diff --git a/src/taskgraph/transforms/run/run_task.py b/src/taskgraph/transforms/run/run_task.py
index 3ce894e39..56204112f 100644
--- a/src/taskgraph/transforms/run/run_task.py
+++ b/src/taskgraph/transforms/run/run_task.py
@@ -79,9 +79,12 @@ def common_setup(config, task, taskdesc, command):
         )
 
         vcs_path = taskdesc["worker"]["env"]["VCS_PATH"]
-        for repo_config in repo_configs.values():
+
+        for repo_config, index in enumerate(repo_configs.values()):
             checkout_path = path.join(vcs_path, repo_config.path)
             command.append(f"--{repo_config.prefix}-checkout={checkout_path}")
+            if index == 0:
+                taskdesc["worker"]["env"].setdefault("PRIMARY_REPO_PATH", checkout_path)
 
         if run["sparse-profile"]:
             command.append(