My personal opinion-psuedoterminals should be enhanced to provide some mechanism for sending out-of-band data, like the existing TIOCPKT but on steroids. Maybe something like… if both ends support it, they can exchange JSON-RPC messages over this out-of-band channel, with it being possible to discover if the other side supports this or not. Possibly this is just a new TIOC* ioctl
Why? Well one reason is escape sequences are really limited and messy. This would enable everyone to gradually and backward-compatibly transition to a more modern alternative. Once you have a JSON-RPC channel, the two ends can use it to negotiate what specific features they support. It would be leveraging patterns already popular with LSP, MCP, etc. And it would be mostly in userspace, only a small kernel enhancement would be required (the kernel doesn’t have to actually understand these JSON-RPC messages just offer a side channel to convey them).
I suppose you could do it without any kernel change if you just put a Unix domain socket in an environment variable: but that would be more fragile, some process will end up with your pty but missing the environment variable or vice versa
Actually I’d add this out-of-band JSON-RPC feature to pipes too, so if I run “foo | bar”, foo and bar can potentially engage in content/feature negotiation with each other