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diff --git a/‎packages/drivers/driver-sql/src/sql-driver-20347-cross-field-class-parity.test.ts‎
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diff --git a/‎packages/drivers/driver-sql/src/sql-driver-20355-cross-field-class-driver-aliases.test.ts‎
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diff --git a/‎packages/drivers/driver-sql/src/sql-driver.ts‎
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diff --git a/‎packages/formula/src/index.ts‎
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@@ -0,0 +1,63 @@
+---
+'@objectstack/plugin-security': minor
+'@objectstack/formula': minor
+'@objectstack/driver-sql': patch
+'@objectstack/lint': patch
+'@objectstack/spec': patch
+---
+
+fix(security)!: the RLS write check refuses a field-to-field comparison the read refuses — one comparison class, one answer per policy (#20355)
+
+Clause-②: yes (narrowing)
+
+<!-- adr-0087: registered rls-predicate-cross-class-field-comparison-refused -->
+
+**BREAKING** — an accept-set narrowing on the row-level write check, shipped as `minor`
+under the launch-window convention (`check-changeset-no-major` refuses `major` until GA;
+breaking-ness is carried by this banner and the ADR-0087 disposition above, not by the
+level). The hand-migration prescription is registered under protocol major 18 as
+`rls-predicate-cross-class-field-comparison-refused`, one ADR-0087 D3 entry for the whole
+family: the authoring arm `os validate` gained in #20347 and this write-check arm.
+
+**What changed.** A row-level policy that compares two fields of no shared comparison
+class — `record.status != record.amount` (text and a number), `record.status !=
+record.photo` (text and a file field), `record.status != record.is_open` (text and a
+formula field), `record.status != record.meta` (text and a json field) — already had
+every read it scopes refused with `INVALID_FILTER` / 400 on the SQL drivers, because
+driver-sql compiles a column-to-column comparison only within one class. The write
+check did not know the rule: it compared the two raw values in-process, so an insert
+or update the policy's `check` judges (or its `using`, standing in as the check) was
+admitted and stored whenever that comparison happened to hold. Measured through
+plugin-security and ObjectQL on SQLite, sqlite-wasm and PostgreSQL. The write check
+now refuses the comparison too, with the read's envelope, `INVALID_FILTER` / 400, for
+every insert (single or array), by-id update and predicate update it judges, and
+nothing is stored. The same-class comparisons it always compared are compared as
+before. The 400 names no column of the policy; the server log names the policy and
+both columns. A comparison against a json or `multiple` field is refused by its
+declared type now, where it used to be judged by the value each record held.
+
+**`@objectstack/formula`.** `matchesFilterCondition(record, filter, options?)` takes an
+optional third argument: `options.fields`, the object's declared columns (`type` and
+`multiple` per field name). Given it, every `{ $field }` comparison between two
+declared columns is judged by `crossFieldComparisonVerdict` from
+`@objectstack/spec/data` before any record is read, and one the platform defines no
+answer for throws `INVALID_FILTER` / 400. Without it the evaluator behaves exactly as
+before. Two new exports go with it: `findCrossFieldClassRefusal(filter, fields)`, the
+pure judgement, and `crossFieldClassRefusalCarriedBy(error)`, which reads the refused
+comparison off the error for a server-side log.
+
+**`@objectstack/driver-sql`.** `crossFieldComparisonClass` reads the same export
+(`crossFieldColumnVerdict`) instead of keeping its own copy of the classification, and
+layers above it only its internal type aliases. Every read answers as before.
+
+**`@objectstack/lint`.** The `rls-predicate-unenforceable` finding for such a
+comparison now states the write answer the runtime gives: the in-process write check
+refuses it by the same classification and stores nothing.
+
+**If a policy of yours is refused.** The platform defines no comparison between those
+two columns on any path, so the policy never protected a read either. Compare a field
+only with a field of the same class — a number with a number, text with text, a
+boolean with a boolean, a date with a date, a datetime with a datetime, a time of day
+with a time of day — or, if the two columns do hold comparable values, correct the
+declaration of the one declared with the wrong type. `os validate` names every such
+comparison.
@@ -0,0 +1,94 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#20355] This driver's cross-field comparison class READS the spec's
+ * classification (`crossFieldColumnVerdict`, `@objectstack/spec/data`) for
+ * every declared `FieldType`, and layers above it only what the spec leaves to
+ * a driver: its internal aliases, which are not `FieldType` members and which
+ * the spec answers `undefined` for.
+ *
+ * The `FieldType` half needs no pin of its own any more — it IS the export,
+ * which `filter-cross-field-comparison-class.test.ts` pins in the spec. The
+ * #20347 parity test that held this driver's private copy equal to the export
+ * over every declared pair retired with the copy. What this file pins is the
+ * alias layer the rewire kept, read off the driver's own column sets:
+ *
+ * | declared `type` | class | why |
+ * |---|---|---|
+ * | `integer` / `int` / `float` | numeric | `NUMERIC_SCALAR_TYPES`' driver-internal aliases |
+ * | `object` / `array` | none — refused | `JSON_COLUMN_TYPES`' driver-internal aliases |
+ *
+ * (The absent-type `string` default is not pinned: `createColumn` refuses a
+ * field that declares no `type`, so no managed table carries one to compare.)
+ *
+ * Each cell is read from what the driver DOES — the comparison compiles and
+ * runs, or it is refused in the `INVALID_FILTER` / 400 envelope — so an alias
+ * the rewire reclassified shows up as an admitted refusal or a refused
+ * admission, never as prose.
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { SqlDriver, withheldFilterDiagnosticOf } from './index.js';
+import type { FilterCondition } from '@objectstack/spec/data';
+
+const OBJ = 'cfc_alias_probe';
+
+const FIELDS: Record<string, Record<string, unknown>> = {
+  id: { name: 'id', type: 'text' },
+  f_number: { name: 'f_number', type: 'number' },
+  f_integer: { name: 'f_integer', type: 'integer' },
+  f_int: { name: 'f_int', type: 'int' },
+  f_float: { name: 'f_float', type: 'float' },
+  f_text: { name: 'f_text', type: 'text' },
+  f_object: { name: 'f_object', type: 'object' },
+  f_array: { name: 'f_array', type: 'array' },
+  f_json: { name: 'f_json', type: 'json' },
+};
+
+type Observed = 'admitted' | 'refused';
+
+const CELLS: ReadonlyArray<[target: string, ref: string, expected: Observed]> = [
+  ['f_integer', 'f_number', 'admitted'],
+  ['f_int', 'f_float', 'admitted'],
+  ['f_float', 'f_number', 'admitted'],
+  ['f_integer', 'f_text', 'refused'],
+  ['f_object', 'f_text', 'refused'],
+  ['f_text', 'f_array', 'refused'],
+  ['f_object', 'f_object', 'refused'],
+  ['f_object', 'f_json', 'refused'],
+];
+
+describe('[#20355] driver-sql cross-field class — the driver-internal aliases above the spec classification', () => {
+  let driver: SqlDriver;
+
+  beforeAll(async () => {
+    driver = new SqlDriver({ client: 'better-sqlite3', connection: { filename: ':memory:' }, useNullAsDefault: true });
+    await driver.initObjects([{ name: OBJ, fields: FIELDS } as never]);
+  });
+
+  afterAll(async () => {
+    await driver.disconnect();
+  });
+
+  async function observe(target: string, ref: string): Promise<Observed> {
+    const where = { [target]: { $eq: { $field: ref } } } as FilterCondition;
+    try {
+      await driver.find(OBJ, { fields: ['id'], where });
+      return 'admitted';
+    } catch (e) {
+      const err = e as { code?: unknown; status?: unknown };
+      expect({ code: err.code, status: err.status }, `${target} vs ${ref}: ${String(e)}`)
+        .toEqual({ code: 'INVALID_FILTER', status: 400 });
+      expect(withheldFilterDiagnosticOf(e), `${target} vs ${ref}: not the cross-field boundary's refusal`)
+        .toMatch(/stored as|no scalar stored column/);
+      return 'refused';
+    }
+  }
+
+  for (const [target, ref, expected] of CELLS) {
+    it(`${target} vs ${ref}: ${expected}, in both orders`, async () => {
+      expect(await observe(target, ref)).toBe(expected);
+      expect(await observe(ref, target)).toBe(expected);
+    });
+  }
+});
@@ -26,6 +26,12 @@ import { STRUCTURED_JSON_TYPES, FILE_REFERENCE_TYPES, MULTI_OPTION_TYPES, NUMERI
 // `os generate migration` reads the SAME table, in both of its formats — that
 // shared table IS the repair, so ⛔ never restate one of its numbers here.
 import { numericColumnFor } from '@objectstack/spec/data';
+// [#20355] The cross-field comparison class, defined once in the spec (#20347,
+// lifted case for case from this driver's #5222 boundary). This driver READS it
+// — `crossFieldComparisonClass` below delegates — so the read it compiles and
+// the write check `@objectstack/formula` evaluates judge one comparison by one
+// rule.
+import { crossFieldColumnVerdict, type CrossFieldComparisonClass } from '@objectstack/spec/data';
 // [#5659] The Filter Protocol's boolean identity reduction — `$and: []` is TRUE,
 // `$or: []` is FALSE, `{}` is a TRUE disjunct, `$not: {}` is FALSE. One
 // implementation for all four consumers, proven against the same
@@ -2729,6 +2735,16 @@ const CROSS_FIELD_COMPARISON_OPERATORS: ReadonlySet<string> = new Set([
  * [#5222] The comparison class a declared field's stored column belongs to, or
  * `null` for a field no column-to-column comparison can be compiled against.
  *
+ * [#20355] The classification is the SPEC'S now — `crossFieldColumnVerdict`
+ * (`@objectstack/spec/data`), lifted case for case from this function by
+ * #20347 — and this function is its reader for the one thing the spec leaves
+ * to a driver: its internal aliases. The write check (`@objectstack/formula`'s
+ * `matchesFilterCondition`, handed the object's declared columns by the RLS
+ * write gate) and the authoring door (`@objectstack/lint`) read the same
+ * export, so a policy's comparison has one answer on the read, on the write
+ * and at `os validate`. The reasoning below is the classification's, kept
+ * here because this driver is where it was measured.
+ *
  * Cross-field comparison is only emitted between two columns of the SAME
  * class. One class = one storage shape on both sides of one row, which is what
  * makes the SQL answer provably the memory evaluator's answer (the cross-path
@@ -2771,19 +2787,22 @@ const CROSS_FIELD_COMPARISON_OPERATORS: ReadonlySet<string> = new Set([
  */
 function crossFieldComparisonClass(
   decl: Record<string, unknown>,
-): 'numeric' | 'text' | 'boolean' | 'date' | 'datetime' | 'time' | null {
+): CrossFieldComparisonClass | null {
   const type = String((decl as { type?: unknown }).type || 'string');
-  if (isMultiValuedColumn(type, decl)) return null;
-  if (type === 'formula') return null;
-  if (JSON_COLUMN_TYPES.has(type) || FILE_REFERENCE_TYPES.has(type)) return null;
+  // [#20355] A declared `FieldType` is the spec's to classify — the one
+  // classification the write check and the authoring door read too. Its
+  // `multiple` reading is `isMultiValueField`'s, the same predicate
+  // `isMultiValuedColumn` asks.
+  const verdict = crossFieldColumnVerdict({ type, multiple: (decl as { multiple?: unknown }).multiple === true });
+  if (verdict !== undefined) return verdict.kind === 'class' ? verdict.class : null;
+  // A type outside `FieldType` is a driver-internal alias the spec does not
+  // judge (its module header: "a driver layers its own aliases above this
+  // table"). This driver's are read off its own column sets, never restated:
+  // `object` / `array` are JSON columns ({@link JSON_COLUMN_TYPES}), `integer` /
+  // `int` / `float` numeric ones ({@link NUMERIC_SCALAR_TYPES}), and everything
+  // else — the absent-type default `string` included — is stored as TEXT.
+  if (JSON_COLUMN_TYPES.has(type)) return null;
   if (NUMERIC_SCALAR_TYPES.has(type)) return 'numeric';
-  if (type === 'boolean' || type === 'toggle') return 'boolean';
-  if (type === 'date') return 'date';
-  if (type === 'datetime') return 'datetime';
-  if (type === 'time') return 'time';
-  // Everything else `createColumn` stores as TEXT: string/text/textarea/html/
-  // markdown/email/url/phone/password, select, lookup/user (row ids),
-  // autonumber, and the unknown-type default.
   return 'text';
 }
 
 
@@ -116,6 +116,11 @@ export { __resetPushdownLimitWarnings } from './cel-to-filter';
 // conditions ARE the red/green line.
 export { isSupportedRlsExpression, sqlPredicateToCel } from './rls-predicate';
 export { matchesFilterCondition } from './matches-filter';
+// #20355 — the write check's comparison-class rule: the spec's cross-field
+// classification, judged over a filter against the object's declared columns,
+// and the reader a caller uses to log the refused comparison server-side.
+export { crossFieldClassRefusalCarriedBy, findCrossFieldClassRefusal } from './matches-filter';
+export type { CrossFieldClassRefusal, MatchesFilterOptions } from './matches-filter';
 // #13594 — the function-EXISTENCE verdict, isolated from the rest of what
 // cel-js's `check()` has an opinion about. Published for the same reason as
 // `firstUndeclaredReference` above and under the same discipline: the answer to