Visual code debugger adapter immediately dies when trying to connect to vscode-hack debugger in local launch mode

Question

I am using the vscode-hack extension on visual code studio on macOS Big Sur. My settings.json includes following launch configuration:

"configurations": [
{
"name": "HHVM: Run Script",
"type": "hhvm",
"request": "launch",
"script": "${file}",
"hhvmPath": "/usr/local/bin/hhvm",
"hhvmArgs": ["--mode", "vsdebug", "--vsDebugPort", 8999],
"cwd":"${workspaceFolder}"
}
]

When I try to debug a Hack script in Visual code debugger, Visual code debugger adapter launches and immediately dies. I have verified launching hhvm with the same args from command line, and it works fine. I am following the instructions here https://github.com/slackhq/vscode-hack/blob/master/docs/debugging.md Does anybody have any suggestions?