Unresponsive window on large mcp tool response - how I fixed it

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1

Where does the bug appear (feature/product)?

Cursor IDE

Describe the Bug

In reference to Window becomes unresponsive and reloads when MCP tool returns large response (markdown lexer regex hang)

Making a new report as that one is locked. I was able to reproduce this happening from an mcp that I built, and then through some investigation, find the root cause and fix it. Dont know if this describes ALL the reasons why it would happen - but in my case, fixing this did fix the issue.

Steps to Reproduce

  1. Very large amount if markdown text being returned by an mcp tool
  2. Somewhere in the markdown there is a broken link. Something like blah blah [this is a link(url) where the closing ] or ) is missing
  3. FWIW, the link was near the beginning of the doc
  4. There is some regex funny business Cursor that would kill the app, whenever this md was retrieved.

Fixing the md to have a fixed link stopped the issue. I also updated by MCP to preemptively scan for issues like this (in a way that would NOT run into the same self-DOS perf issue) and either close unclosed links or remove the opening bracket.

Expected Behavior

Cursor handles malformed markdown without losing it’s … stuff

Operating System

MacOS

Version Information

Version: 3.3.30
VSCode Version: 1.105.1
Commit: 3dc559280adc5f931ade8e25c7b85393842acf30
Date: 2026-05-09T18:28:42.332Z
Layout: editor
Build Type: Stable
Release Track: Default
Electron: 39.8.1
Chromium: 142.0.7444.265
Node.js: 22.22.1
V8: 14.2.231.22-electron.0
OS: Darwin arm64 25.4.0

For AI issues: which model did you use?

Happened on any model - is not an AI issue

Additional Information

Hope this helps

Does this stop you from using Cursor

Sometimes - I can sometimes use Cursor

6

Thanks for narrowing this down. This matches a known issue with Cursor freezing while rendering very large markdown responses from MCP tools, and the malformed-link detail is especially useful.

Your workaround is the right one for now: sanitize large MCP responses before returning them, especially broken markdown links with missing ] or ), or reduce/truncate the response size before Cursor renders it. The malformed-link trigger is useful extra detail for the existing report.