diff --git a/agent_sdks/python/tests/parser/test_payload_fixer.py b/agent_sdks/python/tests/parser/test_payload_fixer.py
new file mode 100644
index 000000000..de209edb3
--- /dev/null
+++ b/agent_sdks/python/tests/parser/test_payload_fixer.py
@@ -0,0 +1,93 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+
+import pytest
+
+from a2ui.parser.payload_fixer import parse_and_fix
+
+
+@pytest.mark.parametrize(
+    "payload,expected",
+    [
+        (
+            '[{"beginRendering": {"surfaceId": "s1", "root": "r1"}}]',
+            [{"beginRendering": {"surfaceId": "s1", "root": "r1"}}],
+        ),
+        (
+            '{"beginRendering": {"surfaceId": "s1", "root": "r1"}}',
+            [{"beginRendering": {"surfaceId": "s1", "root": "r1"}}],
+        ),
+        ("{\u201ckey\u201d: \u201cvalue\u201d}", [{"key": "value"}]),
+        ('{"text": "it\u2019s fine"}', [{"text": "it's fine"}]),
+        ("[]", []),
+    ],
+)
+def test_parse_and_fix_normalization(payload, expected):
+  assert parse_and_fix(payload) == expected
+
+
+@pytest.mark.parametrize(
+    "payload,expected",
+    [
+        ('[{"key": "value",}]', [{"key": "value"}]),
+        ('[{"items": ["a", "b",]}]', [{"items": ["a", "b"]}]),
+        ('[{"a": 1, "b": [1, 2,]}]', [{"a": 1, "b": [1, 2]}]),
+        ('{"a": {"b": 1,}}', [{"a": {"b": 1}}]),
+        ('[{"a": [1, 2, 3,]}]', [{"a": [1, 2, 3]}]),
+    ],
+)
+def test_parse_and_fix_trailing_comma(payload, expected):
+  assert parse_and_fix(payload) == expected
+
+
+def test_parse_and_fix_multiple_messages():
+  payload = (
+      '[{"beginRendering": {"surfaceId": "s1", "root": "r1"}}, {"surfaceUpdate":'
+      ' {"surfaceId": "s1", "components": []}}]'
+  )
+  result = parse_and_fix(payload)
+  assert len(result) == 2
+  assert "beginRendering" in result[0]
+  assert "surfaceUpdate" in result[1]
+
+
+def test_parse_and_fix_logs_warnings_on_trailing_comma_autofix(caplog):
+  payload = '[{"key": "value",}]'
+  with caplog.at_level(logging.WARNING, logger="a2ui.parser.payload_fixer"):
+    result = parse_and_fix(payload)
+  log_messages = [r.message for r in caplog.records]
+  assert any("Initial A2UI payload validation failed" in msg for msg in log_messages)
+  assert any(
+      "Detected trailing commas in LLM output; applied autofix." in msg
+      for msg in log_messages
+  )
+  assert result == [{"key": "value"}]
+
+
+@pytest.mark.parametrize(
+    "payload",
+    [
+        "this is not json at all",
+        "",
+        "{",
+        "[",
+        '{"key": "value"',
+        '[{"key": "value",]',
+    ],
+)
+def test_parse_and_fix_invalid_payload_raises_value_error(payload):
+  with pytest.raises(ValueError, match="Failed to parse JSON"):
+    parse_and_fix(payload)