@@ -0,0 +1,118 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#25522 cut 1] The validation-message localisation helpers of `ObjectQL` —
+ * `validationMessageContext` (the locale and translate hooks a rejected write
+ * is reported through, #3957), `negotiatedMessageLocale` (the locale the
+ * bridged i18n service can actually answer in, #15757) and
+ * `objectDisplayLabel` (an object's display name in the caller's locale,
+ * #7307) — moved out of `engine.ts` whole.
+ *
+ * A MOVE, not a redesign. Each function below is the body of the `ObjectQL`
+ * method of the same name as it stood, re-indented one level and otherwise
+ * unedited: the same negotiation through `resolveBundleLocale`, the same
+ * translation-bundle → declared label → API name ladder, the same two
+ * `catch` blocks that keep a misbehaving i18n service from turning a write
+ * into a 500.
+ *
+ * ## How the moved bodies reach the class
+ *
+ * The shape #23050 set for `rest-server.ts` (its `data-action-routes.ts`
+ * header carries the full reasoning), applied per method. The class keeps
+ * every method — the published `.d.ts` lists each one's name, docblock and
+ * position — and each body is one literal of exactly the
+ * {@link ValidationMessageHost} members that method reads, plus a `.call`;
+ * `objectDisplayLabel` reads no member, takes no host and is called directly.
+ * A method is handed over as a closure that calls it on the engine at call
+ * time, so a test double or spy set on the instance is still the one the body
+ * reaches: `validationMessageContext` reaches `negotiatedMessageLocale`
+ * through the engine, exactly as before.
+ *
+ * The one field the bodies read, `i18nService`, goes as a GETTER, so each read
+ * happens when the body makes it — including the read inside the `translate`
+ * closure, which runs after `validationMessageContext` has returned and sees
+ * whatever service `setI18nService` has installed by then, as it did.
+ * `setI18nService` itself stays in the class: it is the field's setter.
+ *
+ * This module imports the engine only as a TYPE, so there is no module cycle
+ * at run time, and nothing re-exports it, so the published surface does not
+ * move.
+ */
+
+import type { ObjectQL } from './engine.js';
+import { objectLabelKey, resolveBundleLocale } from '@objectstack/spec/system';
+
+/**
+ * The `ObjectQL` members the functions below read — every one of them, and no
+ * other; each function's `this` is the `Pick` of the members IT reads. Each is
+ * typed AS the class member (an indexed access on the class), so a change to a
+ * member's signature on the class is a change here too, and naming a member
+ * the class does not have is a compile error. See the module header for which
+ * member is a getter.
+ */
+export interface ValidationMessageHost {
+  readonly i18nService: ObjectQL['i18nService'];
+  negotiatedMessageLocale: ObjectQL['negotiatedMessageLocale'];
+}
+
+/** The body of `ObjectQL#validationMessageContext`; the method's docblock there is its contract. */
+export function validationMessageContext(
+  this: Pick<ValidationMessageHost, 'i18nService' | 'negotiatedMessageLocale'>,
+  objectName: string,
+  // Only `locale` is read — narrower than `ExecutionContext` so both the
+  // resolved and the input-shaped envelope satisfy it.
+  context?: { locale?: string },
+): { locale?: string; translate?: (key: string, locale: string, params?: Record<string, unknown>) => string; objectName: string } {
+  const t = this.i18nService?.t;
+  return {
+    objectName,
+    locale: this.negotiatedMessageLocale(context?.locale),
+    translate: t ? (key, locale, params) => t.call(this.i18nService, key, locale, params) : undefined,
+  };
+}
+
+/** The body of `ObjectQL#negotiatedMessageLocale`; the method's docblock there is its contract. */
+export function negotiatedMessageLocale(
+  this: Pick<ValidationMessageHost, 'i18nService'>,
+  requested: string | undefined): string | undefined {
+  if (!requested) return requested;
+  const getLocales = this.i18nService?.getLocales;
+  if (typeof getLocales !== 'function') return requested;
+  let available: unknown;
+  try {
+    available = getLocales.call(this.i18nService);
+  } catch {
+    // A misbehaving i18n service must not turn a write into a 500.
+    return requested;
+  }
+  if (!Array.isArray(available) || available.length === 0) return requested;
+  // `resolveBundleLocale` matches against a bundle's KEYS; the offered
+  // locales are exactly that key set, with no data behind them.
+  const offered: Record<string, true> = {};
+  for (const code of available) {
+    if (typeof code === 'string' && code.length > 0) offered[code] = true;
+  }
+  return resolveBundleLocale(offered, requested) ?? requested;
+}
+
+/** The body of `ObjectQL#objectDisplayLabel`; the method's docblock there is its contract. */
+export function objectDisplayLabel(
+  objectName: string,
+  declaredLabel: unknown,
+  ctx: { locale?: string; translate?: (key: string, locale: string, params?: Record<string, unknown>) => string },
+): string {
+  if (ctx.translate && ctx.locale) {
+    const key = objectLabelKey(objectName);
+    try {
+      const translated = ctx.translate(key, ctx.locale);
+      // II18nService echoes the key back on a miss.
+      if (typeof translated === 'string' && translated.length > 0 && translated !== key) {
+        return translated;
+      }
+    } catch {
+      // A misbehaving i18n service must not turn a 409 into a 500.
+    }
+  }
+  const declared = typeof declaredLabel === 'string' ? declaredLabel.trim() : '';
+  return declared.length > 0 ? declared : objectName;
+}
@@ -97,10 +97,11 @@ import {
   isDataMigrationFlagVerified,
   hasMovedFileColumns,
   renderOperationMessage,
-  objectLabelKey,
-  resolveBundleLocale,
   SystemFieldName,
 } from '@objectstack/spec/system';
+// [#25522 cut 1] The validation-message localisation helpers; the methods of the
+// same names below hand each body the members it reads (`ValidationMessageHost`).
+import { negotiatedMessageLocale, objectDisplayLabel, validationMessageContext } from './engine-validation-messages.js';
 import { ExecutionContext, ExecutionContextSchema, type InstalledPackageSource } from '@objectstack/spec/kernel';
 // [#21120] The family-wide stored-metadata-body seam. A `data.record.*` event
 // carries the written row in `after` (and the input patch in `changes`), so a
@@ -10294,12 +10295,13 @@ export class ObjectQL implements IObjectQLEngine {
     // resolved and the input-shaped envelope satisfy it.
     context?: { locale?: string },
   ): { locale?: string; translate?: (key: string, locale: string, params?: Record<string, unknown>) => string; objectName: string } {
-    const t = this.i18nService?.t;
-    return {
-      objectName,
-      locale: this.negotiatedMessageLocale(context?.locale),
-      translate: t ? (key, locale, params) => t.call(this.i18nService, key, locale, params) : undefined,
+    // [#25522 cut 1] The body moved to `engine-validation-messages.ts` (see `ValidationMessageHost`).
+    const readI18nService = () => this.i18nService;
+    const host: ThisParameterType<typeof validationMessageContext> = {
+      get i18nService() { return readI18nService(); },
+      negotiatedMessageLocale: (...a) => this.negotiatedMessageLocale(...a),
     };
+    return validationMessageContext.call(host, objectName, context);
   }
 
   /**
@@ -10333,24 +10335,12 @@ export class ObjectQL implements IObjectQLEngine {
    * EXISTS; inventing one is how a second rule gets born.
    */
   private negotiatedMessageLocale(requested: string | undefined): string | undefined {
-    if (!requested) return requested;
-    const getLocales = this.i18nService?.getLocales;
-    if (typeof getLocales !== 'function') return requested;
-    let available: unknown;
-    try {
-      available = getLocales.call(this.i18nService);
-    } catch {
-      // A misbehaving i18n service must not turn a write into a 500.
-      return requested;
-    }
-    if (!Array.isArray(available) || available.length === 0) return requested;
-    // `resolveBundleLocale` matches against a bundle's KEYS; the offered
-    // locales are exactly that key set, with no data behind them.
-    const offered: Record<string, true> = {};
-    for (const code of available) {
-      if (typeof code === 'string' && code.length > 0) offered[code] = true;
-    }
-    return resolveBundleLocale(offered, requested) ?? requested;
+    // [#25522 cut 1] The body moved to `engine-validation-messages.ts` (see `ValidationMessageHost`).
+    const readI18nService = () => this.i18nService;
+    const host: ThisParameterType<typeof negotiatedMessageLocale> = {
+      get i18nService() { return readI18nService(); },
+    };
+    return negotiatedMessageLocale.call(host, requested);
   }
 
   /**
@@ -10368,20 +10358,8 @@ export class ObjectQL implements IObjectQLEngine {
     declaredLabel: unknown,
     ctx: { locale?: string; translate?: (key: string, locale: string, params?: Record<string, unknown>) => string },
   ): string {
-    if (ctx.translate && ctx.locale) {
-      const key = objectLabelKey(objectName);
-      try {
-        const translated = ctx.translate(key, ctx.locale);
-        // II18nService echoes the key back on a miss.
-        if (typeof translated === 'string' && translated.length > 0 && translated !== key) {
-          return translated;
-        }
-      } catch {
-        // A misbehaving i18n service must not turn a 409 into a 500.
-      }
-    }
-    const declared = typeof declaredLabel === 'string' ? declaredLabel.trim() : '';
-    return declared.length > 0 ? declared : objectName;
+    // [#25522 cut 1] The body moved to `engine-validation-messages.ts`; it reads no member.
+    return objectDisplayLabel(objectName, declaredLabel, ctx);
   }
 
   /**