4 files changed, 65 insertions, 27 deletions
diff --git a/packages/integrations/markdoc/src/index.ts b/packages/integrations/markdoc/src/index.ts
index 4360800a0..627f08c77 100644
--- a/packages/integrations/markdoc/src/index.ts
+++ b/packages/integrations/markdoc/src/index.ts
@@ -10,7 +10,7 @@ import { emitESMImage } from 'astro/assets';
 import { bold, red, yellow } from 'kleur/colors';
 import type * as rollup from 'rollup';
 import { loadMarkdocConfig, type MarkdocConfigResult } from './load-config.js';
-import { applyDefaultConfig } from './runtime.js';
+import { setupConfig } from './runtime.js';
 
 type SetupHookParams = HookParameters<'astro:config:setup'> & {
 	// `contentEntryType` is not a public API
@@ -52,7 +52,7 @@ export default function markdocIntegration(legacyConfig?: any): AstroIntegration
 					async getRenderModule({ entry, viteId }) {
 						const ast = Markdoc.parse(entry.body);
 						const pluginContext = this;
-						const markdocConfig = applyDefaultConfig(userMarkdocConfig, entry);
+						const markdocConfig = setupConfig(userMarkdocConfig, entry);
 
 						const validationErrors = Markdoc.validate(ast, markdocConfig).filter((e) => {
 							return (
@@ -90,7 +90,7 @@ export default function markdocIntegration(legacyConfig?: any): AstroIntegration
 
 						const res = `import { jsx as h } from 'astro/jsx-runtime';
 						import { Renderer } from '@astrojs/markdoc/components';
-						import { collectHeadings, applyDefaultConfig, Markdoc, headingSlugger } from '@astrojs/markdoc/runtime';
+						import { collectHeadings, setupConfig, Markdoc } from '@astrojs/markdoc/runtime';
 import * as entry from ${JSON.stringify(viteId + '?astroContentCollectionEntry')};
 ${
 	markdocConfigResult
@@ -113,16 +113,14 @@ export function getHeadings() {
 		instead of the Content component. Would remove double-transform and unlock variable resolution in heading slugs. */
 		''
 	}
-	headingSlugger.reset();
 	const headingConfig = userConfig.nodes?.heading;
-	const config = applyDefaultConfig(headingConfig ? { nodes: { heading: headingConfig } } : {}, entry);
+	const config = setupConfig(headingConfig ? { nodes: { heading: headingConfig } } : {}, entry);
 	const ast = Markdoc.Ast.fromJSON(stringifiedAst);
 	const content = Markdoc.transform(ast, config);
 	return collectHeadings(Array.isArray(content) ? content : content.children);
 }
 export async function Content (props) {
-	headingSlugger.reset();
-	const config = applyDefaultConfig({
+	const config = setupConfig({
 		...userConfig,
 		variables: { ...userConfig.variables, ...props },
 	}, entry);
diff --git a/packages/integrations/markdoc/src/nodes/heading.ts b/packages/integrations/markdoc/src/nodes/heading.ts
index 8adf57612..19a988b63 100644
--- a/packages/integrations/markdoc/src/nodes/heading.ts
+++ b/packages/integrations/markdoc/src/nodes/heading.ts
@@ -1,10 +1,19 @@
-import Markdoc, { type RenderableTreeNode, type Schema } from '@markdoc/markdoc';
+import Markdoc, { type RenderableTreeNode, type Schema, type ConfigType } from '@markdoc/markdoc';
 import Slugger from 'github-slugger';
 import { getTextContent } from '../runtime.js';
 
-export const headingSlugger = new Slugger();
+type ConfigTypeWithCtx = ConfigType & {
+	// TODO: decide on `ctx` as a convention for config merging
+	ctx: {
+		headingSlugger: Slugger;
+	};
+};
 
-function getSlug(attributes: Record<string, any>, children: RenderableTreeNode[]): string {
+function getSlug(
+	attributes: Record<string, any>,
+	children: RenderableTreeNode[],
+	headingSlugger: Slugger
+): string {
 	if (attributes.id && typeof attributes.id === 'string') {
 		return attributes.id;
 	}
@@ -21,11 +30,11 @@ export const heading: Schema = {
 		id: { type: String },
 		level: { type: Number, required: true, default: 1 },
 	},
-	transform(node, config) {
+	transform(node, config: ConfigTypeWithCtx) {
 		const { level, ...attributes } = node.transformAttributes(config);
 		const children = node.transformChildren(config);
 
-		const slug = getSlug(attributes, children);
+		const slug = getSlug(attributes, children, config.ctx.headingSlugger);
 
 		const render = config.nodes?.heading?.render ?? `h${level}`;
 		const tagProps =
@@ -39,3 +48,16 @@ export const heading: Schema = {
 		return new Markdoc.Tag(render, tagProps, children);
 	},
 };
+
+export function setupHeadingConfig(): ConfigTypeWithCtx {
+	const headingSlugger = new Slugger();
+
+	return {
+		ctx: {
+			headingSlugger,
+		},
+		nodes: {
+			heading,
+		},
+	};
+}
diff --git a/packages/integrations/markdoc/src/nodes/index.ts b/packages/integrations/markdoc/src/nodes/index.ts
index c25b03f27..4cd7e3667 100644
--- a/packages/integrations/markdoc/src/nodes/index.ts
+++ b/packages/integrations/markdoc/src/nodes/index.ts
@@ -1,4 +1,4 @@
 import { heading } from './heading.js';
-export { headingSlugger } from './heading.js';
+export { setupHeadingConfig } from './heading.js';
 
 export const nodes = { heading };
diff --git a/packages/integrations/markdoc/src/runtime.ts b/packages/integrations/markdoc/src/runtime.ts
index 61b38fd02..3164cda13 100644
--- a/packages/integrations/markdoc/src/runtime.ts
+++ b/packages/integrations/markdoc/src/runtime.ts
@@ -4,27 +4,45 @@ import Markdoc, {
 	type RenderableTreeNode,
 } from '@markdoc/markdoc';
 import type { ContentEntryModule } from 'astro';
-import { nodes as astroNodes } from './nodes/index.js';
+import { setupHeadingConfig } from './nodes/index.js';
 
-/** Used to reset Slugger cache on each build at runtime */
+/** Used to call `Markdoc.transform()` and `Markdoc.Ast` in runtime modules */
 export { default as Markdoc } from '@markdoc/markdoc';
-export { headingSlugger } from './nodes/index.js';
 
-export function applyDefaultConfig(
-	config: MarkdocConfig,
-	entry: ContentEntryModule
-): MarkdocConfig {
+/**
+ * Merge user config with default config and set up context (ex. heading ID slugger)
+ * Called on each file's individual transform
+ */
+export function setupConfig(userConfig: MarkdocConfig, entry: ContentEntryModule): MarkdocConfig {
+	const defaultConfig: MarkdocConfig = {
+		// `setupXConfig()` could become a "plugin" convention as well?
+		...setupHeadingConfig(),
+		variables: { entry },
+	};
+	return mergeConfig(defaultConfig, userConfig);
+}
+
+/** Merge function from `@markdoc/markdoc` internals */
+function mergeConfig(configA: MarkdocConfig, configB: MarkdocConfig): MarkdocConfig {
 	return {
-		...config,
-		variables: {
-			entry,
-			...config.variables,
+		...configA,
+		...configB,
+		tags: {
+			...configA.tags,
+			...configB.tags,
 		},
 		nodes: {
-			...astroNodes,
-			...config.nodes,
+			...configA.nodes,
+			...configB.nodes,
+		},
+		functions: {
+			...configA.functions,
+			...configB.functions,
+		},
+		variables: {
+			...configA.variables,
+			...configB.variables,
 		},
-		// TODO: Syntax highlighting
 	};
 }