When connecting from Windows VS Code to gdbserver on Linux, "remote register badly formatted" error #455

Running VSCode 1.97.2 on Windows 10 x64. The project is a C++ one targeting Linux, x64. The launch settings go:

{
    "type": "gdb",
    "request": "attach",
    "name": "Attach to gdbserver",
    "executable": "${workspaceFolder}/foo",
    "target": "sandbox:9999",
    "remote": true,
    "cwd": "${workspaceRoot}"
}

On the Linux box ("sandbox"), I start gdbserver: gdbserver :9999 ./foo

If I try to debug from VS Code, I get the following error:

Failed to attach: Remote register badly formatted: T0506:0000000000000000;07:20e5ffffff7f0000;10:9030fdf7ff7f0000;thread:2b810;core:1;
here: 00000000;07:20e5ffffff7f0000;10:9030fdf7ff7f0000;thread:2b810;core:1; (from target-select remote sandbox:9999)

Some more diagnostics. If I run like this: there is the following output in the debug console window:

"R:\dev\Foo/foo": not in executable format: File format not recognized
warning: limiting remote suggested packet size (18431 bytes) to 16384
warning: while parsing target description (at line 8): Cannot add typed field "CF" to flags
warning: Could not load XML target description; ignoring

Then following some online guidance, I've added gdbpath to the launch settings: "gdbpath": "E:\\cygwin64\\bin\\gdb.exe" and got a different set of output, followed by the same error:

warning:
A handler for the OS ABI "GNU/Linux" is not built into this configuration
of GDB. Attempting to continue with the default
i386:x86-64 settings.

Reading symbols from R:\dev\Foo/foo...
Warning: 'set target-async', an alias for the command 'set mi-async', is deprecated.
Use 'set mi-async'.

Warning: /cygdrive/r/dev/Foo/R:\dev\Foo: No such file or directory.
undefinedBFD: reopening /cygdrive/r/dev/Foo/R:\dev\Foo/foo: No such file or directory
warning: A handler for the OS ABI "GNU/Linux" is not built into this configuration
of GDB. Attempting to continue with the default
i386:x86-64 settings.

warning: Architecture rejected target-supplied description

So it looks like VSCode's extension doesn't talk to gdbserver directly, it talks to gdb which in turn talks to gdbserver. I wonder what the protocol is - is it a socket? If so, can it be remoted?