diff --git a/tests/checks/tmux-bind.fish b/tests/checks/tmux-bind.fish
index 5e84df6d7..a25e8d635 100644
--- a/tests/checks/tmux-bind.fish
+++ b/tests/checks/tmux-bind.fish
@@ -1,39 +1,9 @@
-#RUN: %fish -C 'set -g fish %fish' %s
+#RUN: %fish %s
 #REQUIRES: command -v tmux
 
-set fish (builtin realpath $fish)
-
-# Isolated tmux.
-set -g tmpdir (mktemp -d)
-
-# Don't CD elsewhere, because tmux socket file is relative to CWD. Using
-# absolute path to socket file is prone to 'socket file name too long' error.
-cd $tmpdir
-
-set -g tmux tmux -S .tmux-socket -f /dev/null
-
-set -g sleep sleep .1
-set -q CI && set sleep sleep 1
-
-
-$tmux new-session -x 80 -y 10 -d $fish -C '
-    # This is similar to "tests/interactive.config".
-    function fish_greeting; end
-    function fish_prompt; printf "prompt $status_generation> "; end
-    # No autosuggestion from older history.
-    set fish_history ""
-'
-# Set the correct permissions for the newly created socket to allow future connections.
-# This is required at least under WSL or else each invocation will return a permissions error.
-chmod 777 .tmux-socket
-$sleep # Let fish draw a prompt.
-
 # Test moving around with up-or-search on a multi-line commandline.
-$tmux send-keys 'echo 12' M-Enter 'echo ab' C-p 345 C-n cde
+isolated-tmux send-keys 'echo 12' M-Enter 'echo ab' C-p 345 C-n cde
 $sleep
-$tmux capture-pane -p
+isolated-tmux capture-pane -p
 # CHECK: prompt 0> echo 12345
 # CHECK: echo abcde
-
-$tmux kill-server
-rm -r $tmpdir
diff --git a/tests/checks/tmux-complete.fish b/tests/checks/tmux-complete.fish
index b26e94090..91dfd0a37 100644
--- a/tests/checks/tmux-complete.fish
+++ b/tests/checks/tmux-complete.fish
@@ -1,57 +1,30 @@
-#RUN: %fish -C 'set -g fish %fish' %s
+#RUN: %fish %s
 #REQUIRES: command -v tmux
 
-# Resolve absolute path to fish (if needed) before changing directories
-set fish (builtin realpath $fish)
-
-# Isolated tmux. tmux can't handle session sockets in paths that are too long, and macOS has a very
-# long $TMPDIR, so use a relative path - except macOS doesn't have `realpath --relative-to`...
-# We are cd'd into a unique temp dir created/assigned by the test driver, so this will work so long
-# as `tmux` is only invoked from the same PWD.
-set tmpdir (command mktemp -d ./tmp.XXXXXXXX)
-cd $tmpdir
-set -g tmux tmux -S ./.tmux-socket -f /dev/null
-
-set -g sleep sleep .1
-set -q CI && set sleep sleep 1
-
-
-$tmux new-session -x 80 -y 10 -d $fish -C '
-    # This is similar to "tests/interactive.config".
-    function fish_greeting; end
-    function fish_prompt; printf "prompt $status_generation> "; end
-    # No autosuggestion from older history.
-    set fish_history ""
-'
-# Set the correct permissions for the newly created socket to allow future connections.
-# This is required at least under WSL or else each invocation will return a permissions error.
-chmod 777 .tmux-socket
-$sleep # Let fish draw a prompt.
+isolated-tmux
 
 # Don't escape existing token (#7526).
 echo >file-1
 echo >file-2
-$tmux send-keys 'HOME=$PWD ls ~/' Tab
+isolated-tmux send-keys 'HOME=$PWD ls ~/' Tab
 $sleep
-$tmux capture-pane -p
+isolated-tmux capture-pane -p
 # Note the contents may or may not have the autosuggestion appended - it is a race.
 # CHECK: prompt 0> HOME=$PWD ls ~/file-{{1?}}
 # CHECK: ~/file-1  ~/file-2
 
 # No pager on single smartcase completion (#7738).
-$tmux send-keys C-u C-l 'mkdir cmake CMakeFiles' Enter C-l \
+isolated-tmux send-keys C-u C-l 'mkdir cmake CMakeFiles' Enter C-l \
     'cat cmake' Tab
 $sleep
-$tmux capture-pane -p
+isolated-tmux capture-pane -p
 # CHECK: prompt 1> cat cmake/
 
 # Correct case in pager when prefixes differ in case (#7743).
-$tmux send-keys C-u C-l 'complete -c foo2 -a "aabc aaBd" -f' Enter C-l \
+isolated-tmux send-keys C-u C-l 'complete -c foo2 -a "aabc aaBd" -f' Enter C-l \
     'foo2 A' Tab
 $sleep
-$tmux capture-pane -p
-# The "bc" part is the autosuggestion - we could use "tmux capture-pane -e" to check colors.
+isolated-tmux capture-pane -p
+# The "bc" part is the autosuggestion - we could use "capture-pane -e" to check colors.
 # CHECK: prompt 2> foo2 aabc
 # CHECK: aabc  aaBd
-
-$tmux kill-server
diff --git a/tests/checks/tmux-prompt.fish b/tests/checks/tmux-prompt.fish
index a32342df9..07f5e01af 100644
--- a/tests/checks/tmux-prompt.fish
+++ b/tests/checks/tmux-prompt.fish
@@ -1,49 +1,18 @@
-#RUN: %fish -C 'set -g fish %fish' %s
+#RUN: %fish %s
 #REQUIRES: command -v tmux
 
-set fish (builtin realpath $fish)
-
-# Isolated tmux.
-# Note $XDG_CONFIG_HOME typically has a leading double-dot,
-# so our uvars file will leak across runs; therefore
-# descend more deeply into the tmpdir.
-set -g tmpdir (mktemp -d)/inner1/inner2/
-mkdir -p $tmpdir
-
-# Don't CD elsewhere, because tmux socket file is relative to CWD. Using
-# absolute path to socket file is prone to 'socket file name too long' error.
-cd $tmpdir
-
-set -g tmux tmux -S .tmux-socket -f /dev/null
-
-set -g sleep sleep .1
-set -q CI && set sleep sleep 10
-
-while set -e prompt_var
-end
-
-$tmux new-session -x 80 -y 10 -d $fish -C '
-    # This is similar to "tests/interactive.config".
-    function fish_greeting; end
+set -g isolated_tmux_fish_extra_args -C '
     function fish_prompt; printf "prompt $status_generation> <$prompt_var> "; end
-    # No autosuggestion from older history.
-    set fish_history ""
     function on_prompt_var --on-variable prompt_var
         commandline -f repaint
     end
 '
-# Set the correct permissions for the newly created socket to allow future connections.
-# This is required at least under WSL or else each invocation will return a permissions error.
-chmod 777 .tmux-socket
-$sleep # Let fish draw a prompt.
 
-$tmux capture-pane -p
+isolated-tmux capture-pane -p
 # CHECK: prompt 0> <>
 
+set -q CI && set sleep sleep 10
 set -U prompt_var changed
 $sleep
-$tmux capture-pane -p
+isolated-tmux capture-pane -p
 # CHECK: prompt 0> <changed>
-
-$tmux kill-server
-rm -r $tmpdir
diff --git a/tests/test_functions/isolated-tmux.fish b/tests/test_functions/isolated-tmux.fish
new file mode 100644
index 000000000..3412b839e
--- /dev/null
+++ b/tests/test_functions/isolated-tmux.fish
@@ -0,0 +1,45 @@
+function isolated-tmux --inherit-variable tmpdir
+    set -l tmpdir (mktemp -d)
+    cd $tmpdir
+
+    set -g sleep sleep .1
+    set -q CI && set sleep sleep 1
+
+    # Evil hack - override ourselves, so we only run initialization once.
+    # We could do this outside the function, but then initialization is done too early for a
+    # command that wants to set $isolated_tmux_fish_extra_args first, like
+    #   fish -C 'source tests/test_functions/isolated-tmux.fish' tests/checks/tmux-prompt.fish
+    function isolated-tmux --inherit-variable tmpdir
+        # tmux can't handle session sockets in paths that are too long, and macOS has a very long
+        # $TMPDIR, so use a relative path - except macOS doesn't have `realpath --relative-to`...
+        # Luckily, we don't need to call tmux from other directories, so just make sure no one
+        # does by accident.
+        if test $PWD != $tmpdir
+            echo "error: isolated-tmux must always be run from the same directory." >&2
+            return 1
+        end
+        tmux -S .tmux-socket -f /dev/null $argv
+    end
+
+    function isolated-tmux-cleanup --on-event fish_exit --inherit-variable tmpdir
+        isolated-tmux kill-server
+        rm -r $tmpdir
+    end
+
+    set -l fish (status fish-path)
+    isolated-tmux new-session -x 80 -y 10 -d $fish -C '
+        # This is similar to "tests/interactive.config".
+        function fish_greeting; end
+        function fish_prompt; printf "prompt $status_generation> "; end
+        # No autosuggestion from older history.
+        set fish_history ""
+    ' $isolated_tmux_fish_extra_args
+    # Set the correct permissions for the newly created socket to allow future connections.
+    # This is required at least under WSL or else each invocation will return a permissions error.
+    chmod 777 .tmux-socket
+    $sleep # Let fish draw a prompt.
+
+    if set -q argv[1]
+        isolated-tmux $argv
+    end
+end