diff --git a/.changeset/article-ispartof-array.md b/.changeset/article-ispartof-array.md
new file mode 100644
index 0000000..0715cf6
--- /dev/null
+++ b/.changeset/article-ispartof-array.md
@@ -0,0 +1,14 @@
+---
+'@jdevalk/seo-graph-core': minor
+---
+
+**`buildArticle` now accepts an array for `isPartOf`.**
+
+`ArticleInput.isPartOf` was typed as a single `Reference`, but the shipped
+"Personal blog" recipe in `AGENTS.md` links a posting to both its `WebPage`
+and the `Blog` via `isPartOf: [{ '@id': webPage }, { '@id': blog }]`. The
+builder already emitted the value verbatim at runtime, so the array worked —
+but the type rejected it, forcing callers to add an `as` cast.
+
+The input type is now `Reference | Reference[]`. No runtime change; existing
+single-reference callers are unaffected.
diff --git a/.changeset/builders-return-graphentity.md b/.changeset/builders-return-graphentity.md
new file mode 100644
index 0000000..da9a9d4
--- /dev/null
+++ b/.changeset/builders-return-graphentity.md
@@ -0,0 +1,18 @@
+---
+'@jdevalk/seo-graph-core': minor
+---
+
+**Piece builders now return `GraphEntity` instead of `Record<string, unknown>`.**
+
+`assembleGraph<T extends GraphEntity>(pieces)` requires each piece to be a
+`GraphEntity`, whose `@type` is required. The builders (`buildWebSite`,
+`buildWebPage`, `buildArticle`, `buildBreadcrumbList`, `buildImageObject`,
+`buildVideoObject`, `buildSiteNavigationElement`, `buildPiece`) were declared
+to return `Record<string, unknown>`, which lacks `@type` — so the documented
+pattern `assembleGraph([buildWebSite(...), buildArticle(...)])` failed `tsc` /
+`astro check` under strict mode and forced callers to cast `as GraphEntity[]`.
+
+Every builder already produces an object with a literal `@type`, so the return
+type is widened to `GraphEntity` with no runtime change. Builder results — and
+the arrays returned by `aggregate`/`createSchemaEndpoint` mappers — now compose
+with `assembleGraph` without a cast.
diff --git a/packages/seo-graph-core/src/pieces/article.ts b/packages/seo-graph-core/src/pieces/article.ts
index e60820e..0d2c823 100644
--- a/packages/seo-graph-core/src/pieces/article.ts
+++ b/packages/seo-graph-core/src/pieces/article.ts
@@ -1,7 +1,7 @@
 import type { ArticleLeaf } from 'schema-dts';
 
 import type { IdFactory } from '../ids.js';
-import type { Reference, CreativeWorkFields } from '../types.js';
+import type { Reference, CreativeWorkFields, GraphEntity } from '../types.js';
 import {
     applyCreativeWorkFields,
     spreadRemainingProperties,
@@ -25,8 +25,13 @@ export type ArticleType =
 interface ArticleCoreFields extends CreativeWorkFields {
     /** Canonical URL of the article's page. The @id is `${url}#article`. */
     url: string;
-    /** Reference to the enclosing WebPage (usually ids.webPage(url)). */
-    isPartOf: Reference;
+    /**
+     * Reference to the enclosing entity, usually the WebPage
+     * (`ids.webPage(url)`). Pass an array to link the Article to more than
+     * one parent — e.g. both its WebPage and a Blog
+     * (`[{ '@id': ids.webPage(url) }, { '@id': blogId }]`).
+     */
+    isPartOf: Reference | Reference[];
     /** Author reference. May include a `name` alongside the `@id`. */
     author: Reference;
     /** Publisher reference. Usually the same as the author for personal blogs. */
@@ -67,8 +72,8 @@ export function buildArticle(
     input: ArticleInput,
     ids: IdFactory,
     type: ArticleType = 'Article',
-): Record<string, unknown> {
-    const piece: Record<string, unknown> = {
+): GraphEntity {
+    const piece: GraphEntity = {
         '@type': type,
         '@id': ids.article(input.url),
         isPartOf: input.isPartOf,
diff --git a/packages/seo-graph-core/src/pieces/breadcrumb.ts b/packages/seo-graph-core/src/pieces/breadcrumb.ts
index a7fa01f..bba4c60 100644
--- a/packages/seo-graph-core/src/pieces/breadcrumb.ts
+++ b/packages/seo-graph-core/src/pieces/breadcrumb.ts
@@ -1,6 +1,7 @@
 import type { BreadcrumbListLeaf } from 'schema-dts';
 
 import type { IdFactory } from '../ids.js';
+import type { GraphEntity } from '../types.js';
 import { spreadRemainingProperties } from '../types.js';
 
 export interface BreadcrumbItem {
@@ -27,10 +28,7 @@ const HANDLED_KEYS = new Set<string>(['url', 'items']);
 /**
  * Build a schema.org BreadcrumbList piece.
  */
-export function buildBreadcrumbList(
-    input: BreadcrumbListInput,
-    ids: IdFactory,
-): Record<string, unknown> {
+export function buildBreadcrumbList(input: BreadcrumbListInput, ids: IdFactory): GraphEntity {
     const lastIndex = input.items.length - 1;
     const itemListElement = input.items.map((item, index) => ({
         '@type': 'ListItem',
@@ -43,7 +41,7 @@ export function buildBreadcrumbList(
               : item.url,
     }));
 
-    const piece: Record<string, unknown> = {
+    const piece: GraphEntity = {
         '@type': 'BreadcrumbList',
         '@id': ids.breadcrumb(input.url),
         itemListElement,
diff --git a/packages/seo-graph-core/src/pieces/custom.ts b/packages/seo-graph-core/src/pieces/custom.ts
index 545efec..7e72def 100644
--- a/packages/seo-graph-core/src/pieces/custom.ts
+++ b/packages/seo-graph-core/src/pieces/custom.ts
@@ -1,5 +1,7 @@
 import type { Thing } from 'schema-dts';
 
+import type { GraphEntity } from '../types.js';
+
 /**
  * Build an arbitrary schema.org piece from a raw object.
  *
@@ -26,13 +28,15 @@ export function buildPiece<T extends Thing, TType extends string = string>(
         '@type': TType;
         '@id'?: string;
     },
-): Record<string, unknown>;
+): GraphEntity;
 export function buildPiece(
     raw: Record<string, unknown> & {
         '@type': string | readonly string[];
         '@id'?: string;
     },
-): Record<string, unknown>;
-export function buildPiece(raw: Record<string, unknown>): Record<string, unknown> {
-    return raw;
+): GraphEntity;
+export function buildPiece(raw: Record<string, unknown>): GraphEntity {
+    // The public overloads both require `@type` on `raw`, so the result
+    // always satisfies GraphEntity; the implementation signature is wider.
+    return raw as GraphEntity;
 }
diff --git a/packages/seo-graph-core/src/pieces/image.ts b/packages/seo-graph-core/src/pieces/image.ts
index 092ee09..92b5abf 100644
--- a/packages/seo-graph-core/src/pieces/image.ts
+++ b/packages/seo-graph-core/src/pieces/image.ts
@@ -1,6 +1,7 @@
 import type { ImageObjectLeaf } from 'schema-dts';
 
 import type { IdFactory } from '../ids.js';
+import type { GraphEntity } from '../types.js';
 import { spreadRemainingProperties } from '../types.js';
 
 interface ImageObjectCoreFields {
@@ -32,14 +33,14 @@ const HANDLED_KEYS = new Set<string>([
  * primary image (id = `${pageUrl}#primaryimage`), or `id` for a site-
  * wide image like a personal logo.
  */
-export function buildImageObject(input: ImageObjectInput, ids: IdFactory): Record<string, unknown> {
+export function buildImageObject(input: ImageObjectInput, ids: IdFactory): GraphEntity {
     const resolvedId =
         input.id ?? (input.pageUrl !== undefined ? ids.primaryImage(input.pageUrl) : undefined);
     if (resolvedId === undefined) {
         throw new Error('buildImageObject: either `id` or `pageUrl` is required');
     }
 
-    const piece: Record<string, unknown> = {
+    const piece: GraphEntity = {
         '@type': 'ImageObject',
         '@id': resolvedId,
         url: input.url,
diff --git a/packages/seo-graph-core/src/pieces/navigation.ts b/packages/seo-graph-core/src/pieces/navigation.ts
index f7cab34..d9ed558 100644
--- a/packages/seo-graph-core/src/pieces/navigation.ts
+++ b/packages/seo-graph-core/src/pieces/navigation.ts
@@ -1,7 +1,7 @@
 import type { SiteNavigationElementLeaf } from 'schema-dts';
 
 import type { IdFactory } from '../ids.js';
-import type { Reference } from '../types.js';
+import type { Reference, GraphEntity } from '../types.js';
 import { spreadRemainingProperties } from '../types.js';
 
 export interface NavigationItem {
@@ -27,14 +27,14 @@ const HANDLED_KEYS = new Set<string>(['name', 'isPartOf', 'items']);
 export function buildSiteNavigationElement(
     input: SiteNavigationInput,
     ids: IdFactory,
-): Record<string, unknown> {
+): GraphEntity {
     const hasPart = input.items.map((item) => ({
         '@type': 'SiteNavigationElement',
         name: item.name,
         url: item.url,
     }));
 
-    const piece: Record<string, unknown> = {
+    const piece: GraphEntity = {
         '@type': 'SiteNavigationElement',
         '@id': ids.navigation,
         name: input.name,
diff --git a/packages/seo-graph-core/src/pieces/video.ts b/packages/seo-graph-core/src/pieces/video.ts
index d3840ad..b5eb92f 100644
--- a/packages/seo-graph-core/src/pieces/video.ts
+++ b/packages/seo-graph-core/src/pieces/video.ts
@@ -1,7 +1,7 @@
 import type { VideoObjectLeaf } from 'schema-dts';
 
 import type { IdFactory } from '../ids.js';
-import type { Reference } from '../types.js';
+import type { Reference, GraphEntity } from '../types.js';
 import { spreadRemainingProperties } from '../types.js';
 
 interface VideoObjectCoreFields {
@@ -36,8 +36,8 @@ const HANDLED_KEYS = new Set<string>([
 /**
  * Build a schema.org VideoObject piece.
  */
-export function buildVideoObject(input: VideoObjectInput, ids: IdFactory): Record<string, unknown> {
-    const piece: Record<string, unknown> = {
+export function buildVideoObject(input: VideoObjectInput, ids: IdFactory): GraphEntity {
+    const piece: GraphEntity = {
         '@type': 'VideoObject',
         '@id': ids.videoObject(input.url),
         name: input.name,
diff --git a/packages/seo-graph-core/src/pieces/webpage.ts b/packages/seo-graph-core/src/pieces/webpage.ts
index a9ce6cb..07178a4 100644
--- a/packages/seo-graph-core/src/pieces/webpage.ts
+++ b/packages/seo-graph-core/src/pieces/webpage.ts
@@ -1,7 +1,7 @@
 import type { WebPageLeaf } from 'schema-dts';
 
 import type { IdFactory } from '../ids.js';
-import type { Reference, CreativeWorkFields } from '../types.js';
+import type { Reference, CreativeWorkFields, GraphEntity } from '../types.js';
 import {
     applyCreativeWorkFields,
     spreadRemainingProperties,
@@ -52,12 +52,12 @@ export function buildWebPage(
     input: WebPageInput,
     ids: IdFactory,
     type: WebPageType = 'WebPage',
-): Record<string, unknown> {
+): GraphEntity {
     const potentialAction: ReadonlyArray<Record<string, unknown>> = input.potentialAction ?? [
         { '@type': 'ReadAction', target: [input.url] },
     ];
 
-    const piece: Record<string, unknown> = {
+    const piece: GraphEntity = {
         '@type': type,
         '@id': ids.webPage(input.url),
         url: input.url,
diff --git a/packages/seo-graph-core/src/pieces/website.ts b/packages/seo-graph-core/src/pieces/website.ts
index 2691cbc..f9f9123 100644
--- a/packages/seo-graph-core/src/pieces/website.ts
+++ b/packages/seo-graph-core/src/pieces/website.ts
@@ -1,7 +1,7 @@
 import type { WebSiteLeaf } from 'schema-dts';
 
 import type { IdFactory } from '../ids.js';
-import type { Reference, CreativeWorkFields } from '../types.js';
+import type { Reference, CreativeWorkFields, GraphEntity } from '../types.js';
 import {
     applyCreativeWorkFields,
     spreadRemainingProperties,
@@ -33,8 +33,8 @@ const HANDLED_KEYS = new Set<string>([
  * Build a schema.org WebSite piece. This is the site-wide singleton;
  * every page's WebPage should reference it via `isPartOf`.
  */
-export function buildWebSite(input: WebSiteInput, ids: IdFactory): Record<string, unknown> {
-    const piece: Record<string, unknown> = {
+export function buildWebSite(input: WebSiteInput, ids: IdFactory): GraphEntity {
+    const piece: GraphEntity = {
         '@type': 'WebSite',
         '@id': ids.website,
         url: input.url,
diff --git a/packages/seo-graph-core/test/pieces.test.ts b/packages/seo-graph-core/test/pieces.test.ts
index cd2727c..f2354c9 100644
--- a/packages/seo-graph-core/test/pieces.test.ts
+++ b/packages/seo-graph-core/test/pieces.test.ts
@@ -117,6 +117,36 @@ describe('assembleGraph', () => {
         expect(warn).not.toHaveBeenCalled();
         warn.mockRestore();
     });
+
+    it('composes builder outputs directly, with no cast', () => {
+        // Type-level regression guard: builders return `GraphEntity`, so
+        // their results flow straight into `assembleGraph<T extends
+        // GraphEntity>` without an `as` cast. When the builders returned
+        // `Record<string, unknown>` this file failed `tsc` (the missing
+        // `@type` made the array incompatible with the constraint).
+        const graph = assembleGraph([
+            buildWebSite({ url: siteUrl, name: 'Example', publisher: { '@id': ids.person } }, ids),
+            buildWebPage({ url: postUrl, name: 'My Post', isPartOf: { '@id': ids.website } }, ids),
+            buildArticle(
+                {
+                    url: postUrl,
+                    isPartOf: { '@id': ids.webPage(postUrl) },
+                    author: { '@id': ids.person },
+                    publisher: { '@id': ids.person },
+                    headline: 'My Post',
+                    description: 'desc',
+                    datePublished: new Date('2026-01-01T00:00:00.000Z'),
+                },
+                ids,
+            ),
+        ]);
+        expect(graph['@graph']).toHaveLength(3);
+        expect(graph['@graph'].map((entity) => entity['@type'])).toEqual([
+            'WebSite',
+            'WebPage',
+            'Article',
+        ]);
+    });
 });
 
 describe('buildWebSite', () => {
@@ -398,6 +428,24 @@ describe('buildArticle', () => {
         expect(article.wordCount).toBe(100);
         expect(article.articleBody).toBe('Hello world');
     });
+
+    it('accepts an array of isPartOf references and emits them verbatim', () => {
+        const blogId = `${siteUrl}#blog`;
+        const isPartOf = [{ '@id': ids.webPage(postUrl) }, { '@id': blogId }];
+        const article = buildArticle(
+            {
+                url: postUrl,
+                isPartOf,
+                author: { '@id': ids.person },
+                publisher: { '@id': ids.person },
+                headline: 'Hello',
+                description: 'World',
+                datePublished: new Date('2026-01-01T00:00:00.000Z'),
+            },
+            ids,
+        );
+        expect(article.isPartOf).toEqual(isPartOf);
+    });
 });
 
 describe('buildBreadcrumbList', () => {