diff --git a/utils/arcanist/clang-format.sh b/utils/arcanist/clang-format.sh
--- a/utils/arcanist/clang-format.sh
+++ b/utils/arcanist/clang-format.sh
@@ -51,7 +51,7 @@
 # We do not look for clang-format-diff or clang-format-diff.py in the PATH
 # because whether/how these are installed differs between distributions,
 # and we have an executable copy in the tree anyway.
-arc_base_commit=$(arc which --show-base)
+arc_base_commit=$(/opt/arcanist/bin/arc which --show-base)
 git diff-index -U0 "${arc_base_commit}" "${src_file}" \
   | clang/tools/clang-format/clang-format-diff.py -style file -i -p1