diff --git a/.changeset/28992-scaffold-namespace-pair-cap.md b/.changeset/28992-scaffold-namespace-pair-cap.md
new file mode 100644
index 00000000000..079df59b57c
--- /dev/null
+++ b/.changeset/28992-scaffold-namespace-pair-cap.md
@@ -0,0 +1,13 @@
+---
+'create-objectstack': patch
+'@objectstack/cli': patch
+---
+
+fix(scaffold): a scaffolded namespace is at most 13 characters, so a fresh project from a long name validates with no `namespace-no-customization-pair` warning (#28992)
+
+Clause-②: no
+Metadata-protocol: none
+
+`npx create-objectstack` and `os init` derive `manifest.namespace` from the project name. They truncated it at 20 characters, the manifest grammar's limit, while a namespace longer than 13 has no paired customization namespace (ADR-0146 D3). So `qa-console-first-paint` scaffolded the namespace `qa_console_first_pai`, and the project's first `os validate` warned about it and about the object names it prefixes, none of which the author wrote.
+
+Both scaffolders now cap the namespace at `MAX_CUSTOMIZATION_BASE_NAMESPACE_LENGTH` (13): `qa-console-first-paint` gets `qa_console_fi`, and every object name and file the scaffold derives from the namespace follows it. A truncated namespace never ends in `_`. `os init` reads the constant from `@objectstack/spec/kernel`; `create-objectstack`, which does not depend on the spec at runtime, keeps its own copy, pinned equal to the spec constant by a test. Project names whose namespace already fit are scaffolded exactly as before.
diff --git a/packages/cli/src/commands/init.ts b/packages/cli/src/commands/init.ts
index 36e32addae0..ef45f5993e1 100644
--- a/packages/cli/src/commands/init.ts
+++ b/packages/cli/src/commands/init.ts
@@ -1,7 +1,7 @@
 // Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
 
 import { Args, Command, Flags } from '@oclif/core';
-import { PROTOCOL_MAJOR } from '@objectstack/spec/kernel';
+import { MAX_CUSTOMIZATION_BASE_NAMESPACE_LENGTH, PROTOCOL_MAJOR } from '@objectstack/spec/kernel';
 import chalk from 'chalk';
 import fs from 'fs';
 import path from 'path';
@@ -67,18 +67,26 @@ function pkgVersion(): string {
  *
  * npm names allow hyphens/dots/scopes (e.g. `@acme/my-app`); identifiers don't.
  * We strip the scope, replace separators with `_`, lowercase, prefix a leading
- * digit, truncate to 20, and pad short names so the result always satisfies
- * the regex. Reserved names get a `_app` suffix.
+ * digit, truncate, and pad short names so the result always satisfies the
+ * regex. Reserved names get a `_app` suffix.
+ *
+ * The cap is `MAX_CUSTOMIZATION_BASE_NAMESPACE_LENGTH` (13), not the grammar's
+ * 20: a longer namespace has no paired customization namespace (ADR-0146 D3),
+ * so `os validate` on the fresh project would warn
+ * `namespace-no-customization-pair` about objects its author never wrote.
+ * `create-objectstack`'s `sanitizeNamespace()` restates the same rule, pinned
+ * to the same constant, so both scaffolders agree.
  */
 export function sanitizeNamespace(name: string): string {
+  const max = MAX_CUSTOMIZATION_BASE_NAMESPACE_LENGTH;
   let s = name.replace(/^@[^/]+\//, '');           // drop npm scope
   s = s.toLowerCase().replace(/[^a-z0-9]+/g, '_'); // separators → _
   s = s.replace(/^_+|_+$/g, '');                   // trim underscores
   if (!s) s = 'app';
   if (/^[0-9]/.test(s)) s = 'a' + s;               // must start with a letter
-  if (s.length < 2) s = (s + '_app').slice(0, 20);
-  if (s.length > 20) s = s.slice(0, 20).replace(/_+$/, '');
-  if (['base', 'system', 'sys'].includes(s)) s = (s + '_app').slice(0, 20);
+  if (s.length < 2) s = (s + '_app').slice(0, max);
+  if (s.length > max) s = s.slice(0, max).replace(/_+$/, '');
+  if (['base', 'system', 'sys'].includes(s)) s = (s + '_app').slice(0, max);
   return s;
 }
 
diff --git a/packages/cli/test/init.test.ts b/packages/cli/test/init.test.ts
index 45f40a7a9c3..47eaf3b2fcc 100644
--- a/packages/cli/test/init.test.ts
+++ b/packages/cli/test/init.test.ts
@@ -1,7 +1,7 @@
 // Copyright (c) 2025 ObjectStack. Licensed under the Apache-2.0 license.
 
 import { describe, it, expect } from 'vitest';
-import { PROTOCOL_MAJOR } from '@objectstack/spec/kernel';
+import { PROTOCOL_MAJOR, customizationNamespace } from '@objectstack/spec/kernel';
 import fs from 'fs';
 import os from 'os';
 import path from 'path';
@@ -404,6 +404,26 @@ describe('sanitizeNamespace', () => {
       expect(sanitizeNamespace(input)).toMatch(NS_RE);
     }
   });
+
+  // A namespace too long for a paired customization namespace (ADR-0146 D3)
+  // makes `os validate` warn `namespace-no-customization-pair` on the fresh
+  // scaffold, about objects its author never wrote.
+  it.each([
+    ['qa-console-first-paint', 'qa_console_fi'],
+    ['abcdefghijkl-mnop', 'abcdefghijkl'], // the 13th character is `_`: dropped
+    ['@acme/very-long-project-name', 'very_long_pro'],
+    ['1234567890123456', 'a123456789012'],
+    ['abcdefghijklm', 'abcdefghijklm'],
+  ])('caps %s at the customization-pair limit → %s', (input, expected) => {
+    expect(sanitizeNamespace(input)).toBe(expected);
+  });
+
+  it('always produces a namespace that has a customization pair', () => {
+    for (const input of ['my-app', '123app', 'sys', 'a', '', 'qa-console-first-paint', 'really-long-name-truncated-here', 'abcdefghijkl-mnop']) {
+      const ns = sanitizeNamespace(input);
+      expect(customizationNamespace(ns), ns).toEqual({ ok: true, value: `${ns}_custom` });
+    }
+  });
 });
 
 describe('scaffold rendering — round-trip', () => {
diff --git a/packages/create-objectstack/src/index.ts b/packages/create-objectstack/src/index.ts
index 625a2dc9c1c..461c63dc9e7 100644
--- a/packages/create-objectstack/src/index.ts
+++ b/packages/create-objectstack/src/index.ts
@@ -74,6 +74,7 @@ import { syncObjectStackDeps } from './pkg-utils.js';
 import { copyDir } from './template-copy.js';
 import {
   deriveManifestId,
+  sanitizeNamespace,
   readTemplateNamespace,
   rewriteObjectNamePrefix,
   findStaleNamespacePrefixes,
@@ -107,23 +108,6 @@ function toTitleCase(str: string): string {
   return str.replace(/[-_]/g, ' ').replace(/\b\w/g, (c) => c.toUpperCase());
 }
 
-/**
- * Convert an npm package name into a valid ObjectStack namespace identifier
- * (regex `^[a-z][a-z0-9_]{1,19}$`, reserved: base/system/sys). Mirrors the
- * implementation in `@objectstack/cli` so both scaffolders agree.
- */
-export function sanitizeNamespace(name: string): string {
-  let s = name.replace(/^@[^/]+\//, '');
-  s = s.toLowerCase().replace(/[^a-z0-9]+/g, '_');
-  s = s.replace(/^_+|_+$/g, '');
-  if (!s) s = 'app';
-  if (/^[0-9]/.test(s)) s = 'a' + s;
-  if (s.length < 2) s = (s + '_app').slice(0, 20);
-  if (s.length > 20) s = s.slice(0, 20).replace(/_+$/, '');
-  if (['base', 'system', 'sys'].includes(s)) s = (s + '_app').slice(0, 20);
-  return s;
-}
-
 function readCliVersion(): string {
   try {
     const pkgPath = path.resolve(__dirname, '..', 'package.json');
diff --git a/packages/create-objectstack/src/rewrite-identity.ts b/packages/create-objectstack/src/rewrite-identity.ts
index 35eae3f4e75..8fb4bd565dd 100644
--- a/packages/create-objectstack/src/rewrite-identity.ts
+++ b/packages/create-objectstack/src/rewrite-identity.ts
@@ -116,6 +116,43 @@ export function deriveManifestId(projectName: string): string {
   return `${SCAFFOLD_ID_PREFIX}${s}`;
 }
 
+/**
+ * The longest namespace a scaffold emits: the longest base namespace that
+ * still has a paired customization namespace (`<namespace>_custom`, ADR-0146
+ * D3). A longer one passes the manifest grammar (20 characters) but has no
+ * pair, so `os validate` on the fresh project warns
+ * `namespace-no-customization-pair` about objects its author never wrote.
+ *
+ * It is `MAX_CUSTOMIZATION_BASE_NAMESPACE_LENGTH` from
+ * `@objectstack/spec/kernel`, restated because this package cannot import it:
+ * the spec is a devDependency here, so a value import would be bundled into
+ * the published scaffolder (`check:undeclared-dep-imports`), and making it a
+ * runtime dependency would put zod into a two-dependency `npx` entry point.
+ * `scaffold-namespace-cap.test.ts` pins the two equal, so they cannot drift.
+ */
+export const SCAFFOLD_NAMESPACE_MAX_LENGTH = 13;
+
+/**
+ * Convert an npm package name into a valid ObjectStack namespace identifier
+ * (regex `^[a-z][a-z0-9_]{1,19}$`, reserved: base/system/sys) that is at most
+ * {@link SCAFFOLD_NAMESPACE_MAX_LENGTH} characters, so it has a paired
+ * customization namespace. A longer name is truncated, with no trailing `_`.
+ * Mirrors `sanitizeNamespace()` in `@objectstack/cli`'s `init` command, so
+ * both scaffolders agree.
+ */
+export function sanitizeNamespace(name: string): string {
+  const max = SCAFFOLD_NAMESPACE_MAX_LENGTH;
+  let s = name.replace(/^@[^/]+\//, '');
+  s = s.toLowerCase().replace(/[^a-z0-9]+/g, '_');
+  s = s.replace(/^_+|_+$/g, '');
+  if (!s) s = 'app';
+  if (/^[0-9]/.test(s)) s = 'a' + s;
+  if (s.length < 2) s = (s + '_app').slice(0, max);
+  if (s.length > max) s = s.slice(0, max).replace(/_+$/, '');
+  if (['base', 'system', 'sys'].includes(s)) s = (s + '_app').slice(0, max);
+  return s;
+}
+
 /** Every `*.ts` file under `dir`, recursively. Missing dir → empty. */
 function tsFiles(dir: string, out: string[] = []): string[] {
   if (!fs.existsSync(dir)) return out;
diff --git a/packages/create-objectstack/src/scaffold-namespace-cap.test.ts b/packages/create-objectstack/src/scaffold-namespace-cap.test.ts
new file mode 100644
index 00000000000..744c2038f02
--- /dev/null
+++ b/packages/create-objectstack/src/scaffold-namespace-cap.test.ts
@@ -0,0 +1,64 @@
+// Copyright (c) 2026 ObjectStack contributors. Apache-2.0 license.
+//
+// `sanitizeNamespace()` emits a namespace that has a paired customization
+// namespace (ADR-0146 D3), judged by the spec's own `customizationNamespace`,
+// and its cap is pinned equal to the spec constant it restates.
+
+import { describe, it, expect } from 'vitest';
+import {
+  MAX_CUSTOMIZATION_BASE_NAMESPACE_LENGTH,
+  ManifestSchema,
+  customizationNamespace,
+} from '@objectstack/spec/kernel';
+import { SCAFFOLD_NAMESPACE_MAX_LENGTH, sanitizeNamespace } from './rewrite-identity.js';
+
+/** Names a scaffolder is handed: short, exact-fit, long, scoped, odd. */
+const NAMES = [
+  '', 'x', 'a', '1', 'my-app', 'crm', 'base', 'system', 'sys', 'Base', '@acme/sys',
+  'abcdefghijklm', 'abcdefghijklmn', 'qa-console-first-paint', 'abcdefghijkl-mnop',
+  'abcdefghijk--lmnop', '1234567890123456', '9-lives', '@acme/very-long-project-name',
+  'My Cool App!!', '__leading', 'trailing__', 'a.b.c.d.e.f.g.h.i', 'ünïcödé-name',
+  'abcdefghijklmnopqrstuvwxyz', 'twelve-chars', 'thirteen-char',
+];
+
+describe('the scaffold namespace cap', () => {
+  it('is the spec constant it restates', () => {
+    expect(SCAFFOLD_NAMESPACE_MAX_LENGTH).toBe(MAX_CUSTOMIZATION_BASE_NAMESPACE_LENGTH);
+  });
+
+  it.each(NAMES)('%j sanitizes to a namespace the manifest accepts and that has a customization pair', (name) => {
+    const ns = sanitizeNamespace(name);
+    expect(ManifestSchema.shape.namespace.safeParse(ns).success, ns).toBe(true);
+    expect(['base', 'system', 'sys']).not.toContain(ns);
+    expect(customizationNamespace(ns), ns).toEqual({ ok: true, value: `${ns}_custom` });
+  });
+});
+
+describe('sanitizeNamespace', () => {
+  it.each([
+    ['qa-console-first-paint', 'qa_console_fi'],
+    // The 13th character is `_`: dropped, never left trailing.
+    ['abcdefghijkl-mnop', 'abcdefghijkl'],
+    ['@acme/very-long-project-name', 'very_long_pro'],
+    ['1234567890123456', 'a123456789012'],
+    ['abcdefghijklmn', 'abcdefghijklm'],
+  ])('truncates %j to %j', (name, expected) => {
+    expect(sanitizeNamespace(name)).toBe(expected);
+  });
+
+  // Names that already fit are scaffolded exactly as before.
+  it.each([
+    ['my-app', 'my_app'],
+    ['crm', 'crm'],
+    ['abcdefghijklm', 'abcdefghijklm'],
+    ['thirteen-char', 'thirteen_char'],
+    ['', 'app'],
+    ['x', 'x_app'],
+    ['9-lives', 'a9_lives'],
+    ['sys', 'sys_app'],
+    ['system', 'system_app'],
+    ['@acme/my-app', 'my_app'],
+  ])('keeps %j as %j', (name, expected) => {
+    expect(sanitizeNamespace(name)).toBe(expected);
+  });
+});
diff --git a/packages/create-objectstack/src/scaffold-namespace-customization-pair.test.ts b/packages/create-objectstack/src/scaffold-namespace-customization-pair.test.ts
new file mode 100644
index 00000000000..99ed187026f
--- /dev/null
+++ b/packages/create-objectstack/src/scaffold-namespace-customization-pair.test.ts
@@ -0,0 +1,75 @@
+// Copyright (c) 2026 ObjectStack contributors. Apache-2.0 license.
+//
+// A scaffold's namespace has a paired customization namespace, so the first
+// `os validate` on a fresh project prints no `namespace-no-customization-pair`
+// warning about objects its author never wrote.
+//
+// The rule (ADR-0146 D3, `@objectstack/lint`'s `validateCustomizationNamespace`)
+// warns when `customizationNamespace(manifest.namespace)` answers
+// `base_namespace_too_long`. The verdict here is that same helper's, read from
+// the spec, so the scaffold is judged by the function the ensure door creates
+// the pair with, never by a number restated in this file.
+//
+// Drives the real CLI end to end via `tsx`: index.ts calls `program.parse()` on
+// import (the pattern of scaffold-namespace-extends-template.test.ts).
+
+import { describe, it, expect, afterEach } from 'vitest';
+import { spawnSync } from 'node:child_process';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { customizationNamespace, validateObjectNamespacePrefix } from '@objectstack/spec/kernel';
+import { readTemplateNamespace } from './rewrite-identity.js';
+
+const HERE = path.dirname(fileURLToPath(import.meta.url));
+const PKG_ROOT = path.resolve(HERE, '..');
+const REPO_ROOT = path.resolve(PKG_ROOT, '..', '..');
+const TSX = path.join(REPO_ROOT, 'node_modules', '.bin', 'tsx');
+const INDEX_TS = path.join(PKG_ROOT, 'src', 'index.ts');
+
+/** Every `name: '…'` literal in the `*.ts` files under `dir`. */
+function nameLiterals(dir: string): string[] {
+  const names: string[] = [];
+  for (const rel of fs.readdirSync(dir, { recursive: true, encoding: 'utf8' })) {
+    if (!rel.endsWith('.ts')) continue;
+    const text = fs.readFileSync(path.join(dir, rel), 'utf8');
+    for (const m of text.matchAll(/\bname:\s*(['"`])([a-z0-9_]+)\1/g)) names.push(m[2]);
+  }
+  return names.sort();
+}
+
+let tmp: string | undefined;
+afterEach(() => {
+  if (tmp) fs.rmSync(tmp, { recursive: true, force: true });
+  tmp = undefined;
+});
+
+describe('a scaffold from a long project name', () => {
+  it('`qa-console-first-paint` gets a namespace with a customization pair, and every object follows it', () => {
+    const project = 'qa-console-first-paint';
+    tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'create-objectstack-ns-pair-'));
+    const run = spawnSync(TSX, [INDEX_TS, project, '--skip-install', '--skip-skills'], {
+      cwd: tmp,
+      encoding: 'utf8',
+    });
+    expect(run.status, `${run.stdout}\n${run.stderr}`).toBe(0);
+
+    const projectDir = path.join(tmp, project);
+    const namespace = readTemplateNamespace(projectDir);
+    expect(namespace).toBe('qa_console_fi');
+    expect(customizationNamespace(namespace)).toEqual({ ok: true, value: 'qa_console_fi_custom' });
+
+    // The config's `namespace:` literal is the same value the manifest carries.
+    const config = fs.readFileSync(path.join(projectDir, 'objectstack.config.ts'), 'utf8');
+    expect(/\bnamespace:\s*'([^']+)'/.exec(config)?.[1]).toBe(namespace);
+
+    // Every object the scaffold ships sits under the shortened namespace.
+    const objects = nameLiterals(path.join(projectDir, 'src')).filter((n) => /_/.test(n));
+    expect(objects.length).toBeGreaterThan(0);
+    for (const name of objects.filter((n) => n.startsWith('qa_'))) {
+      expect(validateObjectNamespacePrefix(name, namespace as string), name).toBeNull();
+    }
+    expect(objects.some((n) => n.startsWith(`${namespace}_`))).toBe(true);
+  }, 30_000);
+});