diff --git a/src/becwright/engine.py b/src/becwright/engine.py
index 230ea1a..7c85adb 100644
--- a/src/becwright/engine.py
+++ b/src/becwright/engine.py
@@ -89,7 +89,8 @@ def _run_check(rule: Rule, stdin: str, root: Path) -> RuleResult:
             rule=rule, passed=False,
             output=f"check timed out after {_check_timeout():g}s (its command hung)",
         )
-    output = proc.stdout.strip() or proc.stderr.strip()
+    outputs = [proc.stdout.strip(), proc.stderr.strip()]
+    output = "\n".join(o for o in outputs if o).strip()
     return RuleResult(rule=rule, passed=proc.returncode == 0, output=output)
 
 
diff --git a/tests/test_engine_integration.py b/tests/test_engine_integration.py
index aa7b143..ee21d4c 100644
--- a/tests/test_engine_integration.py
+++ b/tests/test_engine_integration.py
@@ -109,3 +109,14 @@ def test_evaluate_times_out_a_hung_check(tmp_path, monkeypatch):
     assert result.per_rule[0].passed is False
     assert "timed out" in result.per_rule[0].output
     assert result.had_blocking is True
+
+
+def test_evaluate_captures_both_stdout_and_stderr(tmp_path):
+    (tmp_path / "a.py").write_text("x = 1\n", encoding="utf-8")
+    check = f'"{sys.executable}" -c "import sys; print(\'out_msg\'); print(\'err_msg\', file=sys.stderr); sys.exit(1)"'
+    rule = Rule(id="both", paths=("**/*.py",), check=check, severity="blocking")
+    result = evaluate([rule], ["a.py"], tmp_path)
+    assert result.per_rule[0].passed is False
+    assert "out_msg" in result.per_rule[0].output
+    assert "err_msg" in result.per_rule[0].output
+