From 124fdce1a0e13cf0b079666905f6572e7fcfbb58 Mon Sep 17 00:00:00 2001
From: Harivansh Rathi <rathiharivansh@gmail.com>
Date: Thu, 12 Mar 2026 20:21:58 -0400
Subject: [PATCH] sse heartbeat and ui

---
 .../coding-agent/src/core/gateway/runtime.ts  | 19 +++++++++++++++++++
 .../src/core/gateway/vercel-ai-stream.ts      | 10 ++++++----
 2 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/packages/coding-agent/src/core/gateway/runtime.ts b/packages/coding-agent/src/core/gateway/runtime.ts
index bb1b45a..81ec27b 100644
--- a/packages/coding-agent/src/core/gateway/runtime.ts
+++ b/packages/coding-agent/src/core/gateway/runtime.ts
@@ -60,6 +60,7 @@ export type {
 export type { HistoryPart } from "./types.js";
 
 let activeGatewayRuntime: GatewayRuntime | null = null;
+const SSE_HEARTBEAT_MS = 15_000;
 
 type JsonRecord = Record<string, unknown>;
 type AssistantAgentMessage = Extract<AgentMessage, { role: "assistant" }>;
@@ -87,6 +88,18 @@ function mergeRecords(base: JsonRecord, overrides: JsonRecord): JsonRecord {
   return merged;
 }
 
+function startSseHeartbeat(response: ServerResponse): () => void {
+  const timer = setInterval(() => {
+    if (response.writableEnded) {
+      clearInterval(timer);
+      return;
+    }
+    response.write(":\n\n");
+  }, SSE_HEARTBEAT_MS);
+  timer.unref?.();
+  return () => clearInterval(timer);
+}
+
 function readString(value: unknown): string | undefined {
   if (typeof value !== "string") {
     return undefined;
@@ -1178,11 +1191,13 @@ export class GatewayRuntime {
       Connection: "keep-alive",
     });
     response.write("\n");
+    const stopHeartbeat = startSseHeartbeat(response);
 
     const unsubscribe = await this.addSubscriber(sessionKey, (event) => {
       response.write(`data: ${JSON.stringify(event)}\n\n`);
     });
     request.on("close", () => {
+      stopHeartbeat();
       unsubscribe();
     });
   }
@@ -1210,6 +1225,7 @@ export class GatewayRuntime {
       "x-vercel-ai-ui-message-stream": "v1",
     });
     response.write("\n");
+    const stopHeartbeat = startSseHeartbeat(response);
 
     const listener = createVercelStreamListener(response);
     const structuredPartListener =
@@ -1231,6 +1247,7 @@ export class GatewayRuntime {
     let clientDisconnected = false;
     request.on("close", () => {
       clientDisconnected = true;
+      stopHeartbeat();
       stopStreaming();
     });
 
@@ -1258,6 +1275,7 @@ export class GatewayRuntime {
       });
       if (!clientDisconnected) {
         stopStreaming();
+        stopHeartbeat();
         if (result.ok) {
           this.log(`[chat] session=${sessionKey} completed ok`);
           finishVercelStream(response, "stop");
@@ -1278,6 +1296,7 @@ export class GatewayRuntime {
     } catch (error) {
       if (!clientDisconnected) {
         stopStreaming();
+        stopHeartbeat();
         const message = error instanceof Error ? error.message : String(error);
         this.log(`[chat] session=${sessionKey} exception: ${message}`);
         errorVercelStream(response, message);
diff --git a/packages/coding-agent/src/core/gateway/vercel-ai-stream.ts b/packages/coding-agent/src/core/gateway/vercel-ai-stream.ts
index a1a7262..31d930c 100644
--- a/packages/coding-agent/src/core/gateway/vercel-ai-stream.ts
+++ b/packages/coding-agent/src/core/gateway/vercel-ai-stream.ts
@@ -25,7 +25,10 @@ function isTextContent(
 function getAssistantTextParts(
   event: Extract<AgentSessionEvent, { type: "message_end" }>,
 ): Array<{ contentIndex: number; text: string }> {
-  if (event.message.role !== "assistant" || !Array.isArray(event.message.content)) {
+  if (
+    event.message.role !== "assistant" ||
+    !Array.isArray(event.message.content)
+  ) {
     return [];
   }
 
@@ -338,9 +341,8 @@ export function createGatewayStructuredPartListener(
     if (response.writableEnded) return;
     if (event.type !== "structured_part") return;
     writeChunk(response, {
-      type: "structured-part",
-      partType: event.partType,
-      payload: event.payload,
+      type: `data-${event.partType}`,
+      data: event.payload,
     });
   };
 }