fix: tool-error result serialized as {} due to non-enumerable Error properties

Two fixes:
1. MonoDriver: tool-error event now sends error.message instead of raw
   Error object. Error properties are non-enumerable, so JSON.stringify
   produced "{}".
2. Presentation reducer: handleToolResult adds instanceof Error check
   as defense-in-depth, extracting .message before serialization.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: deepracticexs

packages/agentx/src/presentation/reducer.ts

@@ -381,7 +381,12 @@ function handleToolResult(
   data: { toolCallId: string; result: unknown; isError: boolean }
 ): PresentationState {
   const { toolCallId, result, isError } = data;
-  const resultStr = typeof result === "string" ? result : JSON.stringify(result);
+  const resultStr =
+    typeof result === "string"
+      ? result
+      : result instanceof Error
+        ? result.message
+        : JSON.stringify(result);
 
   const conversations = state.conversations.map((conv): Conversation => {
     if (conv.role !== "assistant") return conv;

packages/mono-driver/src/MonoDriver.ts

@@ -299,7 +299,7 @@ export class MonoDriver implements Driver {
             }
             yield createEvent("tool_result", {
               toolCallId: part.toolCallId,
-              result: part.error,
+              result: part.error instanceof Error ? part.error.message : String(part.error),
               isError: true,
             });
             break;