Where does the bug appear (feature/product)?
Cursor IDE
Describe the Bug
After updating Cursor (June 2026 release), a Cursor sandbox Python process begins consuming massive amounts of memory after an Agent task attempts to modify Cursor user rules and encounters an MCP/tool failure.
The process grows from normal memory usage to over 50 GB RAM, causing macOS to display “Your system has run out of application memory” and consume 12-16 GB of swap.
The issue appears related to the Cursor sandbox Python process (cursor_sandbox.py) repeatedly scanning Cursor state data and/or workspace files after the MCP lookup fails.
Killing the Python process immediately restores normal system behavior.
Steps to Reproduce
- Open Cursor on macOS after updating to the latest version.
- Open an Agent chat.
- Ask Cursor to update or modify Cursor user rules.
- Cursor attempts an MCP/tool action.
- MCP lookup fails with a message similar to:
“Tool cursor-app-control-cursor_dialog was not found” - Cursor falls back to Python sandbox operations.
- Agent output shows actions such as:
- List state DB keys
- Inspect composerHeaders
- Find file containing user rules
- Search files in .cursor
- Memory usage rapidly increases.
- Python process grows to 50+ GB RAM and system memory pressure becomes critical.
Expected Behavior
If an MCP tool is unavailable, the Agent should fail gracefully or stop the task.
The sandbox process should have bounded memory usage and should not recursively scan files or consume tens of gigabytes of RAM.
Memory usage should remain stable regardless of MCP failures.
Operating System
MacOS
Version Information
Version: 3.7.27 (Universal)
VS Code Extension API: 1.105.1
Commit: e48ee6102a199492b0c9964699bf011886708ba0
Date: 2026-06-10T01:46:16.942Z
Layout: glass
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
xterm.js: 6.1.0-beta.256
OS: Darwin arm64 25.4.0
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Additional Information
This issue began immediately after updating Cursor.
The problem appears reproducible when asking the Agent to modify Cursor user rules. During execution, the Agent attempted an MCP action and reported:
“Tool cursor-app-control-cursor_dialog was not found”
The Agent then began running Python sandbox operations including:
- Listing state DB keys
- Inspecting composer headers
- Searching for user rule content
- Searching files within .cursor
At that point a Python process associated with cursor_sandbox.py rapidly grew to approximately 53-56 GB RAM.
Symptoms:
- macOS “Your system has run out of application memory”
- Swap usage increased to approximately 12-16 GB
- Memory pressure entered yellow/red
- System became unstable
Investigation:
- The Cursor state database files were small and appeared normal:
- state.vscdb: ~25 KB
- state.vscdb-wal: ~15 MB
- state.vscdb-shm: ~32 KB
Workaround:
- Quit Cursor
- Kill the Python process manually
- Reopen Cursor
Reopening the same Agent task caused the issue to reappear.
Screenshots attached:
- Out of application memory warning
- Activity Monitor showing Python consuming 50+ GB RAM
- Process inspection output
- Agent transcript showing MCP failure and subsequent Python sandbox activity
Does this stop you from using Cursor
Sometimes - I can sometimes use Cursor