--- a/tests/unit/botocore/test_compress.py
+++ b/tests/unit/botocore/test_compress.py
@@ -107,11 +107,17 @@ def request_dict_non_seekable_bytes_stre
 class StaticGzipFile(gzip.GzipFile):
     def __init__(self, *args, **kwargs):
         kwargs['mtime'] = 1
+        # Python 3.15 changed default gzip header output; pin level so these
+        # tests remain byte-stable across runtimes.
+        kwargs['compresslevel'] = 9
         super().__init__(*args, **kwargs)
 
 
 def static_compress(*args, **kwargs):
     kwargs['mtime'] = 1
+    # Python 3.15 changed default gzip header output; pin level so these
+    # tests remain byte-stable across runtimes.
+    kwargs['compresslevel'] = 9
     return gzip.compress(*args, **kwargs)