diff --git a/src/aws_durable_execution_sdk_python_testing/web/handlers.py b/src/aws_durable_execution_sdk_python_testing/web/handlers.py
index 7c42d09..85417e8 100644
--- a/src/aws_durable_execution_sdk_python_testing/web/handlers.py
+++ b/src/aws_durable_execution_sdk_python_testing/web/handlers.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import base64
 import json
 import logging
 from abc import ABC, abstractmethod
@@ -183,6 +184,37 @@ def _no_content_response(
 
     # Removed deprecated _error_response method - use AWS exceptions directly
 
+    def _parse_callback_result_payload(self, request: HTTPRequest) -> bytes:
+        """Parse callback result payload from request body.
+
+        Expects JSON payload with base64-encoded Result field.
+
+        Args:
+            request: The HTTP request containing the JSON payload
+
+        Returns:
+            bytes: The decoded result payload
+
+        Raises:
+            InvalidParameterValueException: If payload parsing fails
+        """
+        if not request.body or not isinstance(request.body, bytes):
+            return b""
+
+        try:
+            payload = json.loads(request.body.decode("utf-8"))
+            if isinstance(payload, dict) and "Result" in payload:
+                result_value = payload["Result"]
+                if isinstance(result_value, str):
+                    return base64.b64decode(result_value)
+            return b""
+        except (json.JSONDecodeError, UnicodeDecodeError) as e:
+            msg = f"Failed to parse JSON payload: {e}"
+            raise InvalidParameterValueException(msg) from e
+        except ValueError as e:
+            msg = f"Failed to decode base64 result: {e}"
+            raise InvalidParameterValueException(msg) from e
+
     def _parse_query_param(self, request: HTTPRequest, param_name: str) -> str | None:
         """Parse a single query parameter from the request.
 
@@ -611,8 +643,7 @@ def handle(self, parsed_route: Route, request: HTTPRequest) -> HTTPResponse:
             callback_route = cast(CallbackSuccessRoute, parsed_route)
             callback_id: str = callback_route.callback_id
 
-            # For binary payload operations, body is raw bytes
-            result_bytes = request.body if isinstance(request.body, bytes) else b""
+            result_bytes: bytes = self._parse_callback_result_payload(request)
 
             callback_response: SendDurableExecutionCallbackSuccessResponse = (  # noqa: F841
                 self.executor.send_callback_success(
diff --git a/tests/web/handlers_test.py b/tests/web/handlers_test.py
index 5cc4fb0..52c2dea 100644
--- a/tests/web/handlers_test.py
+++ b/tests/web/handlers_test.py
@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import base64
+import json
 from typing import TYPE_CHECKING, Any
 from unittest.mock import Mock
 
@@ -2037,13 +2039,15 @@ def test_send_durable_execution_callback_success_handler():
     assert isinstance(route, CallbackSuccessRoute)
     assert route.callback_id == "test-callback-id"
 
-    # Test with valid request body (bytes for callback operations)
+    result_data = base64.b64encode(b"success-result").decode("utf-8")
+    request_body = json.dumps({"Result": result_data}).encode("utf-8")
+
     request = HTTPRequest(
         method="POST",
         path=route,
         headers={"Content-Type": "application/json"},
         query_params={},
-        body=b"success-result",
+        body=request_body,
     )
 
     response = handler.handle(route, request)
@@ -2058,6 +2062,57 @@ def test_send_durable_execution_callback_success_handler():
     )
 
 
+def test_send_durable_execution_callback_success_handler_invalid_json():
+    """Test SendDurableExecutionCallbackSuccessHandler with invalid JSON."""
+    executor = Mock()
+    handler = SendDurableExecutionCallbackSuccessHandler(executor)
+
+    router = Router()
+    route = router.find_route(
+        "/2025-12-01/durable-execution-callbacks/test-callback-id/succeed", "POST"
+    )
+
+    request = HTTPRequest(
+        method="POST",
+        path=route,
+        headers={"Content-Type": "application/json"},
+        query_params={},
+        body=b"invalid-json",
+    )
+
+    response = handler.handle(route, request)
+
+    assert response.status_code == 400
+    assert response.body["Type"] == "InvalidParameterValueException"
+    assert "Failed to parse JSON payload" in response.body["message"]
+
+
+def test_send_durable_execution_callback_success_handler_invalid_base64():
+    """Test SendDurableExecutionCallbackSuccessHandler with invalid base64."""
+    executor = Mock()
+    handler = SendDurableExecutionCallbackSuccessHandler(executor)
+
+    router = Router()
+    route = router.find_route(
+        "/2025-12-01/durable-execution-callbacks/test-callback-id/succeed", "POST"
+    )
+
+    request_body = json.dumps({"Result": "invalid-base64!"}).encode("utf-8")
+    request = HTTPRequest(
+        method="POST",
+        path=route,
+        headers={"Content-Type": "application/json"},
+        query_params={},
+        body=request_body,
+    )
+
+    response = handler.handle(route, request)
+
+    assert response.status_code == 400
+    assert response.body["Type"] == "InvalidParameterValueException"
+    assert "Failed to decode base64 result" in response.body["message"]
+
+
 def test_send_durable_execution_callback_success_handler_empty_body():
     """Test SendDurableExecutionCallbackSuccessHandler with empty body."""
     executor = Mock()