@@ -621,6 +621,12 @@ def _part_to_message_block(
     else:
       tool_result_content = content
 
+    # An MCP CallToolResult reports a failed call with `isError`, spelled
+    # `is_error` in dumps made with MCP SDK 2.x field names.
+    is_error = bool(
+        response_data.get("isError") or response_data.get("is_error")
+    )
+
     return anthropic_types.ToolResultBlockParam(
         tool_use_id=sanitizer.sanitize(
             function_response.id,
@@ -629,7 +635,7 @@ def _part_to_message_block(
         ),
         type="tool_result",
         content=tool_result_content,
-        is_error=False,
+        is_error=is_error,
     )
   elif _is_image_part(part):
     inline_data = part.inline_data
 
@@ -1379,6 +1379,24 @@ def test_part_to_message_block_with_multiple_content_items():
   assert result["content"] == "First part\nSecond part"
 
 
+@pytest.mark.parametrize("error_key", ["isError", "is_error"])
+def test_part_to_message_block_marks_mcp_error_result_as_error(error_key):
+  """An MCP CallToolResult with isError reaches Claude as an error result."""
+  response_part = types.Part.from_function_response(
+      name="mcp_tool",
+      response={
+          "content": [{"type": "text", "text": "file not found"}],
+          error_key: True,
+      },
+  )
+  response_part.function_response.id = "test_id_mcp_error"
+
+  result = part_to_message_block(response_part)
+
+  assert result["content"] == "file not found"
+  assert result["is_error"] is True
+
+
 def test_part_to_message_block_tool_result_with_image():
   """Media a tool attached to its response reaches Claude as an image block."""
   image_data = b"chart-bytes"