads the body or a hash without naming it as a key is refused', () => {
+    const SCHEMAS: Record<string, unknown> = {
+        sys_metadata: SysMetadataObject,
+        sys_metadata_history: SysMetadataHistoryObject,
+        file_blob: { name: 'file_blob', fields: { name: { name: 'name', type: 'text' }, checksum: { name: 'checksum', type: 'text' } } },
+    };
+
+    function makeProtocol() {
+        const find = vi.fn(async () => []);
+        const aggregate = vi.fn(async () => []);
+        const count = vi.fn(async () => 0);
+        // `findData` reads through find / count / aggregate only — no `findOne`
+        // on this double, so a door that started calling it would fail loudly.
+        const engine: any = { registry: { getObject: (n: string) => SCHEMAS[n] }, find, count, aggregate };
+        return { p: new ObjectStackProtocolImplementation(engine), find, aggregate, count };
+    }
+
+    async function refusal(run: () => Promise<unknown>): Promise<any> {
+        try {
+            await run();
+        } catch (e) {
+            return e;
+        }
+        throw new Error('expected a refusal, but the query ran');
+    }
+
+    const shapes: Array<[string, string, (column: string) => Record<string, unknown>]> = [
+        ['a cross-field comparand in `where`', 'filter', (c) => ({ where: { name: { $ne: { $field: c } } } })],
+        ['a cross-field comparand under $not', 'filter', (c) => ({ where: { $not: { type: { $lt: { $field: c } } } } })],
+        ['a direct predicate 33 levels deep', 'filter', (c) => ({ where: deep(33, { [c]: { $contains: 'guess' } }) })],
+        ['a cross-field comparand 33 levels deep', 'filter', (c) => ({ where: deep(33, { name: { $eq: { $field: c } } }) })],
+        [
+            'a cross-field comparand in an aggregation filter',
+            'filter',
+            (c) => ({ groupBy: ['type'], aggregations: [{ function: 'count', alias: 'n', filter: { name: { $ne: { $field: c } } } }] }),
+        ],
+        [
+            'a direct predicate 33 levels deep in an aggregation filter',
+            'filter',
+            (c) => ({ groupBy: ['type'], aggregations: [{ function: 'count', alias: 'n', filter: deep(33, { [c]: { $contains: 'guess' } }) }] }),
+        ],
+    ];
+    const columns: Array<[string, string]> = [
+        ['sys_metadata', 'metadata'],
+        ['sys_metadata', 'checksum'],
+        ['sys_metadata_history', 'metadata'],
+        ['sys_metadata_history', 'previous_checksum'],
+        ['sys_metadata_history', 'change_note'],
+    ];
+    for (const [label, param, build] of shapes) {
+        for (const [object, column] of columns) {
+            it(`${label}, reading '${column}' on '${object}': INVALID_FIELD / 400, and the engine is never asked`, async () => {
+                const { p, find, aggregate, count } = makeProtocol();
+                const err = await refusal(() => p.findData({ object, query: build(column) }));
+                expect(err).toMatchObject({ code: 'INVALID_FIELD', status: 400, param, field: column, object });
+                expect(find).not.toHaveBeenCalled();
+                expect(aggregate).not.toHaveBeenCalled();
+                expect(count).not.toHaveBeenCalled();
+            });
+        }
+    }
+
+    it('control: the same shapes over scalar columns of a family table still run', async () => {
+        const { p, find, aggregate } = makeProtocol();
+        await p.findData({ object: 'sys_metadata', query: { where: { name: { $ne: { $field: 'type' } } } } });
+        await p.findData({ object: 'sys_metadata', query: { where: deep(33, { type: 'view' }) } });
+        expect(find).toHaveBeenCalledTimes(2);
+        await p.findData({
+            object: 'sys_metadata',
+            query: { groupBy: ['type'], aggregations: [{ function: 'count', alias: 'n', filter: { name: { $ne: { $field: 'type' } } } }] },
+        });
+        expect(aggregate).toHaveBeenCalledTimes(1);
+    });
+
+    it('control: an object outside the family reads its own `checksum` through a comparand and at depth', async () => {
+        const { p, find } = makeProtocol();
+        await p.findData({ object: 'file_blob', query: { where: { name: { $ne: { $field: 'checksum' } } } } });
+        await p.findData({ object: 'file_blob', query: { where: deep(33, { checksum: 'sha256:0000' }) } });
+        expect(find).toHaveBeenCalledTimes(2);
+    });
+});