@@ -0,0 +1,35 @@
+---
+'@objectstack/metadata-protocol': minor
+'@objectstack/cli': minor
+'@objectstack/cloud-connection': minor
+---
+
+**BREAKING** feat(metadata-protocol)!: `__c` extension fields only in a base's paired `.custom` package, refused at the save, restore and publish doors, `os validate`, the marketplace install and `duplicatePackage` (ADR-0146 D10, #28564)
+
+Clause-②: yes (narrowing)
+Metadata-protocol: narrowed
+Skills-docs: none — no file under `skills/**` teaches a field spelled `__c`, an `objectExtensions` field name or a `<base>.custom` package; the measured count below found none.
+
+<!-- adr-0087: not-required (no-migration-prescription) The narrowing refuses a field NAME spelling at the write doors: a field ending in `__c` outside a base's paired `.custom` package, and an `objectExtensions[].fields` key without the suffix inside one. No metadata key, accepted value of a key, export or error code is removed or renamed, and no stored shape changes. The remedy is a rename of a field, an authoring choice a person makes (which name a vendor or a customer field should carry); there is no mechanical conversion between two metadata versions for `objectstack migrate meta` to apply. Measured at `2678600d57`: 0 fields spelled `[a-z0-9]__c` in `examples/**`, `packages/*/src` and `skills/**` outside tests (7 textual hits, all in doc comments). The other categories are closed on facts: all three packages publish (not unpublished); no ADR-0087 id is named or touched (not registered or already-registered); no `packages/spec/**` file is touched, so no schema, type or runtime interface moves (not type-surface-only, not runtime-interface-only); and no export is retired (not runtime-export-retired). -->
+
+ADR-0146 D10 reserves the `__c` field suffix to the customer's layer — maintainer, verbatim: 「扩展字段用 __c 后缀，只有 .custom 包能用。」 So a customer's field on a base object is `region__c`, a vendor's next version can never take that name, and nobody mistakes the customer's field for the vendor's. The reservation was declared in `@objectstack/spec` (`EXTENSION_FIELD_SUFFIX`, `isExtensionFieldName`) and read by no door.
+
+**What is refused**, given the package a body is written into:
+
+- **(a) `__c` outside a `.custom` pair.** In a package that is not a base's paired `<base>.custom`, every field spelled `__c`, on its own objects or in its `objectExtensions`: "the `__c` suffix is reserved to a base's paired `.custom` package (ADR-0146 D10); rename the field." A field one vendor package adds to another's object keeps the plain spelling.
+- **(b) An unsuffixed extension field in a `.custom` pair.** Every `objectExtensions[].fields` key without the suffix: "name it `<name>__c`, or change a base field's display properties in the extension's patch (`fieldPatches`)."
+
+A `.custom` pair's own new objects are not judged (their fields are its own), and neither is a write bound to no package.
+
+**Where**, each door in its own envelope, no new code:
+
+- the save door (`saveMetaItem`, so `PUT /api/v1/meta/object/:name`), the restore (`rollbackMetaItem` and `revertCommit`'s restore limb) and the draft promotion (`publishMetaItem`, `publishPackageDrafts`), for an `object` body: `400 VALIDATION_ERROR`, `issues[]` locating each field;
+- `duplicatePackage`, judged against the target id before the copy writes anything: a copy of a `.custom` pair's `__c` objects into a plain id is refused whole, `400 VALIDATION_ERROR`;
+- `os validate`: exit 1, each field's path on `errors` (`code: 'VALIDATION_ERROR'`, `rule`, `packageId`, `message`);
+- the marketplace install (`POST /api/v1/marketplace/install-local`), at the dependency check's position, before the manifest service sees the artifact, on the managed and the unmanaged branch: `422 VALIDATION_ERROR` (the door's existing body refusal), one row per field in `details.refusals`; nothing is registered and no ledger entry is written.
+
+`installPackage` needs no call: its manifest (`PackageInstallManifestSchema`) is closed, its `objects` are glob strings and it has no `objectExtensions` key, so a field body never reaches it.
+
+**Migration.** None needed for this repository: 0 fields affected (measured above). A package that ships a `__c` field outside `.custom`, or a `.custom` extension field without the suffix, renames the field.
+
+`@objectstack/metadata-protocol` exports the judge, `judgeExtensionFieldSuffix`, with the active doors' envelope (`extensionFieldSuffixRefusalError`) and the one-item question those doors ask (`objectBodyExtensionFieldSuffixRefusal`), so every door asks one judgement.
@@ -77,6 +77,7 @@ import { findViewContainerNameRefusals } from '../utils/view-container-names.js'
 // [ADR-0146 D2, #28410] The override write gate's author-time half: the publish
 // door's own judge, walked over the artifact's packages.
 import { findOverrideDeclarationRefusals } from '../utils/override-declarations.js';
+import { findExtensionFieldSuffixRefusals } from '../utils/extension-field-suffix.js';
 // A field `picklist` that names no picklist the stack declares — refused, or
 // reported when the declaring package depends on packages outside the stack.
 // Walked the way the load path registers, like the refusal above.
@@ -690,6 +691,34 @@ export default class Validate extends Command {
         this.exit(1);
       }
 
+      // 3a-iv. [ADR-0146 D10, #28564] The `__c` extension-field reservation,
+      //     at author time: a field spelled `__c` only in a base's paired
+      //     `.custom` package, and every `objectExtensions[].fields` key of a
+      //     `.custom` pair suffixed. The verdict is the save and install doors'
+      //     own (`judgeExtensionFieldSuffix`), so a green validate is an
+      //     artifact they admit; see `utils/extension-field-suffix.ts`.
+      const suffixRefusals = findExtensionFieldSuffixRefusals(result.data as Record<string, unknown>);
+      if (suffixRefusals.length > 0) {
+        if (flags.json) {
+          await emitJson({
+            valid: false,
+            errors: suffixRefusals,
+            warnings: warningsSoFar(),
+            conversions: conversionNotices,
+            duration: timer.elapsed(),
+          });
+          this.exit(1);
+        }
+        const n = suffixRefusals.length;
+        console.log('');
+        printError(`${n} field${n > 1 ? 's break' : ' breaks'} the \`__c\` extension-field reservation (ADR-0146 D10)`);
+        printBulletList(
+          suffixRefusals.map((r) => `${r.path} [${r.code}] ${r.message}`),
+          { noun: 'extension-field refusal(s)', remedy: JSON_FULL_LIST_REMEDY },
+        );
+        this.exit(1);
+      }
+
       // 3b. [#3366] Installable-provider preflight — the shift-left of the
       //     `serve`-time capability check. `os validate` previously only checked
       //     the `requires` tokens against the vocabulary (ADR-0066), never
 
@@ -0,0 +1,62 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [ADR-0146 D10, #28564] `findExtensionFieldSuffixRefusals` — what `os
+ * validate` hands the `__c` reservation's judge.
+ *
+ * The verdict is `@objectstack/metadata-protocol`'s `judgeExtensionFieldSuffix`,
+ * the one the save and install doors ask; the protocol's own file pins its
+ * sentences. What is pinned here is which packages the walk hands it and under
+ * which id: each `packages[]` entry under its own, or the stack itself under
+ * `manifest.id`, every row's message EQUAL to the judge's for the same subject.
+ */
+
+import { describe, expect, it } from 'vitest';
+import { judgeExtensionFieldSuffix } from '@objectstack/metadata-protocol';
+import { findExtensionFieldSuffixRefusals } from './extension-field-suffix.js';
+
+const VENDOR = 'com.acme.crm';
+const PAIR = 'com.acme.crm.custom';
+
+const objects = [{ name: 'crm_account', label: 'Account', fields: { name: { type: 'text' }, region__c: { type: 'text' } } }];
+const pairExtensions = [{ extend: 'crm_account', fields: { region: { type: 'text' }, tier__c: { type: 'text' } } }];
+
+describe('#28564 — the walk os validate hands the `__c` judge', () => {
+  it('(a) a stack under a plain id: its own `__c` field is refused under that id, located in the stack', () => {
+    const rows = findExtensionFieldSuffixRefusals({ manifest: { id: VENDOR }, objects });
+    expect(rows).toEqual([{
+      path: 'objects[0].fields.region__c', code: 'VALIDATION_ERROR', rule: 'reserved_outside_pair', packageId: VENDOR,
+      message: judgeExtensionFieldSuffix({ packageId: VENDOR, objects })[0]!.message,
+    }]);
+  });
+
+  it('(b) a stack under its `.custom` id: an unsuffixed extension field is refused, the suffixed one admitted', () => {
+    const rows = findExtensionFieldSuffixRefusals({ manifest: { id: PAIR }, objectExtensions: pairExtensions });
+    expect(rows.map((r) => [r.path, r.rule, r.packageId])).toEqual([
+      ['objectExtensions[0].fields.region', 'unsuffixed_in_pair', PAIR],
+    ]);
+    expect(rows[0]!.message).toBe(judgeExtensionFieldSuffix({ packageId: PAIR, objectExtensions: pairExtensions })[0]!.message);
+  });
+
+  it('an artifact\'s `packages[]`: each entry judged under its own id, the stack\'s top level not re-judged', () => {
+    const rows = findExtensionFieldSuffixRefusals({
+      manifest: { id: 'com.acme.bundle' },
+      objects,
+      packages: [
+        { manifest: { id: VENDOR, objects } },
+        { manifest: { id: PAIR, objectExtensions: pairExtensions } },
+      ],
+    });
+    expect(rows.map((r) => [r.path, r.rule, r.packageId])).toEqual([
+      ['packages[0].manifest.objects[0].fields.region__c', 'reserved_outside_pair', VENDOR],
+      ['packages[1].manifest.objectExtensions[0].fields.region', 'unsuffixed_in_pair', PAIR],
+    ]);
+  });
+
+  it('CONTROL: a stack naming no package id is not judged; a clean stack yields nothing', () => {
+    expect(findExtensionFieldSuffixRefusals({ objects })).toEqual([]);
+    expect(findExtensionFieldSuffixRefusals({
+      manifest: { id: PAIR }, objects, objectExtensions: [{ extend: 'crm_account', fields: { region__c: { type: 'text' } } }],
+    })).toEqual([]);
+  });
+});
@@ -0,0 +1,52 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * ADR-0146 D10, #28564 — the `__c` extension-field reservation at `os validate`.
+ *
+ * The verdict is `@objectstack/metadata-protocol`'s `judgeExtensionFieldSuffix`,
+ * the one the save, restore, publish, duplicate and install doors ask, so a
+ * green validate is an artifact those doors admit: `__c` fields only in a
+ * base's paired `.custom` package, and every `objectExtensions[].fields` key of
+ * a `.custom` pair suffixed. This module only hands it the artifact's packages
+ * — each `packages[]` entry under its own id, or the stack itself as one
+ * package under `manifest.id` — the reading `findOverrideDeclarationRefusals`
+ * (`./override-declarations.ts`) takes. A stack naming no package id is not
+ * judged, as the judge skips a write bound to no package.
+ */
+
+import { judgeExtensionFieldSuffix, type ExtensionFieldSuffixRule } from '@objectstack/metadata-protocol';
+
+import { artifactPackages } from './artifact-packages.js';
+
+/** One refused field, located. `message` is the doors' sentence, verbatim. */
+export interface ExtensionFieldSuffixRefusalRow {
+  /** Where the field sits in the parsed stack, e.g. `packages[1].manifest.objectExtensions[0].fields.region`. */
+  path: string;
+  /** The code the active doors refuse the same field with. */
+  code: 'VALIDATION_ERROR';
+  /** Which of the two cases this is (`judgeExtensionFieldSuffix`'s `rule`). */
+  rule: ExtensionFieldSuffixRule;
+  /** The package the field is declared in. */
+  packageId: string;
+  message: string;
+}
+
+const isRecord = (v: unknown): v is Record<string, unknown> =>
+  v !== null && typeof v === 'object' && !Array.isArray(v);
+
+/** Every field of the parsed stack the reservation refuses. */
+export function findExtensionFieldSuffixRefusals(parsed: Record<string, unknown>): ExtensionFieldSuffixRefusalRow[] {
+  const entries = artifactPackages(parsed);
+  const subjects = entries.length > 0
+    ? entries.map(({ index, id, body }) => ({
+      packageId: id, objects: body.objects, objectExtensions: body.objectExtensions, at: `packages[${index}].manifest.`,
+    }))
+    : [{
+      packageId: isRecord(parsed.manifest) ? (parsed.manifest.id as string | undefined) : undefined,
+      objects: parsed.objects,
+      objectExtensions: parsed.objectExtensions,
+    }];
+  return subjects.flatMap((subject) => judgeExtensionFieldSuffix(subject).map((r) => ({
+    path: r.path, code: 'VALIDATION_ERROR' as const, rule: r.rule, packageId: r.packageId, message: r.message,
+  })));
+}
@@ -235,13 +235,23 @@ const BUILD_ONLY_GATES: Readonly<Record<string, string>> = {
  * SHARED_NON_REGISTRY_GATES instead.
  *
  * EMPTY is this ledger's steady state: every row is a gap the build carries.
- * Both rows it has held moved to SHARED_NON_REGISTRY_GATES when `compile.ts`
+ * Two rows it has held moved to SHARED_NON_REGISTRY_GATES when `compile.ts`
  * gained the call: `findViewContainerNameRefusals` (#20331 → #20393) and
- * `findOverrideDeclarationRefusals` (#28410 → #28643). The ledger stays,
- * empty, because it is the only honest place the closed roster has for the
- * next validate-only gate.
+ * `findOverrideDeclarationRefusals` (#28410 → #28643). It holds one today,
+ * `findExtensionFieldSuffixRefusals` (#28564), whose card did not name the
+ * build.
  */
-const VALIDATE_ONLY_GATES: Readonly<Record<string, string>> = {};
+const VALIDATE_ONLY_GATES: Readonly<Record<string, string>> = {
+  // [#28564] A real gap in the build's direction, reported and not closed —
+  // the shape `findOverrideDeclarationRefusals` held until #28643 closed it.
+  // Card #28564 (ADR-0146 D10, decided item 7) named the `__c` reservation's
+  // doors — the save, restore and publish doors, `os validate`, the install
+  // doors and the duplicate — and `os build` is not one of them. The artifact
+  // a build emits is still judged where it lands: the marketplace install and
+  // the save and publish doors refuse the same fields with the same judge.
+  findExtensionFieldSuffixRefusals:
+    'card #28564 placed the __c reservation at the save/restore/publish doors, os validate, the install doors and duplicate; os build is a further point it did not name (as #28410 left the override gate until #28643). The install and publish doors refuse the same fields with the same judge.',
+};
 
 /**
  * Everything else the two commands call, and the reason each one is NOT an
 
@@ -0,0 +1,106 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [ADR-0146 D10, #28564] `os validate` asks the `__c` extension-field
+ * reservation — the save and install doors' own judge
+ * (`judgeExtensionFieldSuffix`), over the walk `utils/extension-field-suffix.ts`
+ * takes, whose own file pins which packages it judges. What is pinned here is
+ * the command: case (a) and case (b) each exit 1 with the field's path on
+ * `errors` (and printed without `--json`); the base's paired `.custom`
+ * declaring `region__c` in its extension validates clean.
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { execFile } from 'node:child_process';
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join, resolve } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { childEnv } from './helpers/serve-process.js';
+import { linkSpec } from './helpers/define-stack-fixture.js';
+
+const HERE = resolve(fileURLToPath(import.meta.url), '..');
+const CLI = resolve(HERE, '../bin/run-dev.js');
+const TSX = resolve(HERE, '../../../node_modules/.bin/tsx');
+
+/** A stack under `id` extending the base's `crm_account` with one field named `field`. */
+const stack = (id: string, namespace: string, field: string) => `
+import { defineStack } from '@objectstack/spec';
+
+export default defineStack({
+  manifest: {
+    id: '${id}', name: 'ext', version: '1.0.0', type: 'app', namespace: '${namespace}',
+    dependencies: { 'com.acme.crm': '^1.0.0' },
+  },
+  objectExtensions: [
+    { extend: 'crm_account', fields: { ${field}: { type: 'text', label: 'Region' } } },
+  ],
+}, { strict: false });
+`;
+
+interface Run {
+  code: number;
+  stdout: string;
+  stderr: string;
+}
+
+function runValidate(cwd: string, json = true): Promise<Run> {
+  return new Promise((resolvePromise) => {
+    execFile(
+      TSX,
+      [CLI, 'validate', ...(json ? ['--json'] : [])],
+      // `childEnv()`, never a bulk `process.env` copy (check:cli-test-child-env).
+      { cwd, maxBuffer: 16 * 1024 * 1024, env: childEnv({ NO_COLOR: '1' }) },
+      (err, stdout, stderr) => {
+        resolvePromise({
+          code: err ? (typeof (err as { code?: unknown }).code === 'number' ? (err as unknown as { code: number }).code : 1) : 0,
+          stdout: String(stdout),
+          stderr: String(stderr),
+        });
+      },
+    );
+  });
+}
+
+const dirs: Record<'vendor' | 'pairPlain' | 'pairSuffixed', string> = { vendor: '', pairPlain: '', pairSuffixed: '' };
+
+beforeAll(() => {
+  const make = (key: keyof typeof dirs, source: string) => {
+    dirs[key] = mkdtempSync(join(tmpdir(), `os-validate-ext-suffix-${key}-`));
+    writeFileSync(join(dirs[key], 'objectstack.config.ts'), source);
+    linkSpec(dirs[key]);
+  };
+  make('vendor', stack('com.acme.partner', 'partner', 'region__c'));
+  make('pairPlain', stack('com.acme.crm.custom', 'crm_custom', 'region'));
+  make('pairSuffixed', stack('com.acme.crm.custom', 'crm_custom', 'region__c'));
+});
+
+afterAll(() => {
+  for (const dir of Object.values(dirs)) if (dir) rmSync(dir, { recursive: true, force: true });
+});
+
+describe('#28564 — `os validate` refuses a field the `__c` reservation refuses', () => {
+  it('(a) a plain package extending with `region__c` exits 1, the field\'s path on `errors`', async () => {
+    const run = await runValidate(dirs.vendor);
+    expect(run.code, run.stdout + run.stderr).toBe(1);
+    const payload = JSON.parse(run.stdout) as { valid: boolean; errors: Array<Record<string, unknown>> };
+    expect(payload.valid).toBe(false);
+    expect(payload.errors.map((e) => [e.path, e.code, e.rule, e.packageId])).toEqual([
+      ['objectExtensions[0].fields.region__c', 'VALIDATION_ERROR', 'reserved_outside_pair', 'com.acme.partner'],
+    ]);
+    expect(String(payload.errors[0]!.message)).toContain('the `__c` suffix is reserved to a base\'s paired `.custom` package');
+  }, 120_000);
+
+  it('(b) the `.custom` pair extending with a plain `region` exits 1, the path printed without `--json`', async () => {
+    const run = await runValidate(dirs.pairPlain, false);
+    expect(run.code, run.stdout + run.stderr).toBe(1);
+    expect(run.stdout + run.stderr).toContain('objectExtensions[0].fields.region [VALIDATION_ERROR]');
+    expect(run.stdout + run.stderr).toContain('name it `region__c`');
+  }, 120_000);
+
+  it('CONTROL — the `.custom` pair extending with `region__c` validates clean', async () => {
+    const run = await runValidate(dirs.pairSuffixed);
+    expect(run.code, run.stdout + run.stderr).toBe(0);
+    expect((JSON.parse(run.stdout) as { valid: boolean }).valid).toBe(true);
+  }, 120_000);
+});