diff --git a/nix/agent-modules/otel.nix b/nix/agent-modules/otel.nix
index e4ff3200..95a432cf 100644
--- a/nix/agent-modules/otel.nix
+++ b/nix/agent-modules/otel.nix
@@ -234,7 +234,15 @@ in
         # host-persisted state bind mount: it indexes /var/log/journal,
         # itself fresh on every container recreate, so the two must share
         # a lifetime or the cursor outlives the journal it points into.
-        extensions.file_storage.directory = "/var/lib/opentelemetry-collector";
+        # `create_directory` matters here for a build-time-vs-runtime
+        # reason, not a convenience one: `validateConfigFile` above runs
+        # `otelcol validate` in the nix build sandbox, before systemd's
+        # `StateDirectory=` has ever created this path. Without it the
+        # validator refuses with "directory must exist" and the build fails.
+        extensions.file_storage = {
+          directory = "/var/lib/opentelemetry-collector";
+          create_directory = true;
+        };
 
         receivers.journald = {
           # ⚠️ STATED, and it must stay stated: the receiver's own default