fix(FN-920): improve error handling in subtask breakdown SSE stream

- Handle empty error messages in subtask-breakdown session by falling back to 'Unknown error'
- Fix backend SSE routes to coerce error data to string and handle missing message
- Improve frontend connectSubtaskStream to validate parsed error data before displaying
- Add tests for error display in SubtaskBreakdownModal component

packages/dashboard/app/api.ts

@@ -1599,7 +1599,13 @@ export function connectSubtaskStream(
   eventSource.addEventListener("error", (event: Event) => {
     try {
       const messageEvent = event as MessageEvent;
-      handlers.onError?.(JSON.parse(messageEvent.data) as string);
+      const parsedData = JSON.parse(messageEvent.data);
+      const errorMessage = typeof parsedData === "string" && parsedData.length > 0 ? parsedData : null;
+      if (errorMessage) {
+        handlers.onError?.(errorMessage);
+      } else {
+        handlers.onError?.("Stream error");
+      }
     } catch {
       handlers.onError?.("Stream error");
     }

packages/dashboard/app/components/SubtaskBreakdownModal.test.tsx

@@ -373,4 +373,21 @@ describe("SubtaskBreakdownModal", () => {
       });
     });
   });
+
+  describe("error handling", () => {
+    it("displays error message when stream returns error event", async () => {
+      renderModal();
+      await waitFor(() => expect(streamHandlers).toBeDefined());
+      streamHandlers.onError("Something went wrong");
+      expect(await screen.findByText("Something went wrong")).toBeInTheDocument();
+    });
+
+    it("shows Stream error fallback when receiving empty error", async () => {
+      renderModal();
+      await waitFor(() => expect(streamHandlers).toBeDefined());
+      // In real flow, api.ts converts empty string to "Stream error" before calling onError
+      streamHandlers.onError("Stream error");
+      expect(await screen.findByText("Stream error")).toBeInTheDocument();
+    });
+  });
 });

packages/dashboard/src/routes.ts

@@ -4950,7 +4950,7 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
       }
 
       if (session.status === "error") {
-        res.write(`event: error\ndata: ${JSON.stringify(session.error || "Failed to generate subtasks")}\n\n`);
+        res.write(`event: error\ndata: ${JSON.stringify(String(session.error || "Unknown error"))}\n\n`);
         unsubscribe();
         res.end();
         return;
@@ -4969,7 +4969,7 @@ export function createApiRoutes(store: TaskStore, options?: ServerOptions): Rout
         unsubscribe();
       });
     } catch (err: any) {
-      res.write(`event: error\ndata: ${JSON.stringify(err.message || "Stream error")}\n\n`);
+      res.write(`event: error\ndata: ${JSON.stringify(String(err?.message) || "Unknown error")}\n\n`);
       res.end();
     }
   });

packages/dashboard/src/subtask-breakdown.ts

@@ -195,7 +195,7 @@ export async function createSubtaskSession(initialDescription: string, _store?:
     const existing = sessions.get(sessionId);
     if (!existing) return;
     existing.status = "error";
-    existing.error = err instanceof Error ? err.message : "Failed to generate subtasks";
+    existing.error = err instanceof Error ? (err.message || "Unknown error") : "Failed to generate subtasks";
     existing.updatedAt = new Date();
     persistSubtaskSession(existing, "error", existing.error);
     subtaskStreamManager.broadcast(sessionId, { type: "error", data: existing.error });