diff --git a/codex-rs/core/tests/suite/tool_parallelism.rs b/codex-rs/core/tests/suite/tool_parallelism.rs
index 0df8f8cbc..b5a7a3a17 100644
--- a/codex-rs/core/tests/suite/tool_parallelism.rs
+++ b/codex-rs/core/tests/suite/tool_parallelism.rs
@@ -187,6 +187,8 @@ async fn mixed_parallel_tools_run_in_parallel() -> anyhow::Result<()> {
     .to_string();
     let shell_args = serde_json::to_string(&json!({
         "command": "sleep 0.3",
+        // Avoid user-specific shell startup cost in timing assertions.
+        "login": false,
         "timeout_ms": 1_000,
     }))?;