diff --git a/aws_lambda_powertools/utilities/parameters/ssm.py b/aws_lambda_powertools/utilities/parameters/ssm.py
index 6ce4f853459..a404b29f764 100644
--- a/aws_lambda_powertools/utilities/parameters/ssm.py
+++ b/aws_lambda_powertools/utilities/parameters/ssm.py
@@ -713,7 +713,7 @@ def _split_batch_and_decrypt_parameters(
 
         for parameter, options in parameters.items():
             # NOTE: TypeDict later
-            _overrides = options or {}
+            _overrides = {**(options or {})}
             _overrides["transform"] = _overrides.get("transform") or transform
 
             # These values can be falsy (False, 0)
diff --git a/tests/functional/parameters/_boto3/test_utilities_parameters.py b/tests/functional/parameters/_boto3/test_utilities_parameters.py
index dc7643795c1..bf11dbcf1c4 100644
--- a/tests/functional/parameters/_boto3/test_utilities_parameters.py
+++ b/tests/functional/parameters/_boto3/test_utilities_parameters.py
@@ -1279,6 +1279,40 @@ def test_ssm_provider_get_parameters_by_name_all_decrypt_should_use_get_paramete
         stubber.deactivate()
 
 
+def test_ssm_provider_get_parameters_by_name_does_not_mutate_input_and_honors_new_globals(
+    mock_name,
+    mock_value,
+    config,
+):
+    # GIVEN a reusable parameters dict with a non-empty per-parameter override
+    param = f"/a/{mock_name}"
+    fail = "/does_not_exist"  # stub model doesn't support all-success yet
+    all_params = {param: {"max_age": 0}, fail: {}}
+    names = list(all_params.keys())
+    stub_response = build_get_parameters_stub(params={param: mock_value}, invalid_parameters=[fail])
+
+    provider = parameters.SSMProvider(boto_config=config)
+    stubber = stub.Stubber(provider.client)
+    # first call: global decrypt defaults to False -> no WithDecryption
+    stubber.add_response("get_parameters", stub_response, {"Names": names})
+    # second call: new global decrypt=True must be honored
+    stubber.add_response("get_parameters", stub_response, {"Names": names, "WithDecryption": True})
+    stubber.activate()
+
+    # WHEN get_parameters_by_name is called twice with different global options
+    try:
+        provider.get_parameters_by_name(parameters=all_params, raise_on_error=False)
+
+        # THEN the caller's dict must not be modified
+        assert all_params == {param: {"max_age": 0}, fail: {}}
+
+        # AND a later global decrypt=True must not be overridden by stale values
+        provider.get_parameters_by_name(parameters=all_params, decrypt=True, raise_on_error=False)
+        stubber.assert_no_pending_responses()
+    finally:
+        stubber.deactivate()
+
+
 def test_dynamodb_provider_clear_cache(mock_name, mock_value, config):
     # GIVEN a provider is initialized with a cached value
     provider = parameters.DynamoDBProvider(table_name="test", boto_config=config)