diff --git a/.arcconfig b/.arcconfig
--- a/.arcconfig
+++ b/.arcconfig
@@ -1,5 +1,6 @@
 {
   "phabricator.uri" : "https://reviews.llvm.org/",
   "repository.callsign" : "G",
-  "conduit_uri" : "https://reviews.llvm.org/"
+  "conduit_uri" : "https://reviews.llvm.org/",
+  "base": "git:HEAD^"
 }