diff --git a/swarm-builder/src/nix_build.rs b/swarm-builder/src/nix_build.rs
index 6a5721cb..23c22507 100644
--- a/swarm-builder/src/nix_build.rs
+++ b/swarm-builder/src/nix_build.rs
@@ -346,11 +346,15 @@ mod tests {
             .stdout(Stdio::piped())
             .spawn()
             .expect("sh runs");
-        write!(
+        // The helper exits without reading the request for any op but `get`,
+        // so the write can hit EPIPE. `git` ignores EPIPE here too.
+        let written = write!(
             child.stdin.take().expect("piped"),
             "protocol={protocol}\nhost={host}\npath=agent-configs/atlas\n\n"
-        )
-        .expect("the helper reads its request");
+        );
+        if let Err(e) = written {
+            assert_eq!(e.kind(), std::io::ErrorKind::BrokenPipe, "{e}");
+        }
         let out = child.wait_with_output().expect("the helper exits");
         assert!(out.status.success());
         String::from_utf8(out.stdout).expect("utf-8")