Where does the bug appear (feature/product)?
Cursor IDE
Describe the Bug
After auto-updating from Cursor 3.18.25 to 3.19.7 on Windows 10 x64, all of my local stdio MCP servers stopped working. They were fine on 3.18.25 with the same ~/.cursor/mcp.json config.
Affected servers are local command-based MCPs (e.g. GitHub MCP via github-mcp-server.exe, Jira via uvx mcp-atlassian). Remote HTTP/OAuth MCP plugins (Slack, Atlassian marketplace) still work.
The servers never start. MCP logs show:
MCP stdio sandbox unavailable: MCP server requires sandboxing because MCP Network Controls are enabled, but the sandbox is not supported on this platform or system. The server was not started.; sandboxFailureReasonClass=unsupported_platform
Also at startup:
Sandbox prerequisites configured for stdio MCP: supported=false
This looks like a 3.19.7 regression on Windows. On 3.18.25 (same machine, same settings, networkControlsEnabled=true, mcpNetworkMode=allow_all), stdio MCP sandbox spawned and servers connected successfully. The first failure on my machine was ~2 minutes after a background auto-update from 3.18.25 → 3.19.7 on 2026-09-03 evening.
The MCP server binaries themselves are fine — launching github-mcp-server.exe stdio directly outside Cursor works and connects normally. Tokens and config are not the issue; Cursor never gets far enough to start the process.
Workaround: Downgrade to 3.18.25 and set “update.mode”: “none” in user settings.
Steps to Reproduce
On Windows 10/11 x64, install Cursor 3.19.7 (or auto-update from 3.18.25).
Add one or more stdio MCP servers in ~/.cursor/mcp.json, for example:
{
“mcpServers”: {
“github”: {
“command”: “C:\Program Files\GitHub MCP\github-mcp-server.exe”,
“args”: [“stdio”],
“env”: {
“GITHUB_PERSONAL_ACCESS_TOKEN”: “${env:GITHUB_TOKEN}”,
“GITHUB_HOST”: “https://github.com”
}
}
}
}
Ensure GITHUB_TOKEN (or equivalent) is set in the environment Cursor inherits.
Restart Cursor.
Open Customize → MCPs (or start an agent session that loads MCP).
Observe the GitHub (and any other stdio) server shows an error / 0 tools.
Expected Behavior
MCP servers connect
Operating System
Windows 10/11
Version Information
IDE 3.19.7
Does this stop you from using Cursor
No - Cursor works, but with this issue