Assistant: restore ReconcileSnapshotMiddleware

Retiring it in the middleware audit was premature: MAF 1.10 reuses the
streamed id for plain-text snapshots but deliberately mints a fresh UUID
for post-tool-call assistant text (their #3619), so tool+text turns
still rendered a duplicate assistant bubble (re-verified live). The
user-message duplication that motivated the retirement only fired on
a2ui retry runs, which no longer exist now that a2ui is disabled.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/server.ts

@@ -191,6 +191,68 @@ class StripModelArtifactsMiddleware extends (Middleware as any) {
   }
 }
 
+// ── ReconcileSnapshotMiddleware ──────────────────────────────────────────────
+// MAF 1.10 reuses the streamed id for PLAIN-text snapshots, but still mints a
+// fresh UUID for post-tool-call assistant text — deliberately, per their
+// issue #3619 ("separate message"). ag-ui's snapshot merge appends unknown
+// ids, so tool+text turns still produce a duplicate assistant bubble without
+// this reconciliation (re-verified live on MAF 1.10 / CK 1.62, 2026-07-04).
+// Drops only the orphan text-only assistant message with no streamed
+// counterpart. Caveat: interfered with a2ui retry runs (duplicated the user
+// message) — a2ui is disabled while that pipeline matures; revisit both
+// together.
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+class ReconcileSnapshotMiddleware extends (Middleware as any) {
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  run(input: any, next: any): Observable<any> {
+    const streamedTextIds = new Set<string>();
+
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    return new Observable<any>((observer) => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const sub = (this as any).runNextWithState(input, next).subscribe({
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        next: (eventWithState: any) => {
+          const event = eventWithState.event;
+          const type: string = event?.type ?? "";
+
+          if (type === EventType.TEXT_MESSAGE_START && event?.messageId) {
+            streamedTextIds.add(String(event.messageId));
+          }
+
+          if (type === EventType.MESSAGES_SNAPSHOT) {
+            // eslint-disable-next-line @typescript-eslint/no-explicit-any
+            const msgs: any[] = Array.isArray(event?.messages) ? event.messages : [];
+            const filtered = msgs.filter((m) => {
+              if (m?.role !== "assistant") return true;
+              const id = String(m?.id ?? "");
+              const hasText = typeof m?.content === "string" && m.content.length > 0;
+              const hasToolCalls =
+                (Array.isArray(m?.toolCalls) && m.toolCalls.length > 0) ||
+                (Array.isArray(m?.tool_calls) && m.tool_calls.length > 0);
+              if (hasText && !hasToolCalls && !streamedTextIds.has(id)) {
+                return false; // drop orphan text-only assistant duplicate
+              }
+              return true;
+            });
+            if (filtered.length !== msgs.length) {
+              observer.next({ ...event, messages: filtered });
+              return;
+            }
+          }
+
+          observer.next(event);
+        },
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        error: (err: any) => observer.error(err),
+        complete: () => observer.complete(),
+      });
+      return () => sub.unsubscribe();
+    });
+  }
+}
+
 // ── SuppressRenderToolTextMiddleware ──────────────────────────────────────────
 // When the LLM emits text content in the same response as a render tool call
 // (render_a2ui or render_spending_summary), the text appears as a duplicate
@@ -298,10 +360,7 @@ app.all("/api/copilotkit/*", async (c) => {
   if (process.env.CK_MIDDLEWARES !== "off") {
     agent.use(new SuppressRenderToolTextMiddleware() as any);
     agent.use(new StripModelArtifactsMiddleware() as any);
-    // ReconcileSnapshotMiddleware retired 2026-07-04: MAF 1.9+ reuses the
-    // streamed message id in MESSAGES_SNAPSHOT (verified in _agent_run.py at
-    // python-1.10.0 and in live event logs), and keeping the surgery caused
-    // duplicated user messages on a2ui follow-up runs.
+    agent.use(new ReconcileSnapshotMiddleware() as any);
     agent.use(new DeduplicateToolCallMiddleware() as any);
   }
   if (process.env.CK_DEBUG_EVENTS === "1" || process.env.CK_MIDDLEWARES === "off") {