For me, it was introduced in this commit: 750a94e89fada66681fe4587f92a4adf558d9386
The output of `arc help` is normal on the previous commit, and on that one changes to:
```
PHP Fatal error: Call to undefined function phutil_json_decode() in /home/cosmogia/git/cosmogia/dev_tools/arcanist/src/configuration/Ar
canistConfigurationManager.php on line 202
Fatal error: Call to undefined function phutil_json_decode() in /home/cosmogia/git/cosmogia/dev_tools/arcanist/src/configuration/Arcanis
tConfigurationManager.php on line 202
```
Cheers!