diff --git a/.changeset/fix-doc-kit-1201.md b/.changeset/fix-doc-kit-1201.md
new file mode 100644
index 00000000..9d53c1b3
--- /dev/null
+++ b/.changeset/fix-doc-kit-1201.md
@@ -0,0 +1,8 @@
+---
+'@doc-kit/core': patch
+'@doc-kit/generator-react': patch
+---
+
+Fix custom HTML bundler configurations failing with DataCloneError
+during parallel page generation by excluding main-thread-only
+bundler functions from worker messages.
diff --git a/packages/core/src/generators/types.d.ts b/packages/core/src/generators/types.d.ts
index 8bee0399..c1100a9d 100644
--- a/packages/core/src/generators/types.d.ts
+++ b/packages/core/src/generators/types.d.ts
@@ -70,6 +70,9 @@ declare global {
 
     hasParallelProcessor?: boolean;
 
+    /** Configuration keys excluded from worker-thread messages. */
+    workerConfigOmit?: string[];
+
     /**
      * The unified plugins this generator processes Markdown with, listed as
      * in the `markdown` option, with paths relative to its module. Each list
diff --git a/packages/core/src/threading/__tests__/parallel.test.mjs b/packages/core/src/threading/__tests__/parallel.test.mjs
index 4a2c3653..029b6943 100644
--- a/packages/core/src/threading/__tests__/parallel.test.mjs
+++ b/packages/core/src/threading/__tests__/parallel.test.mjs
@@ -203,6 +203,51 @@ describe('createParallelWorker', () => {
     deepStrictEqual(await result, [0, 1, 2, 3, 4]);
   });
 
+  it('does not clone main-thread-only HTML bundler functions', async () => {
+    const htmlGenerator = await loadGenerator('html');
+    const transferred = [];
+    const pool = {
+      run(task) {
+        const copy = structuredClone(task);
+        transferred.push(copy);
+        return Promise.resolve(copy.input);
+      },
+    };
+
+    const bundler = {
+      buildServer: async () => 'file:///library.mjs',
+      buildClient: async () => ({}),
+      compile: async code => code,
+    };
+
+    const configuration = {
+      threads: 2,
+      chunkSize: 1,
+      html: {
+        title: 'Worker regression test',
+        bundler,
+      },
+    };
+
+    const worker = createParallelWorker(
+      'html',
+      htmlGenerator,
+      pool,
+      configuration
+    );
+
+    const results = await collectStream(worker.stream([1, 2, 3], {}));
+
+    deepStrictEqual(results, [1, 2, 3]);
+    strictEqual(transferred.length, 3);
+    for (const task of transferred) {
+      deepStrictEqual(task.configuration.html, {
+        title: 'Worker regression test',
+      });
+    }
+    strictEqual(configuration.html.bundler, bundler);
+  });
+
   it('propagates a later worker failure without an unhandled rejection', async () => {
     const tasks = [];
     const pool = {
diff --git a/packages/core/src/threading/parallel.mjs b/packages/core/src/threading/parallel.mjs
index 136bac69..641b9e11 100644
--- a/packages/core/src/threading/parallel.mjs
+++ b/packages/core/src/threading/parallel.mjs
@@ -1,6 +1,7 @@
 'use strict';
 
 import logger from '#logger/index.mjs';
+import { omitKeys } from '#utils/misc.mjs';
 
 const parallelLogger = logger.child('parallel');
 
@@ -73,7 +74,13 @@ export default function createParallelWorker(
 ) {
   const { threads, chunkSize } = configuration;
 
-  const { name } = generator;
+  const { name, workerConfigOmit } = generator;
+
+  // Main-thread-only options may contain functions that cannot be cloned.
+  const workerConfiguration = { ...configuration };
+  if (configuration[name]) {
+    workerConfiguration[name] = omitKeys(configuration[name], workerConfigOmit);
+  }
 
   return {
     /**
@@ -110,7 +117,14 @@ export default function createParallelWorker(
       // unhandled rejection while earlier chunks are still running.
       const tasks = chunks.map(indices => {
         const task = pool.run(
-          createTask(items, indices, extra, configuration, specifier, name)
+          createTask(
+            items,
+            indices,
+            extra,
+            workerConfiguration,
+            specifier,
+            name
+          )
         );
 
         return task.then(
diff --git a/packages/react/src/html/index.mjs b/packages/react/src/html/index.mjs
index 607592a3..c3e48c58 100644
--- a/packages/react/src/html/index.mjs
+++ b/packages/react/src/html/index.mjs
@@ -96,5 +96,8 @@ export default {
   // page count, so it is farmed out to the worker pool.
   hasParallelProcessor: true,
 
+  // The bundler runs on the main thread and contains non-cloneable functions.
+  workerConfigOmit: ['bundler'],
+
   processChunk,
 };