Assistant: drop mid-run MESSAGES_SNAPSHOT instead of reconciling it

Prod bug: earlier assistant replies vanished as new turns completed and
only came back on refresh. Debug capture showed why: with persistence
on, the run-end snapshot rebuilds history under STORE ids while the
client holds it under STREAMED ids (MAF re-mints post-tool-call text
ids, their #3619). ag-ui apply treats the snapshot as authoritative —
matching ids get replaced (streamed text wiped by the store's
empty-content toolCall entry) and the store's text copies were being
filtered out by ReconcileSnapshotMiddleware as presumed duplicates, so
prior answers had no surviving representation.

Reconciling id-mismatched snapshots is unwinnable; skip them instead.
During a normal run the client already built the turn from streamed
events and the snapshot adds nothing. Hydration replays (empty
input.messages) still pass through — there the snapshot IS the
conversation, and every reload converges the client to store ids.
ReconcileSnapshotMiddleware is replaced by the much simpler
DropRunMessagesSnapshotMiddleware.

Verified in dev: multi-turn history persists across runs, one card that
survives follow-ups, reload replays everything, no duplicates.

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

frontend/server.ts

@@ -250,31 +250,25 @@ class StripModelArtifactsMiddleware extends (Middleware as any) {
   }
 }
 
-// ── ReconcileSnapshotMiddleware ──────────────────────────────────────────────
+// ── DropRunMessagesSnapshotMiddleware ────────────────────────────────────────
-// MAF 1.10 reuses the streamed id for PLAIN-text snapshots, but still mints a
+// MAF's run-end MESSAGES_SNAPSHOT re-mints ids for post-tool-call text (their
-// fresh UUID for post-tool-call assistant text — deliberately, per their
+// #3619) and, with thread persistence on, rebuilds history under STORE ids.
-// issue #3619 ("separate message"). ag-ui's snapshot merge appends unknown
+// ag-ui's apply treats the snapshot as authoritative: client messages absent
-// ids, so tool+text turns still produce a duplicate assistant bubble without
+// from it are dropped, matching ids are replaced. Because streamed ids and
-// this reconciliation (re-verified live on MAF 1.10 / CK 1.62, 2026-07-04).
+// snapshot ids never agree for tool+text turns, every mid-session snapshot
-// Drops only the orphan text-only assistant message with no streamed
+// either duplicates the current answer (fresh-id append) or wipes earlier
-// counterpart. Caveat: interfered with a2ui retry runs (duplicated the user
+// ones (store-id replace with empty content) — both observed live. During a
-// message) — a2ui is disabled while that pipeline matures; revisit both
+// normal run the client already built the exact turn from streamed events,
-// together.
+// so the snapshot adds nothing: swallow it. Hydration replays (empty
-//
+// input.messages) pass through untouched — there the snapshot IS the
-// Thread persistence (2026-07-04) adds two exemptions: a hydration replay
+// conversation, and the client converges to store ids on every reload.
-// (empty input.messages) is passed through untouched — nothing in it was
-// streamed this run — and messages whose id the client already sent in
-// input.messages are history, not duplicates.
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
-class ReconcileSnapshotMiddleware extends (Middleware as any) {
+class DropRunMessagesSnapshotMiddleware 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>();
+    const isHydration =
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      !Array.isArray(input?.messages) || input.messages.length === 0;
-    const inputMessages: any[] = Array.isArray(input?.messages) ? input.messages : [];
-    const isHydration = inputMessages.length === 0;
-    const clientKnownIds = new Set<string>(inputMessages.map((m) => String(m?.id ?? "")));
 
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     return new Observable<any>((observer) => {
@@ -283,38 +277,9 @@ class ReconcileSnapshotMiddleware extends (Middleware as any) {
         // eslint-disable-next-line @typescript-eslint/no-explicit-any
         next: (eventWithState: any) => {
           const event = eventWithState.event;
-          const type: string = event?.type ?? "";
+          if (event?.type === EventType.MESSAGES_SNAPSHOT && !isHydration) {
-
+            return; // client state from streaming is already correct
-          if (type === EventType.TEXT_MESSAGE_START && event?.messageId) {
-            streamedTextIds.add(String(event.messageId));
           }
-
-          if (type === EventType.MESSAGES_SNAPSHOT && !isHydration) {
-            // 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) &&
-                !clientKnownIds.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
@@ -433,7 +398,7 @@ app.all("/api/copilotkit/*", async (c) => {
   if (process.env.CK_MIDDLEWARES !== "off") {
     agent.use(new SuppressRenderToolTextMiddleware() as any);
     agent.use(new StripModelArtifactsMiddleware() as any);
-    agent.use(new ReconcileSnapshotMiddleware() as any);
+    agent.use(new DropRunMessagesSnapshotMiddleware() as any);
     agent.use(new DeduplicateToolCallMiddleware() as any);
   }
   if (process.env.CK_DEBUG_EVENTS === "1" || process.env.CK_MIDDLEWARES === "off") {