fix: properly serialize error objects in transcript logging (#427)

Co-authored-by: Letta <noreply@letta.com>

src/cli/App.tsx

@@ -945,18 +945,26 @@ export default function App({
   // Also tracks the error in telemetry so we know an error was shown
   const appendError = useCallback(
     (message: string, skipTelemetry = false) => {
+      // Defensive: ensure message is always a string (guards against [object Object])
+      const text =
+        typeof message === "string"
+          ? message
+          : message != null
+            ? JSON.stringify(message)
+            : "[Unknown error]";
+
       const id = uid("err");
       buffersRef.current.byId.set(id, {
         kind: "error",
         id,
-        text: message,
+        text,
       });
       buffersRef.current.order.push(id);
       refreshDerived();
 
       // Track error in telemetry (unless explicitly skipped for user-initiated actions)
       if (!skipTelemetry) {
-        telemetry.trackError("ui_error", message, "error_display", {
+        telemetry.trackError("ui_error", text, "error_display", {
           modelId: currentModelId || undefined,
         });
       }

src/cli/helpers/accumulator.ts

@@ -420,12 +420,19 @@ export function onChunk(b: Buffers, chunk: LettaStreamingResponse) {
       for (const toolReturn of toolReturns) {
         const toolCallId = toolReturn.tool_call_id;
         // Handle both func_response (streaming) and tool_return (SDK) properties
-        const resultText =
+        const rawResult =
           ("func_response" in toolReturn
             ? toolReturn.func_response
             : undefined) ||
-          ("tool_return" in toolReturn ? toolReturn.tool_return : undefined) ||
-          "";
+          ("tool_return" in toolReturn ? toolReturn.tool_return : undefined);
+
+        // Ensure resultText is always a string (guard against SDK returning objects)
+        const resultText =
+          typeof rawResult === "string"
+            ? rawResult
+            : rawResult != null
+              ? JSON.stringify(rawResult)
+              : "";
         const status = toolReturn.status;
 
         // Look up the line by toolCallId

src/cli/helpers/errorFormatter.ts

@@ -98,7 +98,29 @@ export function formatErrorDetails(e: unknown, agentId?: string): string {
     return e.message;
   }
 
-  // Fallback for any other type
+  // Fallback for any other type (e.g., plain objects thrown by SDK or other code)
+  if (typeof e === "object" && e !== null) {
+    const obj = e as Record<string, unknown>;
+
+    // Check common error-like properties
+    if (typeof obj.message === "string") {
+      return obj.message;
+    }
+    if (typeof obj.error === "string") {
+      return obj.error;
+    }
+    if (typeof obj.detail === "string") {
+      return obj.detail;
+    }
+
+    // Last resort: JSON stringify
+    try {
+      return JSON.stringify(e, null, 2);
+    } catch {
+      return "[Error: Unable to serialize error object]";
+    }
+  }
+
   return String(e);
 }