Crash on latest version

[iandol]

tmux 3.6 and tmuxp 1.57 on Ubuntu 24.04 (I use pixi package manager to install). I get this crash which I didn't have before. Happens on multiple systems. Reverting to old versions (system versions from Ubuntu, tmux 3.4 and tmuxp 1.38) and don't see the crash.

Traceback (most recent call last):
  File "/home/cagelab/.pixi/envs/tmuxp/bin/tmuxp", line 9, in <module>
    sys.exit(cli.cli())
             ~~~~~~~^^
  File "/home/cagelab/.pixi/envs/tmuxp/lib/python3.14/site-packages/tmuxp/cli/__init__.py", line 148, in cli
    command_load(
    ~~~~~~~~~~~~^
        args=CLILoadNamespace(**vars(args)),
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        parser=parser,
        ^^^^^^^^^^^^^^
    )
    ^
  File "/home/cagelab/.pixi/envs/tmuxp/lib/python3.14/site-packages/tmuxp/cli/load.py", line 583, in command_load
    load_workspace(
    ~~~~~~~~~~~~~~^
        workspace_file,
        ^^^^^^^^^^^^^^^
    ...<2 lines>...
        **tmux_options,
        ^^^^^^^^^^^^^^^
    )
    ^
  File "/home/cagelab/.pixi/envs/tmuxp/lib/python3.14/site-packages/tmuxp/cli/load.py", line 357, in load_workspace
    _reattach(builder)
    ~~~~~~~~~^^^^^^^^^
  File "/home/cagelab/.pixi/envs/tmuxp/lib/python3.14/site-packages/tmuxp/cli/load.py", line 129, in _reattach
    builder.session.attach_session()
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/home/cagelab/.pixi/envs/tmuxp/lib/python3.14/site-packages/libtmux/session.py", line 827, in attach_session
    raise exc.LibTmuxException(proc.stderr)
libtmux.exc.LibTmuxException: ['open terminal failed: not a terminal']

[0re5ama]

I am facing the same issue. I tried running tmux and I am getting the error: open terminal failed: not a terminal. I think it has got something to do with ncurses update.

[tony]

Does tmux 3.5 work?

I will take a closer look at 3.6 today.

[tony]

@0re5ama @iandol Does just typing tmux by itself work?

I'm getting not a terminal even when I run tmux on 3.6.

❯ tmux
open terminal failed: not a terminal

[tony]

@iandol @0re5ama

Does it work if you fully close tmux and try again? (e.g. killall -9 tmux / killall -9 /usr/local/bin/tmux / killall -9 /usr/bin/tmux?)

That fixed it for me.

From tmux/tmux#4699:

Note that it is very important to restart tmux entirely after upgrading. This is particularly important with this release because one of the libraries that tmux uses changed its protocol. If you see this message when you try to attach it probably means you forgot to restart tmux:

open terminal failed: not a terminal

[tony]

Moving this to a discussion - this looks tmux 3.6 related and I think we have a workaround

[iandol]

@tony -- thanks for the feedback, indeed tmux 3.6 is failing by itself, thus not tmuxp specific. I did a pkill for all tmux processes but this doesn't fix it for me. I will investigate on my system a bit more, but at least using pixi global install tmux=3.5 didn't fix the tmuxp crash.

[tony]

Thank you! Fine to take up a new issue if you come back with more information if you find it's tmuxp related.

Aside: It's worth investigating if tmux is lingering around in any way as a process, e.g. pkill didn't catch it somehow. That particular error is what tmux 3.6 is raising for that specific reason, but yeah its vague enough to not rule out other things.

[iandol]

I couldn't find any rogue process using procs. The error after ensuring tmux isn't lingering is different:

> tmux
server exited unexpectedly
                          Exception: tmux exited with 1
                                                         [tty 21]:1:1-4: tmux

I've just gone back to the system tmux (3.4) and tmuxp (1.38) in the intermi, thanks for your advice and time!