fix(seo-graph-core): accept Reference array for buildArticle isPartOf

.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.

packages/seo-graph-core/src/pieces/article.ts

@@ -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. */

packages/seo-graph-core/test/pieces.test.ts

@@ -398,6 +398,24 @@ 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', () => {