diff --git a/.changeset/page-header-is-container-3900.md b/.changeset/page-header-is-container-3900.md
new file mode 100644
index 0000000000..948a689968
--- /dev/null
+++ b/.changeset/page-header-is-container-3900.md
@@ -0,0 +1,36 @@
+---
+"@object-ui/layout": patch
+---
+
+`page-header` 注册补 `isContainer: true` —— 校验器不再对文档承诺的 children 写法报 `not-a-container`
+
+`PageHeader` 一直**有意**把 `schema.children` 渲染进右侧动作槽(`PageHeader.tsx:182`,
+`record:quick_actions` 嵌在 `page:header.children` 下就是靠它),
+`content/docs/layout/page-header.mdx` 把该槽的优先级(`action` → React `children` →
+`actions` → schema children)写成公开契约,该文档页唯一的 live demo
+(`layout-page-header/pageheader-with-actions`)正是这个形状且实测正常渲染。但
+`packages/layout/src/index.ts` 的注册漏了 `isContainer: true`。
+
+漏这个 flag 从来没有挡住任何渲染 —— 渲染路径根本不读它(`SchemaRenderer` 把
+`children` 从 React props 里剥掉当元数据,而始终把整个节点作为 `schema` 传下去,
+`PageHeader` 自己再把 `schema.children` 放回槽里)。它的消费者在别处:`sdui-parser`
+的 `not-a-container` 诊断、Studio 调色板元数据、react-page 标签表。所以真正的后果是
+**校验器在说谎**:作者照文档写出能正常渲染的 schema,却拿到一条
+"`page-header` does not accept children" 的 warning;信了这条 warning 去掉 children,
+右槽就空掉。而会说谎的 warning 比缺一条 warning 更贵 —— 它训练作者(尤其 AI 作者)
+连真实的 `not-a-container`(那些确实不收子节点的组件)一起无视。
+
+这不是在 spec 之外新开作者面:`children` 是 objectui JSON 协议里**每个节点**的基础属性
+(`sdui-parser/src/validate.ts` 的 `BASE_PROPS` 把它和 `type`/`id`/`className` 并列),
+不是 `PageHeaderProps` 的键。所以这个 flag 回答的是协议层面的"该节点是否接受子节点列表",
+而对这个组件,答案一直是"是"。维护者 2026-08-09 就 objectui#3900 的 A/B 分叉裁定 A 案,
+理由同上。
+
+行为面变化极窄:注册元数据一个布尔位。渲染输出逐字节不变(渲染路径不读该 flag);
+`sdui-parser` 对带 children 的 `page-header` 少报一条 warning;设计器把它当容器对待
+(即它本来的样子)。canonical 的 `page:header`(`@object-ui/components`)不在此列且刻意不动
+—— 那个渲染器完全不读 `schema.children`,所以它没有 `isContainer` 是正确的。
+
+两个方向都已钉住:文档 demo 走应用真实构建的 manifest 后不再产生 `not-a-container`,
+而一个真正不收子节点的组件(`navigation-renderer`)带 children 时诊断照旧触发 ——
+后者是前者的对照,保证这条修复不是把诊断弄哑了。
diff --git a/examples/schema-catalog/package.json b/examples/schema-catalog/package.json
index 9533fde03e..6aba441290 100644
--- a/examples/schema-catalog/package.json
+++ b/examples/schema-catalog/package.json
@@ -33,6 +33,7 @@
     "@object-ui/core": "workspace:*",
     "@object-ui/layout": "workspace:*",
     "@object-ui/react": "workspace:*",
+    "@object-ui/sdui-parser": "workspace:*",
     "typescript": "^6.0.3"
   }
 }
diff --git a/examples/schema-catalog/test/pageheader-with-actions.test.tsx b/examples/schema-catalog/test/pageheader-with-actions.test.tsx
index c0b5ac3d27..c911607e99 100644
--- a/examples/schema-catalog/test/pageheader-with-actions.test.tsx
+++ b/examples/schema-catalog/test/pageheader-with-actions.test.tsx
@@ -10,7 +10,7 @@
  * rendering, and it held a third, already-drifted copy of the component's
  * spacing numbers (objectui#3786 fixed the second copy, in the prose).
  *
- * Two things are pinned here, and they are different facts:
+ * Three things are pinned here, and they are different facts:
  *
  *  1. SHAPE — the example's root node is a `page-header`, and it contains none
  *     of the class strings that only exist inside `PageHeader.tsx`. This is the
@@ -18,16 +18,23 @@
  *  2. RENDER — driven through the real `SchemaRenderer`, the node produces the
  *     header: an `<h1>` title, the subtitle, and BOTH schema children in the
  *     right-hand slot.
+ *  3. VALIDATE — the same JSON, put through the manifest the app really builds
+ *     from the live registry, draws no `not-a-container` diagnostic (#3900).
  *
- * (2) is worth a test rather than an eyeball because the registration for
- * `page-header` (`packages/layout/src/index.ts`) does NOT declare
- * `isContainer: true`, which reads like children could not reach the slot.
- * They do: `isContainer` is registry metadata that the render path never
- * consults (its consumers are `sdui-parser`'s `not-a-container` diagnostic, the
- * Studio palette, and the react-page tag map). `SchemaRenderer` strips
- * `children` from the React props but always passes the whole node as `schema`,
- * and `PageHeader` re-introduces `schema.children` itself. That is a load-
- * bearing coincidence of two files, so it gets a pin.
+ * (2) and (3) are two halves of one contradiction that used to be live. The
+ * render path never consults `isContainer` — `SchemaRenderer` strips `children`
+ * from the React props but always passes the whole node as `schema`, and
+ * `PageHeader` re-introduces `schema.children` itself — so the header rendered
+ * its children correctly while `packages/layout/src/index.ts` omitted
+ * `isContainer: true` from the registration. The flag's consumers are elsewhere:
+ * `sdui-parser`'s `not-a-container` diagnostic, the Studio palette, and the
+ * react-page tag map. So the omission never blocked anything; it made the
+ * VALIDATOR contradict the docs, the demo and the render — an author following
+ * this very page got a warning telling them their working schema was invalid.
+ * #3900 added the flag (maintainer ruling, route A: `children` is a base
+ * property of every protocol node, not a `PageHeaderProps` key, so declaring it
+ * mints no authoring surface outside the spec). (3) is what keeps the two faces
+ * from drifting apart again in either direction.
  *
  * Module-scope imports, not `beforeAll` (AGENTS.md §测试纪律): the child
  * `button` node resolves through `@object-ui/components`' registration
@@ -37,8 +44,11 @@
 import { describe, it, expect, beforeAll } from 'vitest';
 import { render, screen } from '@testing-library/react';
 import '@object-ui/components';
+import { ComponentRegistry } from '@object-ui/core';
 import { SchemaRenderer } from '@object-ui/react';
 import { registerLayout } from '@object-ui/layout';
+import { manifestFromConfigs, validateTree } from '@object-ui/sdui-parser';
+import type { Diagnostic, SchemaElement } from '@object-ui/sdui-parser';
 import { getExample } from '../src/index.js';
 
 const EXAMPLE_ID = 'layout-page-header/pageheader-with-actions';
@@ -59,6 +69,25 @@ beforeAll(() => {
   registerLayout();
 });
 
+/**
+ * The manifest the running app validates against, built the way the app builds
+ * it — keyed by every KNOWN registry tag (bare and namespaced) rather than by
+ * `getAllConfigs()`, whose `.type` is always the namespaced form. Mirrors
+ * `getJsxManifest()` in `packages/components/src/renderers/layout/page.tsx`
+ * (module-private, hence the four lines here): key it off `getAllConfigs()`
+ * instead and the bare `page-header` tag authors write is absent from the
+ * manifest, so every assertion below would pass on `unknown-component` and
+ * never reach the containment check at all.
+ */
+const diagnose = (schema: unknown): Diagnostic[] => {
+  const configs = ComponentRegistry.getKnownTypes().map((t) => {
+    const meta = ComponentRegistry.getMeta(t);
+    return { type: t, namespace: meta?.namespace, isContainer: meta?.isContainer, inputs: meta?.inputs };
+  });
+  const manifest = manifestFromConfigs(configs as unknown as Parameters<typeof manifestFromConfigs>[0]);
+  return validateTree(schema as SchemaElement, manifest).diagnostics;
+};
+
 describe('the page-header docs demo uses the page-header component (#3787)', () => {
   it('is rooted at a `page-header` node', () => {
     const schema = getExample(EXAMPLE_ID).schema as { type?: string };
@@ -88,8 +117,10 @@ describe('the page-header docs demo uses the page-header component (#3787)', ()
     expect(h1?.textContent).toBe('Users');
     expect(screen.getByText('Manage your team members and permissions')).toBeTruthy();
 
-    // Both children reach the right-hand slot despite the registration not
-    // declaring `isContainer` — see the module header.
+    // Both children reach the right-hand slot. This held even while the
+    // registration omitted `isContainer` — the render path never reads the flag
+    // (see the module header), which is why the omission was invisible here and
+    // only the validator complained.
     expect(screen.getByRole('button', { name: 'Export' })).toBeTruthy();
     expect(screen.getByRole('button', { name: 'Add User' })).toBeTruthy();
   });
@@ -114,3 +145,58 @@ describe('the page-header docs demo uses the page-header component (#3787)', ()
     expect(cls).not.toMatch(/\b(sm|md|lg|xl):pb-/);
   });
 });
+
+/**
+ * The validator agrees with the docs, the demo and the render (#3900).
+ *
+ * Two directions, and BOTH are load-bearing. Asserting only the first would
+ * leave the suite green if someone silenced the containment check outright, or
+ * flipped `isContainer` on by default — the test would then be measuring
+ * nothing. The second case is the control that keeps the first one meaningful.
+ */
+describe('the documented demo validates clean of `not-a-container` (#3900)', () => {
+  const CONTAINMENT = 'not-a-container';
+
+  it('reports no `not-a-container` for the demo the docs page ships', () => {
+    const schema = getExample(EXAMPLE_ID).schema as { children?: unknown[] };
+    const diagnostics = diagnose(schema);
+
+    // Reachability BEFORE the absence — an empty result proves nothing if the
+    // containment branch never ran. Two ways it silently would not:
+    // `validateTree` reports `unknown-component` and returns before the
+    // containment check when the tag is missing from the manifest, and the
+    // branch is guarded by `node.children?.length`, so a demo that lost its
+    // children would satisfy the assertion below for the wrong reason.
+    expect(diagnostics.filter((d) => d.code === 'unknown-component')).toEqual([]);
+    expect(schema.children?.length ?? 0).toBeGreaterThan(0);
+
+    // Filtered by code, not asserted against an empty diagnostic list: the demo
+    // also writes `icon`, which the registration's `inputs` still omits even
+    // though `PageHeader.tsx:224` renders it, so a live `unknown-prop` sits in
+    // this list. That is the same family as #3900 on a different key, filed
+    // separately as objectui#3972 and deliberately NOT pinned here — this test
+    // must not go red when that one is fixed.
+    expect(diagnostics.filter((d) => d.code === CONTAINMENT)).toEqual([]);
+  });
+
+  it('still reports `not-a-container` for a component that genuinely takes none', () => {
+    // The control. `navigation-renderer` (registered in the SAME file as the
+    // #3900 change) is driven entirely by its `items` prop and never reads
+    // `schema.children`, so children under it ARE an authoring mistake and the
+    // author must still hear about it. If this component ever legitimately
+    // becomes a container, this assertion goes red — move the control to
+    // another childless registration rather than deleting it.
+    // `items` is deliberately omitted (it is not `required`, so its absence
+    // draws nothing): this control is about containment only, and writing
+    // `items: []` would also draw a `type-mismatch` — the registration declares
+    // that prop `type: 'object'` while `NavigationRendererProps.items` is
+    // `NavigationItem[]`, a separate declaration-face defect filed as
+    // objectui#3972.
+    const codes = diagnose({
+      type: 'navigation-renderer',
+      children: [{ type: 'button', label: 'Nope' }],
+    }).map((d) => d.code);
+
+    expect(codes).toContain(CONTAINMENT);
+  });
+});
diff --git a/packages/layout/src/__tests__/page-header-authorable-keys.test.tsx b/packages/layout/src/__tests__/page-header-authorable-keys.test.tsx
index 8402d0d3e5..0b94b21352 100644
--- a/packages/layout/src/__tests__/page-header-authorable-keys.test.tsx
+++ b/packages/layout/src/__tests__/page-header-authorable-keys.test.tsx
@@ -91,6 +91,36 @@ describe('the `page-header` registration declares the spec key, not a dialect',
   });
 });
 
+describe('the `page-header` registration declares the child slot it renders (objectui#3900)', () => {
+  // Same principle as the `inputs` narrowing above, other direction: the
+  // declaration face must not DENY a surface the component serves either.
+  // `PageHeader.tsx:182` deliberately renders `schema.children` into the
+  // right-hand slot, `content/docs/layout/page-header.mdx` publishes that slot's
+  // precedence, and the docs page's only live demo is exactly that shape — while
+  // the registration omitted `isContainer`, so `sdui-parser`'s `not-a-container`
+  // diagnostic fired on it. Nothing on the render path reads the flag, so the
+  // omission broke no rendering; it made the validator tell authors (AI authors
+  // especially) that a documented, demo-verified schema was invalid, which is
+  // how the true `not-a-container` reports lose their credibility.
+  //
+  // Not an extension of the spec's authoring surface: `children` is a base
+  // property of every node in objectui's JSON protocol (`BASE_PROPS` in
+  // `sdui-parser/src/validate.ts`), not a key of `PageHeaderProps` — hence the
+  // `specKeys` cross-check above neither covers nor contradicts this.
+  //
+  // The end-to-end half of this pin — the real demo JSON through the manifest
+  // the app actually builds, plus the control proving the diagnostic still fires
+  // for a genuinely childless component — lives in
+  // `examples/schema-catalog/test/pageheader-with-actions.test.tsx`, next to the
+  // fixture it validates.
+  it.each([
+    ['page-header', undefined],
+    ['page-header', 'layout'],
+  ])('marks %s (namespace: %s) as a container', (type, namespace) => {
+    expect(ComponentRegistry.getConfig(type, namespace)?.isContainer).toBe(true);
+  });
+});
+
 describe('the runtime `description` fallback stays until the conversion entry lands', () => {
   // NOT an endorsement of the alias — a guard on the ORDER. Removing this read
   // before `page-header-subtitle-alias` exists is the deletion route that was
diff --git a/packages/layout/src/index.ts b/packages/layout/src/index.ts
index 09b9e19dd1..203489e40c 100644
--- a/packages/layout/src/index.ts
+++ b/packages/layout/src/index.ts
@@ -47,14 +47,34 @@ export function registerLayout() {
   // which lives in the framework repo. Narrowing the DECLARATION is
   // unconditional and independent of that: it breaks no consumer, and it stops
   // the registry from teaching the wrong key in the meantime.
+  //
+  // `isContainer: true` is the other half of the same principle, and it is NOT
+  // an extension of the spec's authoring surface (objectui#3900). `children` is
+  // a base property of EVERY node in objectui's JSON protocol — `BASE_PROPS` in
+  // `sdui-parser/src/validate.ts` lists it beside `type`/`id`/`className` — not
+  // a key of the spec's `PageHeaderProps`. So this flag answers the protocol
+  // question "does this node accept a child list?", and for this component the
+  // answer has always been yes: `PageHeader.tsx` deliberately re-introduces
+  // `schema.children` into the right-hand slot (that is how `record:quick_actions`
+  // nests under `page:header.children`), `content/docs/layout/page-header.mdx`
+  // publishes the slot's precedence as contract, and the docs page's only live
+  // demo is exactly that shape.
+  //
+  // Leaving the flag off did not make children illegal — nothing on the render
+  // path reads `isContainer` — it made the VALIDATOR lie: `sdui-parser`'s
+  // `not-a-container` diagnostic fired on the documented, demo-verified,
+  // correctly-rendering write-up. A warning that lies is worse than a missing
+  // one, because it trains authors (AI authors especially) to discount the true
+  // `not-a-container` reports on components that really are childless.
   ComponentRegistry.register('page-header', PageHeader, {
-      namespace: 'layout',
-      label: 'Page Header',
-      category: 'Layout',
-      inputs: [
-          { name: 'title', type: 'string', label: 'Title' },
-          { name: 'subtitle', type: 'string', label: 'Subtitle' }
-      ]
+    namespace: 'layout',
+    label: 'Page Header',
+    category: 'Layout',
+    isContainer: true,
+    inputs: [
+      { name: 'title', type: 'string', label: 'Title' },
+      { name: 'subtitle', type: 'string', label: 'Subtitle' },
+    ],
   });
 
   // Page Card — register ONLY as `layout:page:card`. `skipFallback` keeps this
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index f902fc198f..460ca528e1 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -642,6 +642,9 @@ importers:
       '@object-ui/react':
         specifier: workspace:*
         version: link:../../packages/react
+      '@object-ui/sdui-parser':
+        specifier: workspace:*
+        version: link:../../packages/sdui-parser
       typescript:
         specifier: ^6.0.3
         version: 6.0.3