How to insert socat sniffing between pyserial and actual serial port - linux

Question

I have an app written in python3 that uses pyserial (python3-serial) to communicate with another processor via serial cable. And I'm losing some responses. I have a tap on the line itself and have determined that the responses are indeed being placed on the wire.
So I wanted to sniff the port at a low linux level to see if the responses are showing up there. My python3 apps uses something like
port = serial.Serial(path, baudrate=115200, timeout=0)
where path is something like _port_TD200_ which is a symlink to \dev\ttyS3.
My general approach was to change that link, so that it points at a virtual port, and then bridge the real port to the virtual port. So I used ln -sf to relink the _port_TD200_ to /tmp/ttyV0 and then run this
sudo socat /dev/ttyS3,raw,echo=0 SYSTEM:'tee input.txt | socat - "PTY,link=/tmp/ttyV0,raw,echo=0,waitslave" | tee output.txt'
However, when I try to restart the service that runs the pyserial app... it chokes on the change:
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/serial/serialposix.py", line 265, in open
self.fd = os.open(self.portstr, os.O_RDWR | os.O_NOCTTY | os.O_NONBLOCK)
PermissionError: [Errno 13] Permission denied: '/opt/pilot/_port_TD200_'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/opt/pilot/radio.py", line 19, in __init__
self.port = serial.Serial(path, baudrate=115200, timeout=0)
File "/usr/lib/python3/dist-packages/serial/serialutil.py", line 236, in __init__
self.open()
File "/usr/lib/python3/dist-packages/serial/serialposix.py", line 268, in open
raise SerialException(msg.errno, "could not open port {}: {}".format(self._port, msg))
serial.serialutil.SerialException: [Errno 13] could not open port /opt/pilot/_port_TD200_: [Errno 13] Permission denied: '/opt/pilot/_port_TD200_'
I have a guess that the pyserial open machinery which opens a file descriptor, but ALSO does setserial like things, is not happy because /tmp/ttyV0 is not enough of a real serial port. Is this correct? If so, is there a way to modify the socat incantation to make it possible?
Or is it something else? I admit I don't really understand the socat incantation. I pulled it from this stack exchange question...