fix(mcp): expose CallToolResult.isError in MCPToolResult

src/strands/tools/mcp/mcp_client.py

@@ -732,6 +732,8 @@ def _handle_tool_result(self, tool_use_id: str, call_tool_result: MCPCallToolRes
             content=mapped_contents,
         )
 
+        if call_tool_result.isError:
+            result["isError"] = True
         if call_tool_result.structuredContent:
             result["structuredContent"] = call_tool_result.structuredContent
         if call_tool_result.meta:

src/strands/tools/mcp/mcp_types.py

@@ -61,7 +61,12 @@ class MCPToolResult(ToolResult):
         metadata: Optional arbitrary metadata returned by the MCP tool. This field allows
             MCP servers to attach custom metadata to tool results (e.g., token usage,
             performance metrics, or business-specific tracking information).
+        isError: Whether the MCP tool itself reported an application-level error.
+            True means the tool executed but returned a failure result (application error).
+            Absent means the call succeeded or that an exception was raised before the
+            tool could return a result (protocol or transport error).
     """
 
     structuredContent: NotRequired[dict[str, Any]]
     metadata: NotRequired[dict[str, Any]]
+    isError: NotRequired[bool]

tests/strands/tools/mcp/test_mcp_client.py

@@ -167,6 +167,21 @@ def test_call_tool_sync_with_structured_content(mock_transport, mock_session):
         assert result["structuredContent"]["status"] == "completed"
 
 
+@pytest.mark.parametrize("is_error,expect_is_error_field", [(True, True), (False, False)])
+def test_call_tool_sync_preserves_is_error(mock_transport, mock_session, is_error, expect_is_error_field):
+    """Test that call_tool_sync exposes CallToolResult.isError in MCPToolResult."""
+    mock_content = MCPTextContent(type="text", text="Tool output")
+    mock_session.call_tool.return_value = MCPCallToolResult(isError=is_error, content=[mock_content])
+
+    with MCPClient(mock_transport["transport_callable"]) as client:
+        result = client.call_tool_sync(tool_use_id="test-123", name="test_tool", arguments={})
+
+        if expect_is_error_field:
+            assert result.get("isError") is True
+        else:
+            assert "isError" not in result
+
+
 def test_call_tool_sync_exception(mock_transport, mock_session):
     """Test that call_tool_sync correctly handles exceptions."""
     mock_session.call_tool.side_effect = Exception("Test exception")