From fbbf343286dd24fa34d112f36bdf755d25f09a46 Mon Sep 17 00:00:00 2001
From: Marco Beretta <81851188+berry-13@users.noreply.github.com>
Date: Tue, 10 Nov 2026 13:21:24 +0100
Subject: [PATCH] fix: Keep Untagged Chat Code Fences Plain (#17986)

---
 .../Content/MarkdownErrorBoundary.tsx         |   2 +-
 .../Chat/Messages/Content/MarkdownLite.tsx    |   2 +-
 .../__tests__/MarkdownHighlight.test.tsx      | 108 ++++++++++++++++++
 .../Chat/Messages/Content/markdownConfig.ts   |   2 +-
 4 files changed, 111 insertions(+), 3 deletions(-)
 create mode 100644 client/src/components/Chat/Messages/Content/__tests__/MarkdownHighlight.test.tsx

diff --git a/client/src/components/Chat/Messages/Content/MarkdownErrorBoundary.tsx b/client/src/components/Chat/Messages/Content/MarkdownErrorBoundary.tsx
index 3d65a2ae14..02d9626e0d 100644
--- a/client/src/components/Chat/Messages/Content/MarkdownErrorBoundary.tsx
+++ b/client/src/components/Chat/Messages/Content/MarkdownErrorBoundary.tsx
@@ -68,7 +68,7 @@ class MarkdownErrorBoundary extends React.Component<
         [
           rehypeHighlight,
           {
-            detect: true,
+            detect: false,
             ignoreMissing: true,
             subset: langSubset,
           },
diff --git a/client/src/components/Chat/Messages/Content/MarkdownLite.tsx b/client/src/components/Chat/Messages/Content/MarkdownLite.tsx
index 54bd0eab31..4ff28dcd63 100644
--- a/client/src/components/Chat/Messages/Content/MarkdownLite.tsx
+++ b/client/src/components/Chat/Messages/Content/MarkdownLite.tsx
@@ -34,7 +34,7 @@ const MarkdownLite = memo(
       [
         rehypeHighlight,
         {
-          detect: true,
+          detect: false,
           ignoreMissing: true,
           subset: langSubset,
         },
diff --git a/client/src/components/Chat/Messages/Content/__tests__/MarkdownHighlight.test.tsx b/client/src/components/Chat/Messages/Content/__tests__/MarkdownHighlight.test.tsx
new file mode 100644
index 0000000000..6405b8f106
--- /dev/null
+++ b/client/src/components/Chat/Messages/Content/__tests__/MarkdownHighlight.test.tsx
@@ -0,0 +1,108 @@
+import React from 'react';
+import { RecoilRoot } from 'recoil';
+import ReactMarkdown from 'react-markdown';
+import { render } from '@testing-library/react';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import { getRehypePlugins, getMarkdownComponents } from '../markdownConfig';
+import { ArtifactProvider, CodeBlockProvider } from '~/Providers';
+import MarkdownErrorBoundary from '../MarkdownErrorBoundary';
+import MarkdownLite from '../MarkdownLite';
+import Markdown from '../Markdown';
+
+jest.mock('~/components/Messages/Content/CodeBlock', () => ({
+  __esModule: true,
+  default: ({ lang, codeChildren }: { lang: string; codeChildren: React.ReactNode }) => (
+    <code data-language={lang}>{codeChildren}</code>
+  ),
+}));
+
+const WholeMessage = ({ content }: { content: string }) => (
+  <ArtifactProvider>
+    <CodeBlockProvider>
+      <ReactMarkdown
+        rehypePlugins={
+          getRehypePlugins() as React.ComponentProps<typeof ReactMarkdown>['rehypePlugins']
+        }
+        components={getMarkdownComponents()}
+      >
+        {content}
+      </ReactMarkdown>
+    </CodeBlockProvider>
+  </ArtifactProvider>
+);
+
+const Throw = (): never => {
+  throw new Error('Exercise markdown fallback');
+};
+
+const Fallback = ({
+  content,
+  codeExecution = true,
+}: {
+  content: string;
+  codeExecution?: boolean;
+}) => (
+  <MarkdownErrorBoundary content={content} codeExecution={codeExecution}>
+    <Throw />
+  </MarkdownErrorBoundary>
+);
+
+const renderers = [
+  ['whole message', WholeMessage],
+  ['blocks', (props: { content: string }) => <Markdown {...props} isLatestMessage={false} />],
+  ['lite', MarkdownLite],
+  [
+    'lite without execution',
+    (props: { content: string }) => <MarkdownLite {...props} codeExecution={false} />,
+  ],
+  ['fallback', Fallback],
+  [
+    'fallback without execution',
+    (props: { content: string }) => <Fallback {...props} codeExecution={false} />,
+  ],
+] as const;
+
+const prose = 'Pick up milk, eggs and bread on the way home. Call the dentist: Tuesday at 3.';
+
+describe.each(renderers)('%s highlighting', (_name, Renderer) => {
+  beforeEach(() => {
+    jest.spyOn(console, 'error').mockImplementation(() => {});
+  });
+
+  const renderContent = (content: string) => {
+    const { container } = render(
+      <QueryClientProvider client={new QueryClient()}>
+        <RecoilRoot>
+          <Renderer content={content} />
+        </RecoilRoot>
+      </QueryClientProvider>,
+    );
+    return container.querySelector('code');
+  };
+
+  it.each([
+    ['', prose, true],
+    ['', prose, false],
+    ['', 'const answer = 42;', true],
+    ['text', prose, true],
+    ['plaintext', prose, true],
+    ['unknownlang', prose, true],
+  ])('keeps %s fences plain for %s (closed: %s)', (language, content, closed) => {
+    const code = renderContent(`\`\`\`${language}\n${content}\n${closed ? '```' : ''}`);
+    expect(code).toHaveAttribute('data-language', language || 'text');
+    expect(code?.textContent).toBe(`${content}\n`);
+    expect(code?.querySelector('[class*="hljs-"]')).toBeNull();
+  });
+
+  it('retains explicit Python highlighting', () => {
+    const code = renderContent('```python\nprint("hello")\n```');
+    expect(code).toHaveAttribute('data-language', 'python');
+    expect(code?.querySelector('[class*="hljs-"]')).not.toBeNull();
+  });
+
+  it('keeps inline code inline', () => {
+    const code = renderContent('Use `const answer = 42;` here.');
+    expect(code).not.toHaveAttribute('data-language');
+    expect(code?.textContent).toBe('const answer = 42;');
+  });
+});
diff --git a/client/src/components/Chat/Messages/Content/markdownConfig.ts b/client/src/components/Chat/Messages/Content/markdownConfig.ts
index dbf0fa9700..db082d0207 100644
--- a/client/src/components/Chat/Messages/Content/markdownConfig.ts
+++ b/client/src/components/Chat/Messages/Content/markdownConfig.ts
@@ -65,7 +65,7 @@ export const getRemarkPlugins = (latexParsing = true): PluggableList => [
 
 export const getRehypePlugins = (): PluggableList => [
   [rehypeKatex],
-  [rehypeHighlight, { detect: true, ignoreMissing: true, subset: langSubset }],
+  [rehypeHighlight, { detect: false, ignoreMissing: true, subset: langSubset }],
 ];
 
 export const getMarkdownComponents = (): { [nodeType: string]: ElementType } => ({