@@ -0,0 +1,21 @@
+---
+'@objectstack/objectql': minor
+---
+
+fix(objectql)!: a per-aggregation `filter` and a `having` refuse a non-boolean `$exists` / `$null` with `INVALID_FILTER` / 400, in the words every driver's `where` refuses it in, instead of reading `$exists` by truthiness and dropping `$null` (#20981)
+
+Clause-②: no (narrowing)
+
+<!-- adr-0087: not-required (already-registered filter-query-face-comparands-refused-at-save) this narrows the engine's own in-process evaluator to the rule that registered entry already records: its reason states that every query face refuses a non-boolean $null / $exists flag, its surface names a query having, the data-engine aggregate call's having and an aggregation filter among the carriers, and its replacement is this change's whole migration (a flag is the boolean itself; $null true is "has no value", $exists true is "has a value"). This change makes that statement true on the per-aggregation filter and having positions, which the engine evaluates itself after the driver. No authorable key, spelling, export or published type moves, and no stored row is read or rewritten; a stored filter carrying such a flag is already refused when it is saved, by that entry. -->
+
+**BREAKING**: this narrows what `aggregate` accepts in two positions, `aggregations[i].filter` and `having`, on every driver. A `$exists` or `$null` comparand that is not a boolean (a string such as `"false"`, a number, `null`, an array) is now refused with `INVALID_FILTER` / 400, before any driver is asked for a row, so an empty table refuses it too, at any depth under `$and` / `$or` / `$not`. A plain object or `undefined` there is refused first by the comparand-type check, in its own words, as before; a `{ $field }` reference there, already refused as a reference outside a scalar comparison, is now refused in this entry's words. The published `applyInMemoryAggregation(rows, ast, timezone, fields)` narrows the same way, per row: it throws the same refusal for a row its per-aggregation filter judges on the flag, with or without a `fields` map (an empty `rows` array, or a row a `$or` branch settles first, is not judged there; `engine.aggregate` judges the whole filter once before any row). It ships as `minor` under the launch-window convention for accept-set narrowings. No export or published type changes.
+
+**Why a refusal.** `FieldOperatorsSchema` declares both flags as booleans, and every driver's `where` refuses any other comparand. The engine evaluates a per-aggregation `filter` and a `having` itself, and it read one anyway. Measured through `engine.aggregate` on the in-memory driver and on `SqlDriver` (SQLite), with identical answers: `$exists` was read by truthiness, so `"yes"`, `1` and the string `"false"` selected the rows and groups WITH a value, and `0` / `null` the ones without; and `$null` tested only `true` / `false`, so any other value constrained nothing, and every row and every group came back.
+
+**What an author sees now.** The message `driver-sql` gives the same flag, beginning `Operator "$exists" on field "FIELD" requires a boolean comparand (true or false).`, naming what arrived and the position (`aggregations[1].filter.stage.$exists`, `having.stage.$null`). Unlike a `where` on `SqlDriver`, the field and the value are not withheld: a per-aggregation `filter` and a `having` never carry a merged read scope.
+
+**What to write instead.** Write the boolean itself. `"$exists": true` and `"$null": false` match a field that has a value; `"$exists": false` and `"$null": true` match one that has none.
+
+**Who is affected.** A caller that reaches `engine.aggregate` without the REST query door's schema parse (server-side code, a flow or hook, the analytics bridge that lowers a dataset measure's filter into an aggregation filter, a host calling `applyInMemoryAggregation` directly) and read the count as a real answer. `POST /api/v1/data/:object/query` already refused all three positions with 400 `VALIDATION_FAILED` before the request reached the engine, and still does.
+
+**Unchanged.** `$exists: true` / `false` and `$null: true` / `false` answer exactly as before, on both positions. `$empty` and every other operator, and `where`.
@@ -439,6 +439,8 @@ describe('[#20122] per-aggregation filter — the walker\'s refusals belong to t
     ['a reference as an $in member', () => ({ amount: { $in: [{ $field: 'amount' }] } }), `${AT}.amount.$in`],
     ['a reference as a $nin member', () => ({ amount: { $nin: [1, { $field: 'amount' }] } }), `${AT}.amount.$nin`],
     ['a reference as a $contains pattern', () => ({ stage: { $contains: { $field: 'region' } } }), `${AT}.stage.$contains`],
+    // [#20981] Refused at the same path, as a non-boolean flag first now — the
+    // words are pinned in engine-aggregate-flag-comparand-refusal.test.ts.
     ['a reference under $exists', () => ({ amount: { $exists: { $field: 'amount' } } }), `${AT}.amount.$exists`],
     ['a fractional addDays', () => ({ amount: { $lte: { $field: 'amount', addDays: 1.5 } } }), `${AT}.amount.$lte`],
     ['a string addDays', () => ({ amount: { $lte: { $field: 'amount', addDays: '7' } } }), `${AT}.amount.$lte`],
 
@@ -0,0 +1,284 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// [#20981] A per-aggregation `filter` and a `having` REFUSE a non-boolean
+// `$exists` / `$null` with `INVALID_FILTER` / 400, in the words every driver's
+// `where` refuses it in, before any driver is asked. `true` / `false` answer
+// exactly as before.
+//
+// Measured before this change (`origin/main` `7a606a9a3`) through
+// `engine.aggregate` on driver-memory AND driver-sql (better-sqlite3), over a
+// text column `name` holding `'won'` on one row (amount 10) and no value on two
+// (amounts 1 and 100). The engine evaluates both clauses itself, so the two
+// drivers answered identically:
+//
+// | clause · comparand | `$exists` | `$null` |
+// |:--|:--|:--|
+// | filter · `"yes"` / `1` / `"false"` | sum 10 (the valued row) | sum 111 (every row) |
+// | filter · `0` / `null` | sum 101 (the no-value rows) | sum 111 (every row) |
+// | having · `"yes"` / `1` / `"false"` | the `won` group | both groups |
+// | having · `0` / `null` | the null group | both groups |
+//
+// while the same flag in a `where` is refused 400 on both drivers. After it,
+// every cell above is that 400, and the `true` / `false` control keeps the
+// numbers below.
+//
+// This file is the engine-level cell, over a driver of each `having` path's
+// shape — `rows` (no `aggregate()`: the engine reads rows and lowers in memory,
+// the face that evaluates `aggregations[i].filter`) and `native` (the driver
+// aggregates, the engine applies `having`). Every refusal precedes the driver,
+// so the backend does not enter into it; the same answers were measured on
+// driver-memory and driver-sql above. This package does not depend on either
+// driver, and the in-memory driver's test consumers are a closed census
+// (`check:driver-memory-census`), so the SQLite cell — beside its live `where`
+// twin, through `POST /data/:object/query` — is `packages/rest`'s
+// `aggregation-flag-comparand-refusal.test.ts`.
+
+import { describe, it, expect } from 'vitest';
+import type { EngineAggregateOptions, FilterCondition } from '@objectstack/spec/data';
+import { ObjectQL } from './engine.js';
+import { applyHaving, matchesAggregationFilter } from './having-filter.js';
+import { applyInMemoryAggregation } from './in-memory-aggregation.js';
+
+const OBJECT = 'os20981_deal';
+
+const FIELDS = {
+  name: { type: 'text' },
+  amount: { type: 'number' },
+};
+
+// `a` holds a value; `b` holds null and `c` does not carry the column — both
+// have NO value, the reading `$exists` and `$null` share (#5298).
+const ROWS = [
+  { id: 'a', name: 'won', amount: 10 },
+  { id: 'b', name: null, amount: 1 },
+  { id: 'c', amount: 100 },
+];
+
+type Path = 'native' | 'rows';
+
+/** A driver of one `having` path's shape, counting every read of the object. */
+function makeDriver(path: Path, rows: ReadonlyArray<Record<string, unknown>>) {
+  const reads = { aggregate: 0, find: 0 };
+  const driver: any = {
+    name: `${path}-recorder`,
+    version: '0.0.0',
+    supports: {},
+    async connect() {}, async disconnect() {}, async checkHealth() { return true; }, async execute() { return null; },
+    async find() { reads.find += 1; return rows.map((r) => ({ ...r })); },
+    async findOne() { return null; },
+    async create(_o: string, d: any) { return d; },
+    async update(_o: string, _id: string, d: any) { return d; },
+    async delete() { return true; },
+    async count() { return rows.length; },
+    async bulkCreate(_o: string, r: any[]) { return r; },
+    async bulkUpdate() { return []; }, async bulkDelete() {},
+    async beginTransaction() { return { commit: async () => {}, rollback: async () => {} }; },
+    async commit() {}, async rollback() {},
+  };
+  if (path === 'native') {
+    driver.aggregate = async (_o: string, ast: any) => { reads.aggregate += 1; return applyInMemoryAggregation([...rows], ast); };
+  }
+  return { driver, reads };
+}
+
+async function makeEngine(path: Path, rows: ReadonlyArray<Record<string, unknown>> = ROWS) {
+  const { driver, reads } = makeDriver(path, rows);
+  const engine = new ObjectQL();
+  engine.registerDriver(driver, true);
+  await engine.init();
+  (engine.registry as any).registerObject({ name: OBJECT, fields: FIELDS });
+  return { engine, reads };
+}
+
+/** `s` sums `amount` over the rows `filter` selects; `n` counts every row. Always the rows path. */
+const filterQuery = (filter: unknown): EngineAggregateOptions => ({
+  aggregations: [
+    { function: 'count', alias: 'n' },
+    { function: 'sum', field: 'amount', alias: 's', filter: filter as FilterCondition },
+  ],
+});
+
+/** Groups by `name`; on the rows path a filtered aggregation forces the engine's own lowering. */
+const havingQuery = (path: Path, having: unknown): EngineAggregateOptions => ({
+  groupBy: ['name'],
+  aggregations: path === 'native'
+    ? [{ function: 'count', alias: 'n' }]
+    : [{ function: 'count', alias: 'n' }, { function: 'count', alias: 'all', filter: { amount: { $gte: 0 } } }],
+  having: having as FilterCondition,
+});
+
+interface Refusal extends Error { code?: unknown; status?: unknown }
+
+async function refusalOf(run: () => Promise<unknown>): Promise<Refusal> {
+  try {
+    await run();
+  } catch (e) {
+    return e as Refusal;
+  }
+  throw new Error('expected engine.aggregate to refuse this clause, but it answered');
+}
+
+function syncRefusalOf(run: () => unknown): Refusal {
+  try {
+    run();
+  } catch (e) {
+    return e as Refusal;
+  }
+  throw new Error('expected a refusal, but the clause was answered');
+}
+
+/**
+ * The comparand doors' refusal: the ADR-0112 identity, then the drivers' first
+ * three sentences — the operator, the field, what arrived (in the drivers'
+ * `describeFilterOperand (safeShapePreview)` rendering) and where, and the
+ * declaration — then the reason that names the backends.
+ */
+function expectFlagRefusal(err: Refusal, op: '$exists' | '$null', received: string, at: string): void {
+  expect(err.code, err.message).toBe('INVALID_FILTER');
+  expect(err.status, err.message).toBe(400);
+  expect(err.message.startsWith(
+    `Operator "${op}" on field "name" requires a boolean comparand (true or false). `
+    + `Received ${received} at ${at}. `
+    + `@objectstack/spec FieldOperatorsSchema declares ${op} as a boolean. It is refused rather than coerced `,
+  ), err.message).toBe(true);
+  expect(err.message).toContain('OPPOSITE directions');
+  expect(err.message).toContain('Note "false" the STRING is truthy');
+}
+
+const OPS = ['$exists', '$null'] as const;
+
+/** The card's five non-booleans, each with the rendering the drivers give it. */
+const NON_BOOLEANS: ReadonlyArray<readonly [string, unknown, string]> = [
+  ['"yes"', 'yes', 'string ("yes")'],
+  ['1', 1, 'number (1)'],
+  ['"false" (truthy)', 'false', 'string ("false")'],
+  ['0', 0, 'number (0)'],
+  ['null', null, 'null (null)'],
+  // Not one of the card's five, but reaching the same gate: no earlier door
+  // refuses a list here, so it met the old `!!target` read like any other value.
+  ['[true] (a list)', [true], 'array ([true])'],
+];
+
+describe('[#20981] a non-boolean $exists / $null — refused before any driver read, on both positions', () => {
+  for (const op of OPS) {
+    for (const [label, comparand, received] of NON_BOOLEANS) {
+      it(`aggregations[1].filter ${op}: ${label} — 400, the drivers' words, on an empty and a populated table`, async () => {
+        for (const rows of [[], ROWS]) {
+          const { engine, reads } = await makeEngine('rows', rows);
+          const err = await refusalOf(() => engine.aggregate(OBJECT, filterQuery({ name: { [op]: comparand } })));
+          expectFlagRefusal(err, op, received, `aggregations[1].filter.name.${op}`);
+          expect(reads, 'the verdict is the filter\'s: no row was read').toEqual({ aggregate: 0, find: 0 });
+        }
+      });
+
+      it(`having ${op}: ${label} — 400, the drivers' words, on both having paths`, async () => {
+        for (const path of ['native', 'rows'] as const) {
+          for (const rows of [[], ROWS]) {
+            const { engine, reads } = await makeEngine(path, rows);
+            const err = await refusalOf(() => engine.aggregate(OBJECT, havingQuery(path, { name: { [op]: comparand } })));
+            expectFlagRefusal(err, op, received, `having.name.${op}`);
+            expect(reads, `${path}: no row was read`).toEqual({ aggregate: 0, find: 0 });
+          }
+        }
+      });
+    }
+  }
+
+  // Where the flag sits must not change the verdict: the walk is row-independent,
+  // so a branch the per-row walk would short-circuit past is judged too.
+  const POSITIONS: ReadonlyArray<readonly [string, (flag: Record<string, unknown>) => Record<string, unknown>, string]> = [
+    ['nested in $and', (flag) => ({ $and: [{ amount: { $gte: 0 } }, { name: flag }] }), '.$and[1].name'],
+    ['behind a $or branch that already holds', (flag) => ({ $or: [{ amount: { $gte: 0 } }, { name: flag }] }), '.$or[1].name'],
+    ['under $not', (flag) => ({ $not: { name: flag } }), '.$not.name'],
+    ['beside a boolean sibling flag', (flag) => ({ name: { $empty: false, ...flag } }), '.name'],
+  ];
+
+  for (const [name, wrap, tail] of POSITIONS) {
+    it(`${name}: refused at that position, in the per-aggregation filter and in having`, async () => {
+      const filterEngine = await makeEngine('rows');
+      expectFlagRefusal(
+        await refusalOf(() => filterEngine.engine.aggregate(OBJECT, filterQuery(wrap({ $exists: 'yes' })))),
+        '$exists', 'string ("yes")', `aggregations[1].filter${tail}.$exists`,
+      );
+      // `having` reads the aggregated row, which carries `name` (the groupBy)
+      // but not `amount` — so its wrapper's other branch tests `n` instead.
+      const having = JSON.parse(JSON.stringify(wrap({ $null: 1 })).replaceAll('"amount"', '"n"'));
+      const havingEngine = await makeEngine('native');
+      expectFlagRefusal(
+        await refusalOf(() => havingEngine.engine.aggregate(OBJECT, havingQuery('native', having))),
+        '$null', 'number (1)', `having${tail}.$null`,
+      );
+      expect(filterEngine.reads.find + havingEngine.reads.aggregate).toBe(0);
+    });
+  }
+
+  it('a { $field } reference in a flag\'s slot is a non-boolean first, as in $empty\'s', async () => {
+    const { engine } = await makeEngine('rows');
+    expectFlagRefusal(
+      await refusalOf(() => engine.aggregate(OBJECT, filterQuery({ name: { $exists: { $field: 'amount' } } }))),
+      '$exists', 'object ({"$field":"amount"})', 'aggregations[1].filter.name.$exists',
+    );
+    const native = await makeEngine('native');
+    expectFlagRefusal(
+      await refusalOf(() => native.engine.aggregate(OBJECT, havingQuery('native', { name: { $null: { $field: 'n' } } }))),
+      '$null', 'object ({"$field":"n"})', 'having.name.$null',
+    );
+  });
+});
+
+describe('[#20981] the control — true / false answer exactly as before, on both positions and both paths', () => {
+  // `a` (10) has a value; `b` (1) and `c` (100) have none.
+  const SUMS: ReadonlyArray<readonly [string, Record<string, unknown>, number]> = [
+    ['$exists: true', { $exists: true }, 10],
+    ['$exists: false', { $exists: false }, 101],
+    ['$null: true', { $null: true }, 101],
+    ['$null: false', { $null: false }, 10],
+  ];
+
+  for (const [name, flag, sum] of SUMS) {
+    it(`aggregations[1].filter ${name} sums ${sum}`, async () => {
+      const { engine, reads } = await makeEngine('rows');
+      expect(await engine.aggregate(OBJECT, filterQuery({ name: flag }))).toEqual([{ n: 3, s: sum }]);
+      expect(reads.find).toBe(1);
+    });
+
+    const kept = sum === 10 ? ['won'] : ['null'];
+    it(`having ${name} keeps the ${kept[0]} group on both paths`, async () => {
+      for (const path of ['native', 'rows'] as const) {
+        const { engine } = await makeEngine(path);
+        const groups = await engine.aggregate(OBJECT, havingQuery(path, { name: flag }));
+        expect(groups.map((g: any) => String(g.name)), path).toEqual(kept);
+      }
+    });
+  }
+});
+
+describe('[#20981] the published row evaluators refuse per row — the floor for a caller that evaluates rows itself', () => {
+  it('applyInMemoryAggregation with fields: $exists "yes" and $null 1 are refused, true / false answer', () => {
+    for (const [op, comparand, received] of [['$exists', 'yes', 'string ("yes")'], ['$null', 1, 'number (1)']] as const) {
+      const err = syncRefusalOf(() => applyInMemoryAggregation(
+        ROWS.map((r) => ({ ...r })),
+        filterQuery({ name: { [op]: comparand } }),
+        undefined,
+        FIELDS,
+      ));
+      expectFlagRefusal(err, op, received, `aggregations[1].filter.name.${op}`);
+    }
+    expect(applyInMemoryAggregation(ROWS.map((r) => ({ ...r })), filterQuery({ name: { $exists: true } }), undefined, FIELDS))
+      .toEqual([{ n: 3, s: 10 }]);
+    expect(applyInMemoryAggregation(ROWS.map((r) => ({ ...r })), filterQuery({ name: { $null: true } }), undefined, FIELDS))
+      .toEqual([{ n: 3, s: 101 }]);
+  });
+
+  it('applyHaving and matchesAggregationFilter refuse a row that reaches the flag, with or without a value', () => {
+    const groups = [{ name: 'won', n: 1 }, { name: null, n: 2 }];
+    for (const row of groups) {
+      expectFlagRefusal(syncRefusalOf(() => applyHaving([row], { name: { $exists: 'false' } } as never)),
+        '$exists', 'string ("false")', 'having.name.$exists');
+      expectFlagRefusal(syncRefusalOf(() => matchesAggregationFilter(row, { name: { $null: 0 } } as never, 2)),
+        '$null', 'number (0)', 'aggregations[2].filter.name.$null');
+    }
+    expect(applyHaving(groups, { name: { $null: false } })).toEqual([{ name: 'won', n: 1 }]);
+    expect(applyHaving(groups, { name: { $exists: false } })).toEqual([{ name: null, n: 2 }]);
+  });
+});
@@ -646,6 +646,8 @@ describe('[#20099] having — a { $field } reference resolves against the aggreg
     ['a reference as a $nin member', () => ({ total: { $nin: [1, { $field: 'max_cap' }] } }), 'having.total.$nin'],
     ['a reference as a $contains pattern', () => ({ customer_id: { $contains: { $field: 'customer_id' } } }), 'having.customer_id.$contains'],
     ['a reference as a $startsWith pattern', () => ({ customer_id: { $startsWith: { $field: 'customer_id' } } }), 'having.customer_id.$startsWith'],
+    // [#20981] These two are refused at the same path as a non-boolean flag
+    // first now — the words are pinned in engine-aggregate-flag-comparand-refusal.test.ts.
     ['a reference under $exists', () => ({ total: { $exists: { $field: 'max_cap' } } }), 'having.total.$exists'],
     ['a reference under $null', () => ({ total: { $null: { $field: 'max_cap' } } }), 'having.total.$null'],
     ['a reference naming no column', () => ({ total: { $gt: { $field: 'nope' } } }), 'having.total.$gt'],