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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']
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@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

Replies: 7 comments · 4 replies

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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.

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Does tmux 3.5 work?

I will take a closer look at 3.6 today.

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@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
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@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
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Answer selected by tony
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Moving this to a discussion - this looks tmux 3.6 related and I think we have a workaround

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@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.

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2 replies
@tony
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tony Nov 30, 2025
Maintainer

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
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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!

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I tried to update to tmux V3.6a and the latest tmuxp (V1.62) but I am having similar crashes:

ε ⚡️tmuxp load 0
[Loading] /Users/ian/.tmuxp/0.yaml
Traceback (most recent call last):
                                    File "/Users/ian/.pixi/envs/tmuxp/bin/tmuxp", line 9, in <module>
                                                                                                         sys.exit(cli.cli())
                                                                                                                                         ~~~~~~~^^
                                                                                                                                                    File "/Users/ian/.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 "/Users/ian/.pixi/envs/tmuxp/lib/python3.14/site-packages/tmuxp/cli/load.py", line 562, in command_load
                                                                                                                                                                 load_workspace(
                                                                                                                                                                                    ~~~~~~~~~~~~~~^
             workspace_file,
                                    ^^^^^^^^^^^^^^^
                                                       ...<2 lines>...
                                                                              **tmux_options,
                                                                                                     ^^^^^^^^^^^^^^^
                                                                                                                        )
                                                                                                                             ^
                                                                                                                                File "/Users/ian/.pixi/envs/tmuxp/lib/python3.14/site-packages/tmuxp/cli/load.py", line 372, in load_workspace
                                                    _load_attached(builder, detached)
                                                                                         ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
                                                                                                                            File "/Users/ian/.pixi/envs/tmuxp/lib/python3.14/site-packages/tmuxp/cli/load.py", line 152, in _load_attached
                                                builder.session.attach()
                                                                            ~~~~~~~~~~~~~~~~~~~~~~^^
                                                                                                      File "/Users/ian/.pixi/envs/tmuxp/lib/python3.14/site-packages/libtmux/session.py", line 330, in attach
                   raise exc.LibTmuxException(proc.stderr)
                                                          libtmux.exc.LibTmuxException: ['server exited unexpectedly']
                                                                                                                      Exception: tmuxp exited with 1
                                                                                                                                                      [tty 10]:1:1-12: tmuxp load 0

tmux itself is working again, and I've tried to pkill tmux before running tmuxp after the update...

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2 replies
@iandol
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The session is:

session_name: '0'
windows:
- focus: 'true'
  layout: 2ccc,190x62,0,0{95x62,0,0,0,94x62,96,0,1}
  options: {}
  panes:
  - elvish
  - focus: 'true'
    shell_command: elvish
  start_directory: /Users/ian
  window_name: elvish
@tony
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tony Dec 20, 2025
Maintainer

I think this should be a new discussion rather than continuing the original thread. tmux 3.6a was reported upstream as fixing the earlier terminal protocol issue (the not a terminal error), but this is still reproducible after a full restart with newer tmuxp.

The failure mode appears different now:

  • tmux itself starts and attaches correctly
  • tmuxp fails specifically during attach
  • the error has changed from open terminal failed: not a terminal to server exited unexpectedly

That suggests this is no longer the original tmux 3.6 upgrade problem.

If you open a new discussion, please include:

  • tmuxp debug-info
  • tmux version and install method (pixi / system / pacman / brew / pip)
  • your tmux config (or confirmation it’s empty)
  • confirmation that no tmux processes are running (ps -ef | grep '[t]mux')
  • whether tmux new -s test works reliably before invoking tmuxp

Additionally:

  • Try the tmuxp configuration without the layout, to rule out layout parsing or geometry issues
  • Check whether tmux produced a core dump and, if so, inspect it

That should let us narrow down whether this is tmuxp, libtmux, or a tmux 3.6a regression without conflating multiple issues.

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Converted from issue

This discussion was converted from issue #994 on November 30, 2025 14:23.

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