diff --git a/src/web-ui/src/flow_chat/timeline/highlight.worker.test.ts b/src/web-ui/src/flow_chat/timeline/highlight.worker.test.ts
new file mode 100644
index 0000000000..5f14c8f9f1
--- /dev/null
+++ b/src/web-ui/src/flow_chat/timeline/highlight.worker.test.ts
@@ -0,0 +1,63 @@
+// @vitest-environment node
+import { fileURLToPath } from 'node:url';
+import { runInNewContext } from 'node:vm';
+import { build } from 'vite';
+import { expect, it } from 'vitest';
+
+const root = fileURLToPath(new URL('../../../', import.meta.url));
+const entry = fileURLToPath(new URL('./highlight.worker.ts', import.meta.url));
+
+type WorkerSandbox = {
+  self: WorkerSandbox;
+  Prism?: { disableWorkerMessageHandler?: boolean };
+  onmessage?: (event: { data: unknown }) => void;
+  postMessage: (message: unknown) => void;
+  addEventListener: (type: string, listener: (event: { data: unknown }) => void) => void;
+};
+
+it('uses the app worker protocol without Prism installing its JSON worker handler', async () => {
+  const result = await build({
+    root,
+    configFile: false,
+    publicDir: false,
+    logLevel: 'silent',
+    build: {
+      write: false,
+      minify: true,
+      rollupOptions: { input: entry, preserveEntrySignatures: 'strict' },
+    },
+  });
+  if ('on' in result) throw new Error('Unexpected watch build');
+
+  const output = (Array.isArray(result) ? result : [result]).flatMap(bundle => bundle.output);
+  const worker = output.find(asset => asset.type === 'chunk' && /highlight\.worker-.*\.js$/.test(asset.fileName));
+  if (!worker || worker.type !== 'chunk') throw new Error('Highlight worker asset was not emitted');
+
+  const replies: unknown[] = [];
+  const listeners: Array<(event: { data: unknown }) => void> = [];
+  const sandbox = {
+    postMessage: (message: unknown) => replies.push(message),
+    addEventListener: (type: string, listener: (event: { data: unknown }) => void) => {
+      if (type === 'message') listeners.push(listener);
+    },
+  } as unknown as WorkerSandbox;
+  runInNewContext(
+    'class WorkerGlobalScope {}; globalThis.WorkerGlobalScope = WorkerGlobalScope; globalThis.self = globalThis; Object.setPrototypeOf(globalThis, WorkerGlobalScope.prototype);',
+    sandbox,
+  );
+
+  runInNewContext(worker.code, sandbox, { timeout: 2000 });
+
+  expect(sandbox.Prism?.disableWorkerMessageHandler).toBe(true);
+  expect(listeners).toHaveLength(0);
+  expect(sandbox.onmessage).toBeTypeOf('function');
+
+  const message = { id: 7, content: 'const answer: number = 42;', language: 'typescript' };
+  expect(() => {
+    for (const listener of listeners) listener({ data: message });
+    sandbox.onmessage!({ data: message });
+  }).not.toThrow();
+
+  expect(replies).toHaveLength(1);
+  expect(replies[0]).toMatchObject({ id: 7 });
+}, 20_000);
diff --git a/src/web-ui/src/flow_chat/timeline/highlight.worker.ts b/src/web-ui/src/flow_chat/timeline/highlight.worker.ts
index fb141718e2..2a6cd1bd59 100644
--- a/src/web-ui/src/flow_chat/timeline/highlight.worker.ts
+++ b/src/web-ui/src/flow_chat/timeline/highlight.worker.ts
@@ -1,3 +1,4 @@
+import './prismWorkerSetup';
 import Prism from 'prismjs';
 import 'prismjs/components/prism-typescript';
 import 'prismjs/components/prism-jsx';
diff --git a/src/web-ui/src/flow_chat/timeline/prismWorkerSetup.ts b/src/web-ui/src/flow_chat/timeline/prismWorkerSetup.ts
new file mode 100644
index 0000000000..135e384f95
--- /dev/null
+++ b/src/web-ui/src/flow_chat/timeline/prismWorkerSetup.ts
@@ -0,0 +1,12 @@
+/**
+ * Prism installs its own JSON-based worker protocol when it detects a worker.
+ * This worker uses the app's structured message protocol instead.
+ */
+const workerGlobal = globalThis as typeof globalThis & {
+  Prism?: { disableWorkerMessageHandler?: boolean };
+};
+
+workerGlobal.Prism = {
+  ...(workerGlobal.Prism ?? {}),
+  disableWorkerMessageHandler: true,
+};