fix(editor): Remove trailing newline from AI Assistant code blocks (no-changelog) (#40731)

Co-authored-by: Claude Sonnet 5.5 <[email protected]>

packages/frontend/@n8n/design-system/src/components/AskAssistantChat/messages/useMarkdown.test.ts

@@ -0,0 +1,29 @@
+import { useMarkdown } from './useMarkdown';
+
+describe('useMarkdown', () => {
+	const { renderMarkdown } = useMarkdown();
+
+	it('should not add a trailing newline inside a fenced code block', () => {
+		const html = renderMarkdown('```\nhttps://example.com/webhook/abc\n```');
+
+		expect(html).toContain('<code>https://example.com/webhook/abc</code>');
+	});
+
+	it('should keep newlines between lines of a fenced code block', () => {
+		const html = renderMarkdown('```\nline 1\nline 2\n```');
+
+		expect(html).toContain('<code>line 1\nline 2</code>');
+	});
+
+	it('should keep the language class on a fenced code block', () => {
+		const html = renderMarkdown('```js\nconst a = 1;\n```');
+
+		expect(html).toContain('<code class="language-js">const a = 1;</code>');
+	});
+
+	it('should keep an intentional blank last line in a fenced code block', () => {
+		const html = renderMarkdown('```\nline 1\n\n```');
+
+		expect(html).toContain('<code>line 1\n</code>');
+	});
+});

packages/frontend/@n8n/design-system/src/components/AskAssistantChat/messages/useMarkdown.ts

@@ -17,6 +17,15 @@ export function useMarkdown() {
 		},
 	});
 
+	// markdown-it ends fenced code with a newline. Triple-click copy would include it.
+	const renderFence = md.renderer.rules.fence;
+	md.renderer.rules.fence = (tokens, idx, options, env, self) => {
+		tokens[idx].content = tokens[idx].content.replace(/\n$/, '');
+		return renderFence
+			? renderFence(tokens, idx, options, env, self)
+			: self.renderToken(tokens, idx, options);
+	};
+
 	function renderMarkdown(content: string) {
 		try {
 			return md.render(content);