diff --git a/.changeset/28858-extension-legacy-key-fold.md b/.changeset/28858-extension-legacy-key-fold.md
new file mode 100644
index 00000000000..dc3156ce44e
--- /dev/null
+++ b/.changeset/28858-extension-legacy-key-fold.md
@@ -0,0 +1,14 @@
+---
+'@objectstack/objectql': patch
+---
+
+fix(objectql): an object extension whose `extend` folds to a legacy `ns__name` key is filed under that key, and never opens a second, owner-less object entry (#28858)
+
+Clause-②: no — no accept set moves and nothing is exported; an extension that already registered now lands on the object it names. Measured reach: no package or example in this repository or in hotcrm spells an `extend` that folds.
+Metadata-protocol: none
+
+`getObject` reads a short name as a legacy `<ns>__<name>` key when that is the only object it can mean (an object a package still ships as `crm__case`, which `ObjectSchema` accepts). The registry used to file an extension under the name it is written with. So a manifest `objectExtensions` entry or a stored `object_extension` row with `extend: 'case'` opened a second entry, `case`, with no owner, beside `crm__case`. Its fields reached no read of the object, and nothing checked the entry.
+
+- **Now.** Every entry that registers an extension (the manifest loop and the stored row's hydration, both through `registerObject`) files it under the key `getObject` reads its `extend` as. The fields land on `crm__case`, and no `case` entry opens. The spelling is named once per spelling and package at `warn`, with the key to write.
+- **Never a boot refusal.** The manifest loop and the hydration run while packages are still registering. The `object_extension` item door keeps refusing the folded spelling at save (#28834).
+- **Unchanged.** An `extend` spelled with the registered key, or naming an object registered under that exact name, registers as before, with no line. `computeFQN` and `parseFQN` are untouched. A legacy owner that registers after the extension is not yet there to fold to, so that extension is filed as written, as before.
diff --git a/packages/objectql/src/registry-extension-legacy-key-fold.test.ts b/packages/objectql/src/registry-extension-legacy-key-fold.test.ts
new file mode 100644
index 00000000000..65d0294655b
--- /dev/null
+++ b/packages/objectql/src/registry-extension-legacy-key-fold.test.ts
@@ -0,0 +1,112 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [#28858, ADR-0146 D10] An `extend` that `getObject` folds to a legacy
+ * `<ns>__<name>` key never opens a second object entry.
+ *
+ * A vendor package still ships its Case under the legacy key `crm__case`
+ * (the published `ObjectSchema` accepts the name), so `getObject('case')`
+ * folds to it. A second package extends it as `extend: 'case'`. The
+ * extension is filed under the key the object is registered under, its field
+ * lands on the vendor's object, no owner-less `case` entry opens beside it,
+ * and the spelling is named once at `warn`. Never a boot refusal (AGENTS.md
+ * *Startup registry reads*). The `object_extension` item door still refuses
+ * the folded spelling at save (#28853, pinned in `@objectstack/runtime`).
+ */
+
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { SchemaRegistry } from './registry.js';
+import { ObjectQL } from './engine.js';
+
+const VENDOR = 'com.vendor.crm';
+const EXT = 'com.acme.case-region';
+const LEGACY_KEY = 'crm__case';
+const SHORT = 'case';
+
+const vendorCase = () => ({
+  name: LEGACY_KEY,
+  label: 'Case',
+  fields: { subject: { name: 'subject', type: 'text', label: 'Subject' } },
+});
+
+const region = { region: { name: 'region', type: 'text', label: 'Region' } };
+
+function registry(): SchemaRegistry {
+  const reg = new SchemaRegistry({ multiTenant: false, searchCompanion: false } as never);
+  reg.logLevel = 'silent';
+  return reg;
+}
+
+const spellingLines = (warn: { mock: { calls: unknown[][] } }): string[] =>
+  warn.mock.calls.map((c) => String(c[0])).filter((l) => l.includes(`'${LEGACY_KEY}'`) && l.includes(EXT));
+
+afterEach(() => vi.restoreAllMocks());
+
+describe('#28858 — an extension whose `extend` folds to a legacy key lands on the object it folds to', () => {
+  it('a manifest objectExtensions entry: the field is on the vendor\'s object, no second entry opens, named once', () => {
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const ql = new ObjectQL();
+    (ql.registry as SchemaRegistry).logLevel = 'silent';
+    ql.registerApp({ id: VENDOR, name: 'CRM', version: '1.0.0', objects: [vendorCase()] });
+    ql.registerApp({
+      id: EXT, name: 'Case region', version: '1.0.0',
+      objectExtensions: [{ extend: SHORT, fields: region }],
+    });
+
+    const reg = ql.registry as SchemaRegistry;
+    expect(Object.keys((reg.getObject(LEGACY_KEY) as any).fields)).toEqual(expect.arrayContaining(['subject', 'region']));
+    expect(reg.getObjectContributors(SHORT)).toEqual([]);
+    expect(reg.getObjectContributors(LEGACY_KEY).map((c) => [c.packageId, c.ownership]))
+      .toEqual([[VENDOR, 'own'], [EXT, 'extend']]);
+    expect(reg.getAllObjects().map((o) => o.name).filter((n) => n === SHORT || n === LEGACY_KEY)).toEqual([LEGACY_KEY]);
+    // The short name still reads the one object, with the field.
+    expect(Object.keys((reg.getObject(SHORT) as any).fields)).toContain('region');
+
+    // Re-reading and re-registering name it no more than once.
+    reg.invalidateAll();
+    reg.getObject(LEGACY_KEY);
+    ql.registerApp({ id: EXT, name: 'Case region', version: '1.0.0', objectExtensions: [{ extend: SHORT, fields: region }] });
+    const lines = spellingLines(warn);
+    expect(lines).toHaveLength(1);
+    expect(lines[0]).toContain(`'${SHORT}'`);
+  });
+
+  it('registerObjectExtension (the stored row\'s verb) files under the resolved key and answers it', () => {
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const reg = registry();
+    reg.registerObject(vendorCase() as any, VENDOR, undefined, 'own');
+
+    const filed = reg.registerObjectExtension({ extend: SHORT, fields: region, _provenance: 'org' }, EXT);
+
+    expect(filed).toBe(LEGACY_KEY);
+    expect(reg.getObjectContributors(SHORT)).toEqual([]);
+    expect(Object.keys((reg.getObject(LEGACY_KEY) as any).fields)).toContain('region');
+    expect(spellingLines(warn)).toHaveLength(1);
+    // The withdrawal the row delete makes reaches the same entry.
+    expect(reg.withdrawObjectExtension(SHORT, EXT)).toBe(true);
+    expect(Object.keys((reg.getObject(LEGACY_KEY) as any).fields)).not.toContain('region');
+  });
+
+  it('control: an extension spelled with the registered key registers as before, with no warn', () => {
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const reg = registry();
+    reg.registerObject(vendorCase() as any, VENDOR, undefined, 'own');
+
+    expect(reg.registerObjectExtension({ extend: LEGACY_KEY, fields: region }, EXT)).toBe(LEGACY_KEY);
+    expect(Object.keys((reg.getObject(LEGACY_KEY) as any).fields)).toContain('region');
+    expect(reg.getObjectContributors(SHORT)).toEqual([]);
+    expect(spellingLines(warn)).toEqual([]);
+  });
+
+  it('control: an object registered under the exact name keeps its extensions, even beside a legacy key that shares the short name', () => {
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const reg = registry();
+    reg.registerObject(vendorCase() as any, VENDOR, undefined, 'own');
+    reg.registerObject({ name: SHORT, label: 'Support case', fields: { title: { name: 'title', type: 'text', label: 'Title' } } } as any, 'com.other.support', undefined, 'own');
+
+    expect(reg.registerObjectExtension({ extend: SHORT, fields: region }, EXT)).toBe(SHORT);
+    expect(reg.getObjectContributors(SHORT).map((c) => c.ownership)).toEqual(['own', 'extend']);
+    expect(Object.keys((reg.getObject(LEGACY_KEY) as any).fields)).not.toContain('region');
+    expect(spellingLines(warn)).toEqual([]);
+  });
+});
diff --git a/packages/objectql/src/registry.ts b/packages/objectql/src/registry.ts
index 8827e579eb5..9761f56f108 100644
--- a/packages/objectql/src/registry.ts
+++ b/packages/objectql/src/registry.ts
@@ -4285,6 +4285,12 @@ export class SchemaRegistry {
     if (ownership === 'extend') {
       const extendRefusal = extendContributionRefusal(schema, packageId);
       if (extendRefusal) throw extendRefusal;
+      // [#28858] …filed under the key the object it extends is registered
+      // under — see {@link extensionTargetKey}. Every entry that registers an
+      // extension reaches this line: the manifest `objectExtensions` loop and
+      // a stored `object_extension` row ({@link registerObjectExtension}).
+      const target = this.extensionTargetKey(String(schema.name ?? ''), packageId);
+      if (target !== schema.name) schema = { ...schema, name: target };
     }
     // [ADR-0144 D6] A `summary` roll-up that reads a `record`-mode amount —
     // `sum` / `avg` / `min` / `max` of a child field whose rows each carry
@@ -6183,6 +6189,46 @@ export class SchemaRegistry {
     );
   }
 
+  /**
+   * [#28858] Spellings of an `extend` already named, keyed
+   * `spelled|resolved|package`. Per registry, for the reason
+   * {@link shadowedExtensionQueueWarned} gives.
+   */
+  private extensionSpellingWarned = new Set<string>();
+
+  /**
+   * [#28858, ADR-0146 D10] The key an extension of `name` is filed under: the
+   * key {@link getObject} reads `name` as. That is `name` itself whenever an
+   * entry is registered under it, or nothing is registered that `name` folds
+   * to. It differs only for a short name `getObject` folds to a legacy
+   * `<ns>__<name>` key ({@link resolveObjectKey}); filing such an extension
+   * under the short name would open a second, owner-less entry beside the
+   * object it meant, which nothing serves and nothing checks.
+   *
+   * The folded spelling is named once at `warn`, with the key to write. ⛔
+   * Never a refusal: the manifest loop and the boot hydration run while
+   * packages are still registering (AGENTS.md *Startup registry reads*). The
+   * `object_extension` item door refuses the same spelling at save (#28834),
+   * where the author is present to fix it. A legacy owner that registers
+   * AFTER the extension is not yet there to fold to, so that extension is
+   * filed under its own spelling, as before.
+   */
+  private extensionTargetKey(name: string, packageId: string | undefined): string {
+    if (name === '' || this.objectContributors.has(name)) return name;
+    const resolved = this.resolveObjectKey(name);
+    if (resolved === undefined || resolved === name) return name;
+    const key = `${name}|${resolved}|${packageId ?? ''}`;
+    if (!this.extensionSpellingWarned.has(key)) {
+      this.extensionSpellingWarned.add(key);
+      console.warn(
+        `[Registry] An extension${packageId ? ` from package "${packageId}"` : ''} names its object '${name}', ` +
+          `which is registered as '${resolved}': the extension is filed under '${resolved}', the object it ` +
+          `reads as. Write \`extend: '${resolved}'\` to name the object by the key it is registered under.`,
+      );
+    }
+    return resolved;
+  }
+
   /**
    * [#28574] Withdraw ONE package's `'extend'` contribution to an object — the
    * delete of that package's `object_extension` item. The owner, the overlay
diff --git a/packages/runtime/src/domains/meta-object-extension-item-door-fit.test.ts b/packages/runtime/src/domains/meta-object-extension-item-door-fit.test.ts
index cf3f0e3b560..ea91611ad35 100644
--- a/packages/runtime/src/domains/meta-object-extension-item-door-fit.test.ts
+++ b/packages/runtime/src/domains/meta-object-extension-item-door-fit.test.ts
@@ -29,7 +29,7 @@
  * sentence and that no row was left.
  */
 
-import { describe, it, expect, beforeAll, afterAll, afterEach } from 'vitest';
+import { describe, it, expect, beforeAll, afterAll, afterEach, vi } from 'vitest';
 import { mkdtempSync, rmSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
@@ -199,3 +199,41 @@ describe('an object_extension item names its target by the key it is registered
         expect(engine.registry.getObjectContributors(LEGACY_SHORT)).toEqual([]);
     });
 });
+
+describe('a stored object_extension row whose extend folds to a legacy key lands on that object at the boot hydration (#28858)', () => {
+    it('the field is on the legacy-keyed object, no second entry opens, and the spelling is named once', async () => {
+        const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        try {
+            const { engine, p } = await boot();
+            // A row stored under the short spelling: the item door refuses
+            // that spelling now (#28834), so it is a row written before it.
+            // Stored as the registered key, then respelled at rest.
+            await p.saveMetaItem({ type: 'object_extension', name: LEGACY_KEY, packageId: PAIR, item: region(LEGACY_KEY), mode: 'draft' });
+            await p.publishMetaItem({ type: 'object_extension', name: LEGACY_KEY, packageId: PAIR });
+            const stored: any[] = ((await engine.find('sys_metadata', { where: { type: 'object_extension' }, context: { isSystem: true } })) ?? [])
+                .filter((r: any) => r.package_id === PAIR);
+            expect(stored).toHaveLength(1);
+            const body = typeof stored[0].metadata === 'string' ? JSON.parse(stored[0].metadata) : stored[0].metadata;
+            await engine.update('sys_metadata', {
+                id: stored[0].id,
+                name: LEGACY_SHORT,
+                metadata: JSON.stringify({ ...body, extend: LEGACY_SHORT }),
+            }, { context: { isSystem: true } });
+            // A fresh process: the registry holds no contribution of the pair.
+            engine.registry.withdrawObjectExtension(LEGACY_KEY, PAIR);
+            expect(engine.registry.getObjectContributors(LEGACY_KEY).map((c: any) => c.packageId)).toEqual([BASE]);
+
+            const result = await p.loadMetaFromDb();
+            expect(result.errors).toBe(0);
+            expect(engine.registry.getObjectContributors(LEGACY_SHORT)).toEqual([]);
+            expect(engine.registry.getObjectContributors(LEGACY_KEY).map((c: any) => [c.packageId, c.ownership]))
+                .toEqual([[BASE, 'own'], [PAIR, 'extend']]);
+            expect(Object.keys((engine.registry.getObject(LEGACY_KEY) as any)?.fields ?? {})).toEqual(expect.arrayContaining(['region__c', 'subject']));
+            const lines = warn.mock.calls.map((c) => String(c[0]))
+                .filter((l) => l.includes(`'${LEGACY_SHORT}'`) && l.includes(`'${LEGACY_KEY}'`) && l.includes(PAIR));
+            expect(lines).toHaveLength(1);
+        } finally {
+            warn.mockRestore();
+        }
+    });
+});