diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 3d4dff3185ca95..b3b6aa3bae2919 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -2331,7 +2331,7 @@ _git_mergetool ()
 		return
 		;;
 	--*)
-		__gitcomp "--tool= --prompt --no-prompt --gui --no-gui"
+		__gitcomp "--tool= --tool-help --prompt --no-prompt --gui --no-gui"
 		return
 		;;
 	esac
diff --git a/git-difftool--helper.sh b/git-difftool--helper.sh
index dd0c9a5b7f2b07..d32e47cc09eab0 100755
--- a/git-difftool--helper.sh
+++ b/git-difftool--helper.sh
@@ -61,9 +61,7 @@ launch_merge_tool () {
 		export BASE
 		eval $GIT_DIFFTOOL_EXTCMD '"$LOCAL"' '"$REMOTE"'
 	else
-		initialize_merge_tool "$merge_tool"
-		# ignore the error from the above --- run_merge_tool
-		# will diagnose unusable tool by itself
+		initialize_merge_tool "$merge_tool" || exit 1
 		run_merge_tool "$merge_tool"
 	fi
 }
@@ -87,9 +85,7 @@ if test -n "$GIT_DIFFTOOL_DIRDIFF"
 then
 	LOCAL="$1"
 	REMOTE="$2"
-	initialize_merge_tool "$merge_tool"
-	# ignore the error from the above --- run_merge_tool
-	# will diagnose unusable tool by itself
+	initialize_merge_tool "$merge_tool" || exit 1
 	run_merge_tool "$merge_tool" false
 
 	status=$?
diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index 1ff26170ffcff8..9a00fabba276a1 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -159,14 +159,18 @@ check_unchanged () {
 }
 
 valid_tool () {
-	setup_tool "$1" && return 0
+	setup_tool "$1" 2>/dev/null && return 0
 	cmd=$(get_merge_tool_cmd "$1")
 	test -n "$cmd"
 }
 
 setup_user_tool () {
 	merge_tool_cmd=$(get_merge_tool_cmd "$tool")
-	test -n "$merge_tool_cmd" || return 1
+	if test -z "$merge_tool_cmd"
+	then
+		echo >&2 "error: ${TOOL_MODE}tool.$tool.cmd not set for tool '$tool'"
+		return 1
+	fi
 
 	diff_cmd () {
 		( eval $merge_tool_cmd )
@@ -255,10 +259,11 @@ setup_tool () {
 
 	# Now let the user override the default command for the tool.  If
 	# they have not done so then this will return 1 which we ignore.
-	setup_user_tool
+	setup_user_tool 2>/dev/null
 
 	if ! list_tool_variants | grep -q "^$tool$"
 	then
+		echo "error: unknown ${tool%[0-9]} variant '$tool'" >&2
 		return 1
 	fi
 
@@ -474,7 +479,7 @@ get_merge_tool_path () {
 	merge_tool="$1"
 	if ! valid_tool "$merge_tool"
 	then
-		echo >&2 "Unknown merge tool $merge_tool"
+		echo >&2 "Unknown $TOOL_MODE tool $merge_tool"
 		exit 1
 	fi
 	if diff_mode
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 22b3a85b3e960e..82a88107850f2a 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -898,4 +898,12 @@ test_expect_success 'mergetool with guiDefault' '
 	git commit -m "branch1 resolved with mergetool"
 '
 
+test_expect_success 'mergetool with non-existent tool' '
+	test_when_finished "git reset --hard" &&
+	git checkout -b test$test_count branch1 &&
+	test_must_fail git merge main &&
+	yes "" | test_must_fail git mergetool --tool=absent >out 2>&1 &&
+	test_grep -i "not set for tool" out
+'
+
 test_done