old setting was deprecated. this setting worked for me
@ -9,7 +9,7 @@ file](https://jj-vcs.github.io/jj/v0.26.0/config/#user-config-file).
```toml
[ui]
diff.tool = ["difft", "--color=always", "$left", "$right"]
diff-formatter = ["difft", "--color=always", "$left", "$right"]
```