diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8d5bcb2ccb..52fdd09be5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@
 - Syntax highlighting for Python files using uv as script runner in shebang #3689 (@janlarres)
 
 ## Bugfixes
+- Fix fish completions failing with `The expanded command was empty` when the completion file is first sourced inside a function, e.g. under a `<TAB>` binding that calls `commandline --function complete`, see #4025 (@kokhlo)
 - Track strikethrough (SGR 9 / 29) in the ANSI style tracker so it is re-emitted like bold, dim, italic, and underline, and is no longer dropped after the first highlighted token when displaying ANSI input with syntax highlighting, see #4015 (@mikehasa)
 - Allow boolean flags to be given more than once, so that a flag set in the config file can also be passed on the command line without erroring, see #3912 (@logarithmone1128)
 - Use parsed CLI arguments to detect number flags, see #3908 (@cuishuang)
diff --git a/assets/completions/bat.fish.in b/assets/completions/bat.fish.in
index b2e356640f..38ff1270fe 100644
--- a/assets/completions/bat.fish.in
+++ b/assets/completions/bat.fish.in
@@ -3,7 +3,12 @@
 # ($XDG_CONFIG_HOME is usually set to ~/.config)
 
 # `bat` is `batcat` on Debian and Ubuntu
-set bat {{PROJECT_EXECUTABLE}}
+# This has to be a global: fish sources completion files in the scope of the
+# caller, so an unscoped `set` here would be local to whatever function
+# happened to trigger the first completion (e.g. a custom <TAB> binding that
+# calls `commandline --function complete`). The helpers below run later, in a
+# different scope, and would then see an empty `$bat`.
+set -g bat {{PROJECT_EXECUTABLE}}
 
 # Helper functions: