diff --git a/t/test-lib.sh b/t/test-lib.sh
index 9e9862b0f1..5db495fda9 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -481,6 +481,9 @@ test_failure_ () {
 			"$(cat "$GIT_TEST_TEE_OUTPUT_FILE")")"
 		>"$GIT_TEST_TEE_OUTPUT_FILE"
 		junit_insert="$junit_insert</failure>"
+		junit_insert="$junit_insert<system-err>$(xml_attr_encode \
+			"$(cat "$GIT_TEST_TEE_OUTPUT_FILE.err")")</system-err>"
+		>"$GIT_TEST_TEE_OUTPUT_FILE.err"
 		write_junit_xml_testcase "$1" "      $junit_insert"
 	fi
 	test_failure=$(($test_failure + 1))
@@ -765,9 +768,12 @@ test_start_ () {
 	then
 		junit_start=$(test-tool date getnanos)
 
-		# truncate output
-		test -z "$GIT_TEST_TEE_OUTPUT_FILE" ||
-		>"$GIT_TEST_TEE_OUTPUT_FILE"
+		# append to future <system-err>; truncate output
+		test -z "$GIT_TEST_TEE_OUTPUT_FILE" || {
+			cat "$GIT_TEST_TEE_OUTPUT_FILE" \
+				>>"$GIT_TEST_TEE_OUTPUT_FILE.err"
+			>"$GIT_TEST_TEE_OUTPUT_FILE"
+		}
 	fi
 }