fix(web): prevent LaTeX from being cut at \right] or \big) in agent o… (#13155)

### What problem does this PR solve?

- Use negative lookbehind (?<![a-zA-Z]) so \] and \) inside commands
(e.g. \right], \big)) are not treated as block/inline delimiters
- Use greedy matching to capture up to the last valid delimiter, fixing
truncated formulas (e.g. C_{seq}(y|x) = \frac{1}{|y|} ...)
- Add unit tests for preprocessLaTeX

Closes #13134


### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

web/src/utils/__tests__/chat.test.ts

@@ -0,0 +1,34 @@
+import { preprocessLaTeX } from '../chat';
+
+describe('preprocessLaTeX', () => {
+  it('converts block \\[ \\] to $$ $$', () => {
+    expect(preprocessLaTeX('\\[ x + y \\]')).toBe('$$x + y$$');
+  });
+
+  it('converts inline \\( \\) to $ $', () => {
+    expect(preprocessLaTeX('\\( a \\)')).toBe('$a$');
+  });
+
+  it('does not cut block math at \\right] (Closes #13134)', () => {
+    const content = '\\[ C_{seq}(y|x) = \\frac{1}{|y|} \\sum_{t=1}^{|y|} \\right] \\]';
+    const result = preprocessLaTeX(content);
+    expect(result).toContain('\\right]');
+    expect(result).toContain('\\frac{1}{|y|}');
+    expect(result).toBe(
+      '$$ C_{seq}(y|x) = \\frac{1}{|y|} \\sum_{t=1}^{|y|} \\right] $$',
+    );
+  });
+
+  it('does not cut inline math at \\big) or nested parens', () => {
+    const content = '\\( f(x) + \\big) \\)';
+    const result = preprocessLaTeX(content);
+    expect(result).toContain('\\big)');
+    expect(result).toBe('$ f(x) + \\big) $');
+  });
+
+  it('handles multiple block equations', () => {
+    const content = 'First \\[ a \\] then \\[ b \\right] c \\]';
+    const result = preprocessLaTeX(content);
+    expect(result).toBe('First $$a$$ then $$ b \\right] c $$');
+  });
+});

web/src/utils/chat.ts

@@ -39,14 +39,24 @@ export const buildMessageUuidWithRole = (
 
 // Preprocess LaTeX equations to be rendered by KaTeX
 // ref: https://github.com/remarkjs/react-markdown/issues/785
+//
+// Delimiter matching: we only treat \] and \) as block/inline endings when they
+// are not part of a LaTeX command (e.g. \right], \big), \left)). Use a negative
+// lookbehind (?<![a-zA-Z]) so that \] or \) preceded by a letter (command name)
+// is not considered the closing delimiter. Use greedy matching so we match up to
+// the last valid delimiter and avoid cutting at the first \] or \) inside the
+// equation (e.g. \frac{1}{|y|} or \right]).
+
+const BLOCK_MATH_RE = /\\\[([\s\S]*)(?<![a-zA-Z])\\\]/g;
+const INLINE_MATH_RE = /\\\(([\s\S]*)(?<![a-zA-Z])\\\)/g;
 
 export const preprocessLaTeX = (content: string) => {
   const blockProcessedContent = content.replace(
-    /\\\[([\s\S]*?)\\\]/g,
+    BLOCK_MATH_RE,
     (_, equation) => `$$${equation}$$`,
   );
   const inlineProcessedContent = blockProcessedContent.replace(
-    /\\\(([\s\S]*?)\\\)/g,
+    INLINE_MATH_RE,
     (_, equation) => `$${equation}$`,
   );
   return inlineProcessedContent;