On macOS, communicating with other interpreters using the send command is
limited to interpreters within the same process. This limitation is not mentioned
in the manual page for "send".

Also, some tests in the test file "send.test" involve interpreters in other
processes. Some of these tests are skipped for aqua, but not all, notably
send-8.18 and send-8.19.

A probably related limitation holds for [winfo interps]. Under macOS, this command
does not list interpreters in another process. As is the case with [send], this
limitation is not mentioned in the docs. Furthermore, the test file "winfo.test"
doesn't contain any tests that exercise multiple interpreters, whether in the same
process or not, taking into account the different behaviour on macOS compared
to that on other unix derivatives.

See also the post to the TCLCORE mailing list on 15-aug-2025 with subject
"Advice requested regarding [send] on macOS".