@@ -0,0 +1,14 @@
+---
+'@objectstack/objectql': patch
+'@objectstack/metadata-protocol': patch
+---
+
+fix(objectql,metadata-protocol): an aggregate query that would answer a wrong table is refused 400, and an empty aggregate list no longer answers "not registered"
+
+Clause-②: no
+Metadata-protocol: none
+
+- **Date bucket on a non-date field.** `engine.aggregate` refuses a `groupBy` entry whose `dateGranularity` buckets a field the object declares neither a `date` nor a `datetime`: `INVALID_FIELD` / 400, `path: 'groupBy.N.dateGranularity'`. Before, every row fell into one `null` bucket (a `number` field into a `1970-01` bucket) under 200. A `formula` or undeclared field is not judged.
+- **Two columns under one name.** `engine.aggregate` refuses a query whose aggregated row would carry two values under one column: an aggregation `alias` used twice, an alias equal to a group column (the field name, or the entry's own `alias`), or two group entries projecting one column. `INVALID_QUERY` / 400, `path` at the second position (`aggregations.1.alias`, `groupBy.1.field`). Before, the last value written won under 200: a count overwritten by a sum, or the group keys replaced by counts. Give the entry its own `alias`.
+- Both refusals run at the engine's `aggregate` entry, before any driver is asked, so the REST query door, its dispatcher twin, the analytics ObjectQL strategy and the MCP data bridge answer alike.
+- **Empty aggregate list.** `POST /data/:object/query` with `aggregations: []` (or `groupBy: []`, or both) and no non-empty axis beside it answers `400 INVALID_QUERY` naming `query.aggregations` (or `query.groupBy`) as empty. Before, it answered `404 OBJECT_NOT_FOUND`, "Object '…' is not registered", for a registered object. An empty list beside a non-empty axis is served as before.
@@ -0,0 +1,75 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#25819] An aggregate axis sent as an EMPTY list, with no non-empty axis
+ * beside it, is refused `INVALID_QUERY` / 400 by `findData`, naming the empty
+ * key, before the engine is asked.
+ *
+ * Measured on `origin/main` `cb0a2fcf4f` through `findData` over the real
+ * engine: `{ aggregations: [] }` (with or without a `where`), `{ groupBy: [] }`
+ * and both together routed to `engine.find`, which refuses an option it does
+ * not execute with an uncoded error ("find('acct') does not recognise option
+ * 'aggregations'…"); the REST door's unknown-object heuristic reads the quoted
+ * object name in it as `404 OBJECT_NOT_FOUND`, "Object 'acct' is not
+ * registered". The wire half is pinned over the real engine in
+ * `@objectstack/objectql`'s `engine-aggregate-query-shape-door.test.ts`.
+ */
+
+import { describe, it, expect, vi } from 'vitest';
+import { ObjectStackProtocolImplementation } from './protocol.js';
+
+const SCHEMA = {
+    name: 'acct',
+    fields: {
+        name: { name: 'name', type: 'text' },
+        status: { name: 'status', type: 'text' },
+    },
+};
+
+function makeProtocol() {
+    const find = vi.fn(async () => [] as unknown[]);
+    const aggregate = vi.fn(async () => [] as unknown[]);
+    const engine = {
+        registry: { getObject: (n: string) => (n === 'acct' ? SCHEMA : undefined) },
+        find,
+        aggregate,
+        count: vi.fn(async () => 0),
+    };
+    return { p: new ObjectStackProtocolImplementation(engine as any), find, aggregate };
+}
+
+describe('[#25819] findData — an empty aggregate axis is refused for what it is', () => {
+    const REFUSED: ReadonlyArray<readonly [Record<string, unknown>, string, string]> = [
+        [{ aggregations: [] }, 'aggregations', "'query.aggregations' is an empty list"],
+        [{ aggregations: [], where: { status: 'open' } }, 'aggregations', "'query.aggregations' is an empty list"],
+        [{ groupBy: [] }, 'groupBy', "'query.groupBy' is an empty list"],
+        [{ groupBy: [], aggregations: [] }, 'aggregations', "'query.groupBy' and 'query.aggregations' are empty lists"],
+    ];
+
+    for (const [query, param, words] of REFUSED) {
+        it(`refuses ${JSON.stringify(query)} with INVALID_QUERY / 400 — the engine is never asked`, async () => {
+            const { p, find, aggregate } = makeProtocol();
+            const err: any = await p.findData({ object: 'acct', query } as never).then(() => null, (e) => e);
+            expect({ code: err?.code, status: err?.status, param: err?.param, object: err?.object })
+                .toEqual({ code: 'INVALID_QUERY', status: 400, param, object: 'acct' });
+            expect(err.message).toContain(words);
+            expect(find).not.toHaveBeenCalled();
+            expect(aggregate).not.toHaveBeenCalled();
+        });
+    }
+
+    it('CONTROL an empty list beside a non-empty axis is served as before, through engine.aggregate', async () => {
+        const { p, find, aggregate } = makeProtocol();
+        await p.findData({ object: 'acct', query: { groupBy: ['status'], aggregations: [] } } as never);
+        await p.findData({ object: 'acct', query: { groupBy: [], aggregations: [{ function: 'count', alias: 'n' }] } } as never);
+        expect(aggregate).toHaveBeenCalledTimes(2);
+        expect(find).not.toHaveBeenCalled();
+    });
+
+    it('CONTROL a query with neither axis is the plain row read', async () => {
+        const { p, find, aggregate } = makeProtocol();
+        await p.findData({ object: 'acct', query: { where: { status: 'open' } } } as never);
+        expect(find).toHaveBeenCalledTimes(1);
+        expect(aggregate).not.toHaveBeenCalled();
+    });
+});
@@ -13463,6 +13463,37 @@ export class ObjectStackProtocolImplementation implements
         // rows returned ungrouped, looking exactly like a served query.
         this.assertGroupByFieldsExist(request.object, options.groupBy);
         this.assertAggregationFieldsExist(request.object, options.aggregations);
+        // [#25819] An aggregate axis sent as an EMPTY list, with no non-empty
+        // axis beside it, asks for an aggregate of nothing. The routing below
+        // reads it as no aggregate and handed the key to `engine.find`, which
+        // refuses an option it does not execute with an uncoded error whose
+        // words carry the quoted object name — read at the REST door as the
+        // unknown-object heuristic: `404 OBJECT_NOT_FOUND`, "Object '…' is not
+        // registered", for a registered object. No shipped caller sends `[]`
+        // meaning the plain rows, so it is refused for what it is. An empty
+        // list beside a non-empty one is served as before (`groupBy` with
+        // `aggregations: []` lists the groups).
+        {
+            const emptyAxes = (['groupBy', 'aggregations'] as const)
+                .filter((k) => Array.isArray(options[k]) && options[k].length === 0);
+            const servedAxis = (['groupBy', 'aggregations'] as const)
+                .some((k) => Array.isArray(options[k]) && options[k].length > 0);
+            if (emptyAxes.length > 0 && !servedAxis) {
+                const named = emptyAxes.map((k) => `'query.${k}'`).join(' and ');
+                const err: any = new Error(
+                    `${named} ${emptyAxes.length > 1 ? 'are empty lists' : 'is an empty list'} on object `
+                    + `'${request.object}': an aggregate query names at least one aggregation, or a non-empty `
+                    + `'groupBy' to list its groups. The query was NOT run. Send e.g. { "aggregations": `
+                    + `[{ "function": "count", "alias": "n" }] }, or omit ${emptyAxes.map((k) => `'${k}'`).join(' and ')} `
+                    + 'for the plain rows.',
+                );
+                err.status = 400;
+                err.code = 'INVALID_QUERY';
+                err.param = emptyAxes[emptyAxes.length - 1];
+                err.object = request.object;
+                throw err;
+            }
+        }
         // [#21086] A stored metadata body is served only as its type's read
         // projection (see `redactStoredMetadataRows` below), and a GROUP KEY
         // cannot be projected — so grouping by the body column of a
 
@@ -0,0 +1,177 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#25818] Two shapes of an aggregate query that the engine's `aggregate`
+ * answered with a plausible table under 200, now refused before any driver is
+ * asked:
+ *
+ * 1. a `groupBy` entry that date-buckets (`{ field, dateGranularity }`) a field
+ *    the object does not declare a date or datetime —
+ *    {@link assertDateBucketTargetsAreTemporal};
+ * 2. two columns of the aggregated row under one name: an aggregation alias
+ *    used twice, an aggregation alias equal to a group column, or two group
+ *    entries projecting one column — {@link assertAggregatedColumnsAreDistinct}.
+ *
+ * ## What ran before this door, measured on `origin/main` `cb0a2fcf4f`
+ *
+ * Through `findData` (the `POST /api/v1/data/:object/query` door) over three
+ * rows, `status` holding `open`, `open`, `won`:
+ *
+ * | query | answer |
+ * |:--|:--|
+ * | `groupBy: [{ field: 'name', dateGranularity: 'month' }]` (a `text` field) | `200 [{ name: null, n: 3 }]`: every row in one `null` bucket |
+ * | `groupBy: [{ field: 'amount', dateGranularity: 'month' }]` (a `number` field) | `200 [{ amount: '1970-01', n: 3 }]`: the numbers read as epoch milliseconds |
+ * | `groupBy: [{ field: 'at_time', dateGranularity: 'day' }]` (a `time` field) | `200 [{ at_time: null, n: 3 }]` |
+ * | `aggregations: [{ count, alias: 'n' }, { sum amount, alias: 'n' }]` | `200 [{ n: 60 }]`: the count overwritten by the sum |
+ * | `groupBy: ['status'], aggregations: [{ count, alias: 'status' }]` | `200 [{ status: 2 }, { status: 1 }]`: the group keys replaced by counts |
+ * | `groupBy: [{ field: 'opened_on', dateGranularity: 'month' }, { field: 'opened_on', dateGranularity: 'year' }]` | `200 [{ opened_on: '2026', n: 1 }, { opened_on: '2026', n: 2 }]`: the month key overwritten by the year |
+ *
+ * The ingress door already refuses an unknown `groupBy` field, because an
+ * unknown column "puts every row in ONE bucket keyed null"; these are that
+ * hazard, and its overwrite twin, reached by a declared field.
+ *
+ * ## Where it stands
+ *
+ * At the entry of `engine.aggregate`, beside the structured-JSON group door and
+ * the aggregate × field-type door, so it holds for every caller that reaches
+ * the engine: the REST query door and its dispatcher twin (both through
+ * `findData`), the analytics ObjectQL strategy, the MCP data bridge, a flow or
+ * a hook. After those two doors, so a `json` field under a `dateGranularity`
+ * keeps the structured-JSON refusal's words.
+ *
+ * ## What "a date field" is
+ *
+ * The declared type's class, read by `declaredFieldClasses` (having-filter.ts)
+ * — the reading `in-memory-aggregation.ts` buckets a `date` field's calendar
+ * day by. A `date` (`CALENDAR_DATE_TYPES`) and a `datetime` (`INSTANT_TYPES`)
+ * pass. **Not judged:** a field the declaration cannot type — undeclared (the
+ * ingress door answers an unknown one first; a system column the map does not
+ * declare is bucketed as before), a `formula`, or a registry-less host.
+ *
+ * ## The column names
+ *
+ * A group column is the entry's name, or `alias ?? field` for the object form
+ * — the name `applyInMemoryAggregation` and `driver-sql` project it under. An
+ * aggregation column is its `alias`. Entries that name nothing are left to the
+ * shape gates that judge them.
+ *
+ * `INVALID_FIELD` for the date bucket (a verdict about the named field's type,
+ * as the structured-JSON group door gives) and `INVALID_QUERY` for a column
+ * collision (the query names one column twice; no field is wrong). Each
+ * refusal carries `path`, the position it judged (`groupBy.0.dateGranularity`,
+ * `aggregations.1.alias`).
+ *
+ * @see https://github.com/objectstack-ai/objectstack/issues/25818
+ */
+
+import { StandardErrorCode } from '@objectstack/spec/api';
+import { declaredFieldClasses } from './having-filter.js';
+
+type ShapeRefusal = Error & {
+  code?: string; status?: number; httpStatus?: number;
+  field?: string; fields?: string[]; object?: string; param?: string; path?: string;
+};
+
+function refusal(
+  code: 'INVALID_FIELD' | 'INVALID_QUERY',
+  message: string,
+  extra: Partial<ShapeRefusal>,
+): ShapeRefusal {
+  const err = new Error(message) as ShapeRefusal;
+  err.code = StandardErrorCode.enum[code];
+  err.status = 400;
+  // …and `httpStatus`, the same number under ADR-0112 D5's spelling — what a
+  // consumer holding the THROWN error reads; `status` stays for the HTTP doors.
+  err.httpStatus = 400;
+  Object.assign(err, extra);
+  return err;
+}
+
+/**
+ * Refuse a `groupBy` entry whose `dateGranularity` buckets a field the object
+ * declares neither a date nor a datetime — `INVALID_FIELD` / 400, naming the
+ * position, the field and its declared type. See the module header.
+ */
+export function assertDateBucketTargetsAreTemporal(
+  object: string,
+  schema: unknown,
+  groupBy: unknown,
+): void {
+  if (!Array.isArray(groupBy) || groupBy.length === 0) return;
+  const fields = (schema as { fields?: unknown } | undefined)?.fields;
+  if (!fields || typeof fields !== 'object' || Array.isArray(fields)) return;
+  let classes: ReturnType<typeof declaredFieldClasses> | undefined;
+  for (const [i, entry] of groupBy.entries()) {
+    if (entry === null || typeof entry !== 'object' || Array.isArray(entry)) continue;
+    const { field, dateGranularity } = entry as { field?: unknown; dateGranularity?: unknown };
+    if (dateGranularity == null || typeof field !== 'string') continue;
+    classes ??= declaredFieldClasses(fields);
+    const cls = classes.get(field);
+    if (cls === undefined || cls === 'date' || cls === 'datetime') continue;
+    const type = String((fields as Record<string, { type?: unknown }>)[field]?.type);
+    const path = `groupBy.${i}.dateGranularity`;
+    throw refusal(
+      'INVALID_FIELD',
+      `aggregate('${object}'): ${path} buckets '${field}' by '${String(dateGranularity)}', but '${field}' `
+      + `is a declared ${type} field — only a date or datetime field can be date-bucketed. The query was `
+      + `NOT run. Group by '${field}' without 'dateGranularity', or bucket a date or datetime field. A `
+      + 'value that is not a date has no calendar position: every row fell into one null bucket (or, '
+      + 'for a number, an epoch bucket) while the answer looked served.',
+      { field, fields: [field], object, param: 'groupBy', path },
+    );
+  }
+}
+
+/** The column a `groupBy` entry projects: its name, or `alias ?? field`. */
+function groupColumnOf(entry: unknown): string | undefined {
+  if (typeof entry === 'string') return entry;
+  if (entry === null || typeof entry !== 'object' || Array.isArray(entry)) return undefined;
+  const { alias, field } = entry as { alias?: unknown; field?: unknown };
+  const name = alias ?? field;
+  return typeof name === 'string' && name.length > 0 ? name : undefined;
+}
+
+/** `groupBy.N` for a name, `groupBy.N.alias` / `groupBy.N.field` for the object form. */
+function groupColumnPath(entry: unknown, i: number): string {
+  if (typeof entry === 'string') return `groupBy.${i}`;
+  return (entry as { alias?: unknown }).alias != null ? `groupBy.${i}.alias` : `groupBy.${i}.field`;
+}
+
+/**
+ * Refuse a query whose aggregated row would carry two columns under one name —
+ * `INVALID_QUERY` / 400, naming both positions. See the module header.
+ */
+export function assertAggregatedColumnsAreDistinct(
+  object: string,
+  groupBy: unknown,
+  aggregations: unknown,
+): void {
+  const seen = new Map<string, string>();
+  const claim = (column: string, path: string, param: 'groupBy' | 'aggregations', what: string) => {
+    const first = seen.get(column);
+    if (first === undefined) {
+      seen.set(column, path);
+      return;
+    }
+    throw refusal(
+      'INVALID_QUERY',
+      `aggregate('${object}'): ${path} names the column '${column}', which ${first} already names — `
+      + `one row cannot carry two values under one column. The query was NOT run. ${what} A repeated `
+      + 'column name kept only the last value written under it: a count overwritten by a sum, or the '
+      + 'group keys replaced by the numbers, while the answer looked served.',
+      { object, param, path, fields: [column] },
+    );
+  };
+  for (const [i, entry] of (Array.isArray(groupBy) ? groupBy : []).entries()) {
+    const column = groupColumnOf(entry);
+    if (column === undefined) continue;
+    claim(column, groupColumnPath(entry, i), 'groupBy',
+      "Give one of the group entries its own 'alias'.");
+  }
+  for (const [i, entry] of (Array.isArray(aggregations) ? aggregations : []).entries()) {
+    const alias = (entry as { alias?: unknown } | null)?.alias;
+    if (typeof alias !== 'string' || alias.length === 0) continue;
+    claim(alias, `aggregations.${i}.alias`, 'aggregations',
+      "Give each aggregation an 'alias' of its own, distinct from every group column.");
+  }
+}