fix(gateway): flush only pending delta tail before emitChatFinal

- Send text.slice(lastBroadcastLen) instead of full accumulated text
- Prevents duplication for naive concat clients
- Incorporates silent lead fragment + heartbeat streaming guards
- Update test to assert on content correctness, not emission count

src/gateway/server-chat.agent-events.test.ts

@@ -548,7 +548,16 @@ describe("agent event handler", () => {
 
     emitLifecycleEnd(handler, "run-heartbeat-alert");
 
-    const payload = expectSingleFinalChatPayload(broadcast) as {
+    // The flush-before-final path may emit a delta with the pending tail
+    // followed by the authoritative final.  Assert on content, not count.
+    const chatCalls = chatBroadcastCalls(broadcast);
+    expect(chatCalls.length).toBeGreaterThanOrEqual(1);
+
+    const finalCall = chatCalls.find(
+      (c) => (c[1] as { state?: string }).state === "final",
+    );
+    expect(finalCall).toBeDefined();
+    const payload = finalCall![1] as {
       message?: { content?: Array<{ text?: string }> };
     };
     expect(payload.message?.content?.[0]?.text).toBe(

src/gateway/server-chat.ts

@@ -358,13 +358,24 @@ export function createAgentEventHandler({
     const text = normalizedHeartbeatText.text.trim();
     const shouldSuppressSilent =
       normalizedHeartbeatText.suppress || isSilentReplyText(text, SILENT_REPLY_TOKEN);
+    const shouldSuppressSilentLeadFragment = isSilentReplyLeadFragment(text);
+    const shouldSuppressHeartbeatStreaming = shouldHideHeartbeatChatOutput(
+      clientRunId,
+      sourceRunId,
+    );
     // Flush any throttled delta so streaming clients receive the complete text
     // before the final event.  The 150 ms throttle in emitChatDelta may have
     // suppressed the most recent chunk, leaving the client with stale text.
     // Only flush if the buffer has grown since the last broadcast to avoid duplicates.
-    if (text && !shouldSuppressSilent) {
+    if (
+      text &&
+      !shouldSuppressSilent &&
+      !shouldSuppressSilentLeadFragment &&
+      !shouldSuppressHeartbeatStreaming
+    ) {
       const lastBroadcastLen = chatRunState.deltaLastBroadcastLen.get(clientRunId) ?? 0;
       if (text.length > lastBroadcastLen) {
+        const pendingTail = text.slice(lastBroadcastLen);
         const flushPayload = {
           runId: clientRunId,
           sessionKey,
@@ -372,12 +383,13 @@ export function createAgentEventHandler({
           state: "delta" as const,
           message: {
             role: "assistant",
-            content: [{ type: "text", text }],
+            content: [{ type: "text", text: pendingTail }],
             timestamp: Date.now(),
           },
         };
         broadcast("chat", flushPayload, { dropIfSlow: true });
         nodeSendToSession(sessionKey, "chat", flushPayload);
+        chatRunState.deltaLastBroadcastLen.set(clientRunId, text.length);
       }
     }
     chatRunState.deltaLastBroadcastLen.delete(clientRunId);