5 files changed, 55 insertions, 11 deletions
diff --git a/packages/markdown/remark/src/frontmatter-injection.ts b/packages/markdown/remark/src/frontmatter-injection.ts
new file mode 100644
index 000000000..921d01297
--- /dev/null
+++ b/packages/markdown/remark/src/frontmatter-injection.ts
@@ -0,0 +1,41 @@
+import type { Data, VFile } from 'vfile';
+import type { MarkdownAstroData } from './types.js';
+
+function isValidAstroData(obj: unknown): obj is MarkdownAstroData {
+	if (typeof obj === 'object' && obj !== null && obj.hasOwnProperty('frontmatter')) {
+		const { frontmatter } = obj as any;
+		try {
+			// ensure frontmatter is JSON-serializable
+			JSON.stringify(frontmatter);
+		} catch {
+			return false;
+		}
+		return typeof frontmatter === 'object' && frontmatter !== null;
+	}
+	return false;
+}
+
+export class InvalidAstroDataError extends TypeError {}
+
+export function safelyGetAstroData(vfileData: Data): MarkdownAstroData | InvalidAstroDataError {
+	const { astro } = vfileData;
+
+	if (!astro || !isValidAstroData(astro)) {
+		return new InvalidAstroDataError();
+	}
+
+	return astro;
+}
+
+export function toRemarkInitializeAstroData({
+	userFrontmatter,
+}: {
+	userFrontmatter: Record<string, any>;
+}) {
+	return () =>
+		function (tree: any, vfile: VFile) {
+			if (!vfile.data.astro) {
+				vfile.data.astro = { frontmatter: userFrontmatter };
+			}
+		};
+}
diff --git a/packages/markdown/remark/src/index.ts b/packages/markdown/remark/src/index.ts
index 66889108e..480cc9e38 100644
--- a/packages/markdown/remark/src/index.ts
+++ b/packages/markdown/remark/src/index.ts
@@ -8,7 +8,7 @@ import rehypeIslands from './rehype-islands.js';
 import rehypeJsx from './rehype-jsx.js';
 import toRemarkContentRelImageError from './remark-content-rel-image-error.js';
 import remarkEscape from './remark-escape.js';
-import { remarkInitializeAstroData } from './remark-initialize-astro-data.js';
+import { toRemarkInitializeAstroData } from './frontmatter-injection.js';
 import remarkMarkAndUnravel from './remark-mark-and-unravel.js';
 import remarkMdxish from './remark-mdxish.js';
 import remarkPrism from './remark-prism.js';
@@ -45,13 +45,14 @@ export async function renderMarkdown(
 		isAstroFlavoredMd = false,
 		isExperimentalContentCollections = false,
 		contentDir,
+		frontmatter: userFrontmatter = {},
 	} = opts;
 	const input = new VFile({ value: content, path: fileURL });
 	const scopedClassName = opts.$?.scopedClassName;
 
 	let parser = unified()
 		.use(markdown)
-		.use(remarkInitializeAstroData)
+		.use(toRemarkInitializeAstroData({ userFrontmatter }))
 		.use(isAstroFlavoredMd ? [remarkMdxish, remarkMarkAndUnravel, remarkUnwrap, remarkEscape] : []);
 
 	if (extendDefaultPlugins || (remarkPlugins.length === 0 && rehypePlugins.length === 0)) {
diff --git a/packages/markdown/remark/src/internal.ts b/packages/markdown/remark/src/internal.ts
new file mode 100644
index 000000000..0ab7e34bb
--- /dev/null
+++ b/packages/markdown/remark/src/internal.ts
@@ -0,0 +1,5 @@
+export {
+	InvalidAstroDataError,
+	safelyGetAstroData,
+	toRemarkInitializeAstroData,
+} from './frontmatter-injection.js';
diff --git a/packages/markdown/remark/src/remark-initialize-astro-data.ts b/packages/markdown/remark/src/remark-initialize-astro-data.ts
deleted file mode 100644
index 37af8aeaf..000000000
--- a/packages/markdown/remark/src/remark-initialize-astro-data.ts
+++ /dev/null
@@ -1,9 +0,0 @@
-import type { VFile } from 'vfile';
-
-export function remarkInitializeAstroData() {
-	return function (tree: any, vfile: VFile) {
-		if (!vfile.data.astro) {
-			vfile.data.astro = { frontmatter: {} };
-		}
-	};
-}
diff --git a/packages/markdown/remark/src/types.ts b/packages/markdown/remark/src/types.ts
index 15465d950..f52fd4bf5 100644
--- a/packages/markdown/remark/src/types.ts
+++ b/packages/markdown/remark/src/types.ts
@@ -11,6 +11,10 @@ import type { VFile } from 'vfile';
 
 export type { Node } from 'unist';
 
+export type MarkdownAstroData = {
+	frontmatter: Record<string, any>;
+};
+
 export type RemarkPlugin<PluginParameters extends any[] = any[]> = unified.Plugin<
 	PluginParameters,
 	mdast.Root
@@ -58,6 +62,8 @@ export interface MarkdownRenderingOptions extends AstroMarkdownOptions {
 	isExperimentalContentCollections?: boolean;
 	/** Used to prevent relative image imports from `src/content/` */
 	contentDir: URL;
+	/** Used for frontmatter injection plugins */
+	frontmatter?: Record<string, any>;
 }
 
 export interface MarkdownHeading {