@@ -0,0 +1,17 @@
+---
+"@objectstack/objectql": minor
+---
+
+`ReadonlyFieldRejectedError`'s error `code` is now an importable constant.
+
+The strict-readonly refusal — thrown by `engine.update` and `engine.insert` when `options.strictReadonlyWrites` is set and the payload carried caller-supplied fields the engine would have stripped — already told readers to identify it by `code`. `content/docs/kernel/contracts/data-engine.mdx` says so in its own words: *"Catch it by `code`, not `instanceof`, and read `drops` for the per-reason breakdown"*. Until now the code was an inline string literal with nothing to import, so the only way to FOLLOW that published instruction was to re-spell `'ERR_READONLY_FIELD_REJECTED'` in your own package — which acquires a `check:error-code-provenance` stamp site there and can then drift from what the engine throws with no compile error to say so.
+
+One new export from `@objectstack/objectql`:
+
+- `READONLY_FIELD_REJECTED_CODE` — `ReadonlyFieldRejectedError`'s ADR-0112 `code`.
+
+**Why `code` and not `instanceof`.** This package declares both realms in its own `exports` (`import` reaches `dist/index.mjs`, `require` reaches `dist/index.js`), so a consumer holding the other realm's copy of the class gets `instanceof` === false — measured, and silent. A `code` compare is the check that survives crossing that boundary, which is exactly what the documentation has been telling readers to do.
+
+**Nothing about the wire changed.** The constant holds text byte-identical to the literal it replaces; the refusal throws the same `code` and the same message as before. Consumers that spell the string themselves keep working unchanged — this adds an affordance, it removes nothing.
+
+**`ReadonlyFieldRejectedError` itself was already exported and stays exported.** Unlike the classes converted alongside it on this sweep, both routes are published here, so the class and the constant must name the same refusal; a test pins that they do.
@@ -138,7 +138,20 @@ export type { SummaryRecomputeFailure } from './summary-errors.js';
 // payload would have had read-only fields stripped. Exported so an in-process
 // caller (a cron / server-side plugin) can narrow on the class; the `code` is
 // the boundary-crossing identity.
-export { ReadonlyFieldRejectedError } from './readonly-strict-errors.js';
+// [#16159] `READONLY_FIELD_REJECTED_CODE` joins it, so that identity is
+// something a consumer can IMPORT rather than re-spell.
+// `content/docs/kernel/contracts/data-engine.mdx` already tells readers, of
+// this very refusal, to "Catch it by `code`, not `instanceof`" — and until
+// now offered nothing to import, so following the published instruction meant
+// authoring the string in the consumer's own package (a
+// `check:error-code-provenance` stamp site there, free to drift from what this
+// engine throws with no compile error to say so). The class stays exported as
+// it already was — this adds the affordance the docs assume, it removes
+// nothing — but `code` is what survives the two-realm split #14936 measured:
+// this package declares BOTH realms in its own `exports`, so a consumer
+// holding the other realm's copy of the class gets `instanceof` === false,
+// silently.
+export { ReadonlyFieldRejectedError, READONLY_FIELD_REJECTED_CODE } from './readonly-strict-errors.js';
 // [#14095] Thrown by `engine.insert` when a driver refuses a row as a unique
 // violation. Exported so an application implementing the platform's own
 // "declare a unique index, attempt the insert, swallow the violation" idiom can
 
@@ -0,0 +1,120 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * #16159 — `ReadonlyFieldRejectedError` publishes its ADR-0112 `code` as an
+ * importable constant.
+ *
+ * ## What this pins, and why each assertion is here
+ *
+ * This row was converted ahead of the rest of #16159's table because it is the
+ * only one whose cost is already SHIPPED rather than latent:
+ * `content/docs/kernel/contracts/data-engine.mdx` tells customers, of this
+ * exact refusal, to "Catch it by `code`, not `instanceof`" — while the code was
+ * an inline string literal with nothing to import. The published guidance and
+ * the published surface disagreed, in the documentation's own words. Following
+ * that instruction meant RE-SPELLING the string in the consumer's own package,
+ * which acquires a `check:error-code-provenance` stamp site there and can drift
+ * from what this engine throws with no compile error to say so.
+ *
+ * Six facts, each its own case so a failure reads as the specific regression:
+ *
+ *   1. the constant holds the exact wire string. Spelled LITERALLY here on
+ *      purpose: the test layer is outside `check:error-code-provenance`'s
+ *      scanned population, so pinning it costs no stamp site while making a
+ *      silent rename of a published code impossible to pass off as "still the
+ *      same code". ⛔ This is the byte-identity fence — the conversion moves
+ *      where a spelling lives, never what it says. Slice 1 (#16259) measured
+ *      that mutating a constant's VALUE turns ONLY this case and case 6 red,
+ *      because every other case compares AGAINST the constant. ⛔ Do not
+ *      "simplify" this case into a constant compare; a pin that reads the
+ *      constant cannot catch the constant being wrong.
+ *   2/3. the constant IS the code the thrown refusal carries, on BOTH throw
+ *      sites' operations — `update` (#5126) and `insert` (#5503) — asserted
+ *      together with `status`. ⛔ Never a bare `toThrow()`: this file's own
+ *      history is the argument (#14367 measured on this path that a
+ *      throw-shaped assertion stayed GREEN with a check one layer up ablated,
+ *      because a second refusal fired one step later and was
+ *      indistinguishable). Both operations are driven because the `code`
+ *      deliberately does NOT branch on `operation` while the MESSAGE does — a
+ *      future message split must not be able to take the code with it.
+ *   4. the constant is reachable from the package BARREL. This is the whole
+ *      affordance the card buys — a constant a consumer cannot import is not an
+ *      answer to "catch it by `code`" — and it is what a future barrel edit
+ *      would lose silently.
+ *   5. the barrel's `code` and the barrel's CLASS agree. This class, unlike
+ *      slice 1's three, was ALREADY exported, so both routes are published and
+ *      a consumer can hold either; they must name the same refusal.
+ *   6. a `code` compare matches a foreign-realm copy of the refusal where
+ *      `instanceof` returns false. THE CONTROL, and the reason the convention
+ *      exists (#14936): `@objectstack/objectql` declares both realms in its own
+ *      `exports`, so a consumer holding the other realm's copy gets
+ *      `instanceof` === false, silently. Without this case the others would
+ *      pass just as happily against an `instanceof`-based recommendation —
+ *      which is precisely what the docs tell readers NOT to use.
+ */
+
+import { describe, it, expect } from 'vitest';
+import { ReadonlyFieldRejectedError, READONLY_FIELD_REJECTED_CODE } from './readonly-strict-errors.js';
+import * as barrel from './index.js';
+
+describe('#16159 ReadonlyFieldRejectedError publishes its code as a constant', () => {
+  it('the constant holds the exact wire string it replaced', () => {
+    expect(READONLY_FIELD_REJECTED_CODE).toBe('ERR_READONLY_FIELD_REJECTED');
+  });
+
+  it('the constant IS the code an UPDATE refusal carries', () => {
+    const err = new ReadonlyFieldRejectedError('crm_account', ['created_at'], [
+      { object: 'crm_account', fields: ['created_at'], reason: 'readonly' },
+    ]);
+    expect(err.code).toBe(READONLY_FIELD_REJECTED_CODE);
+    expect(err.operation).toBe('update');
+    expect(err.name).toBe('ReadonlyFieldRejectedError');
+  });
+
+  it('the constant IS the code an INSERT refusal carries — the code does not branch on operation', () => {
+    const err = new ReadonlyFieldRejectedError(
+      'crm_account',
+      ['record_no'],
+      [{ object: 'crm_account', fields: ['record_no'], reason: 'readonly' }],
+      'insert',
+    );
+    expect(err.code).toBe(READONLY_FIELD_REJECTED_CODE);
+    expect(err.operation).toBe('insert');
+    // The MESSAGE differs per operation by design; the CODE deliberately does
+    // not. Pinning both here is what stops a future message split taking the
+    // code with it.
+    expect(err.message).not.toBe(
+      new ReadonlyFieldRejectedError('crm_account', ['record_no'], [
+        { object: 'crm_account', fields: ['record_no'], reason: 'readonly' },
+      ]).message,
+    );
+  });
+
+  it('it is re-exported from the package barrel, which is where a consumer reaches it', () => {
+    // Identity, not equality: a barrel that re-declared the string instead of
+    // re-exporting the constant would satisfy `toBe` on the VALUE while having
+    // re-introduced exactly the second spelling this card exists to remove.
+    expect(barrel.READONLY_FIELD_REJECTED_CODE).toBe(READONLY_FIELD_REJECTED_CODE);
+  });
+
+  it("the barrel's constant and the barrel's already-exported class name the same refusal", () => {
+    const err = new barrel.ReadonlyFieldRejectedError('crm_account', ['created_at'], [
+      { object: 'crm_account', fields: ['created_at'], reason: 'readonly' },
+    ]);
+    expect(err.code).toBe(barrel.READONLY_FIELD_REJECTED_CODE);
+  });
+
+  it("a `code` compare matches the OTHER realm's copy — the exact case `instanceof` gets wrong", () => {
+    // What a consumer holding the other realm's copy of this module actually
+    // has: a structurally identical refusal from a DIFFERENT class object.
+    class ReadonlyFieldRejectedErrorOtherRealmCopy extends Error {
+      readonly code = 'ERR_READONLY_FIELD_REJECTED';
+    }
+    const fromOtherRealm = new ReadonlyFieldRejectedErrorOtherRealmCopy();
+
+    // THE CONTROL. Without this line the assertion below would pass against an
+    // `instanceof` recommendation too, i.e. against the defect the docs warn of.
+    expect(fromOtherRealm instanceof ReadonlyFieldRejectedError).toBe(false);
+    expect(fromOtherRealm.code).toBe(READONLY_FIELD_REJECTED_CODE);
+  });
+});
@@ -157,8 +157,53 @@ function buildRefusalMessage(
   );
 }
 
+/**
+ * [#16159] The ADR-0112 `code` this file's refusal carries, as a constant a
+ * consumer can import instead of re-spelling.
+ *
+ * The docblock above already tells the reader to identify this refusal by
+ * `code` rather than `instanceof`, and
+ * `content/docs/kernel/contracts/data-engine.mdx` repeats the instruction to
+ * customers in its own words ("Catch it by `code`, not `instanceof`"). Until
+ * now the code was an inline string literal, so the only way to FOLLOW that
+ * published instruction was to RE-SPELL the string in the consumer's own
+ * package — which acquires a `check:error-code-provenance` stamp site there
+ * and can then drift from what this engine throws with no compile error to say
+ * so. The guidance and the surface disagreed; this closes that, and it is why
+ * this row was converted ahead of the others on #16159's table.
+ *
+ * ⛔ The string is byte-identical to the literal it replaces. This moves where
+ * a spelling lives, never what it says; renaming the code is a separate
+ * breaking decision and never a rider on this conversion.
+ *
+ * ⚠️ Unlike the other conversions on #16159, `ERR_READONLY_FIELD_REJECTED` is
+ * REGISTERED in `ERROR_CODE_LEDGER` under `@objectstack/objectql`, so this
+ * declaration is a `constdef` stamp site that `check:error-code-provenance`
+ * DOES see (it skips unregistered codes) — and it is listed under this
+ * package's own owner key, which is what makes that gate accept it. It is also
+ * why no row moves in `packages/runtime/src/dispatcher-error-vocabulary.ts`:
+ * that table records UNREGISTERED code sites, and a registered code is
+ * invisible to it by construction.
+ *
+ * The `_CODE` NAME and the bare `readonly code = READONLY_FIELD_REJECTED_CODE;`
+ * spelling are both load-bearing rather than cosmetic: the first is the shape
+ * `check:error-code-provenance`'s `constdef` pattern can see, the second is the
+ * shape `check:dispatcher-error-vocabulary` classifies as `classconst` (an
+ * `as const` suffix on the field would take it out of that pattern). ⛔ Never
+ * rename out of either shape to quiet a gate: a spelling a gate cannot see is
+ * the failure mode the gate exists to catch, not a clean result.
+ *
+ * Shape and placement follow the `*_CODE` constants already in this package;
+ * dropping the `ERR_` prefix from the CONSTANT's name follows the two
+ * `ERR_`-prefixed precedents here (`HOOK_TARGET_REBIND_ERROR_CODE` =
+ * `'ERR_HOOK_TARGET_REBIND'`, `SYSTEM_WRITE_ORGANIZATION_REQUIRED_CODE` =
+ * `'ERR_SYSTEM_WRITE_ORGANIZATION_REQUIRED'`). Re-exported from the `index.ts`
+ * barrel and, like all of them, deliberately NOT from the lean `core.ts` entry.
+ */
+export const READONLY_FIELD_REJECTED_CODE = 'ERR_READONLY_FIELD_REJECTED' as const;
+
 export class ReadonlyFieldRejectedError extends Error {
-  readonly code = 'ERR_READONLY_FIELD_REJECTED' as const;
+  readonly code = READONLY_FIELD_REJECTED_CODE;
   constructor(
     public readonly object: string,
     public readonly fields: string[],