diff --git a/codex-rs/core/tests/suite/truncation.rs b/codex-rs/core/tests/suite/truncation.rs
index ed9e7cde3..7d611f55f 100644
--- a/codex-rs/core/tests/suite/truncation.rs
+++ b/codex-rs/core/tests/suite/truncation.rs
@@ -244,11 +244,16 @@ async fn tool_call_output_exceeds_limit_truncated_chars_limit() -> Result<()> {
         "expected truncated shell output to be plain text"
     );
 
-    assert_eq!(output.len(), 9976); // ~10k characters
-    let truncated_pattern = r#"(?s)^Exit code: 0\nWall time: 0 seconds\nTotal output lines: 100000\nOutput:\n.*?…\d+ chars truncated….*$"#;
+    let truncated_pattern = r#"(?s)^Exit code: 0\nWall time: [0-9]+(?:\.[0-9]+)? seconds\nTotal output lines: 100000\nOutput:\n.*?…\d+ chars truncated….*$"#;
 
     assert_regex_match(truncated_pattern, &output);
 
+    let len = output.len();
+    assert!(
+        (9_900..=10_000).contains(&len),
+        "expected ~10k chars after truncation, got {len}"
+    );
+
     Ok(())
 }