Improve event stream resilience

packages/web/server/lib/event-stream/protocol.js

@@ -1,6 +1,9 @@
 export const MESSAGE_STREAM_GLOBAL_WS_PATH = '/api/global/event/ws';
 export const MESSAGE_STREAM_DIRECTORY_WS_PATH = '/api/event/ws';
 export const MESSAGE_STREAM_WS_HEARTBEAT_INTERVAL_MS = 15 * 1000;
+// Per-client pending outbound WS buffer, not a payload or stream-size limit.
+// Healthy clients stay near 0; this only trips when a client is far behind.
+export const MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES = 4 * 1024 * 1024;
 
 export function parseSseEventEnvelope(block) {
   if (!block || typeof block !== 'string') {
@@ -64,8 +67,23 @@ export function sendMessageStreamWsFrame(socket, payload) {
     return false;
   }
 
+  if (typeof socket.bufferedAmount === 'number' && socket.bufferedAmount > MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES) {
+    try {
+      socket.close(1013, 'Message stream client is too slow');
+    } catch {
+    }
+    return false;
+  }
+
   try {
     socket.send(JSON.stringify(payload));
+    if (typeof socket.bufferedAmount === 'number' && socket.bufferedAmount > MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES) {
+      try {
+        socket.close(1013, 'Message stream client is too slow');
+      } catch {
+      }
+      return false;
+    }
     return true;
   } catch {
     return false;

packages/web/server/lib/event-stream/protocol.test.js

@@ -3,6 +3,7 @@ import { describe, expect, it } from 'vitest';
 import {
   MESSAGE_STREAM_DIRECTORY_WS_PATH,
   MESSAGE_STREAM_GLOBAL_WS_PATH,
+  MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES,
   parseSseEventEnvelope,
   sendMessageStreamWsEvent,
   sendMessageStreamWsFrame,
@@ -63,6 +64,30 @@ describe('event stream protocol helpers', () => {
     expect(rawPayload).toBe('{"type":"ready"}');
   });
 
+  it('closes slow websocket clients instead of adding more buffered data', () => {
+    let closeCall = null;
+    let sendCalls = 0;
+    const socket = {
+      readyState: 1,
+      bufferedAmount: MESSAGE_STREAM_WS_MAX_BUFFERED_BYTES + 1,
+      send() {
+        sendCalls += 1;
+      },
+      close(code, reason) {
+        closeCall = { code, reason };
+      },
+    };
+
+    const sent = sendMessageStreamWsFrame(socket, { type: 'ready' });
+
+    expect(sent).toBe(false);
+    expect(sendCalls).toBe(0);
+    expect(closeCall).toEqual({
+      code: 1013,
+      reason: 'Message stream client is too slow',
+    });
+  });
+
   it('serializes event frames with routing metadata', () => {
     let rawPayload = null;
     const socket = {

packages/web/server/lib/opencode/proxy.js

@@ -6,6 +6,43 @@ import {
   shouldForwardProxyResponseHeader,
 } from '../../proxy-headers.js';
 
+export const waitForSseDrain = (res, signal) => new Promise((resolve) => {
+  if (signal?.aborted || res.writableEnded || res.destroyed) {
+    resolve();
+    return;
+  }
+
+  const cleanup = () => {
+    res.off?.('drain', onDone);
+    res.off?.('close', onDone);
+    res.off?.('error', onDone);
+    signal?.removeEventListener?.('abort', onDone);
+  };
+  const onDone = () => {
+    cleanup();
+    resolve();
+  };
+
+  res.once?.('drain', onDone);
+  res.once?.('close', onDone);
+  res.once?.('error', onDone);
+  signal?.addEventListener?.('abort', onDone, { once: true });
+});
+
+export const writeSseChunkWithBackpressure = async (res, value, signal) => {
+  if (!value || value.length === 0 || signal?.aborted || res.writableEnded || res.destroyed) {
+    return false;
+  }
+
+  const flushed = res.write(value);
+  if (flushed !== false) {
+    return true;
+  }
+
+  await waitForSseDrain(res, signal);
+  return !signal?.aborted && !res.writableEnded && !res.destroyed;
+};
+
 export const registerOpenCodeProxy = (app, deps) => {
   const {
     fs,
@@ -131,7 +168,10 @@ export const registerOpenCodeProxy = (app, deps) => {
           break;
         }
         if (value && value.length > 0) {
-          res.write(value);
+          const canContinue = await writeSseChunkWithBackpressure(res, value, abortController.signal);
+          if (!canContinue) {
+            break;
+          }
         }
       }
 

packages/web/server/lib/scheduled-tasks/routes.js

@@ -209,7 +209,22 @@ export const registerScheduledTaskRoutes = (app, dependencies) => {
     } catch {
     }
 
+    const heartbeat = setInterval(() => {
+      try {
+        writeSseEvent(res, {
+          type: 'openchamber:heartbeat',
+          properties: {
+            timestamp: Date.now(),
+          },
+        });
+      } catch {
+        clearInterval(heartbeat);
+        clients.delete(res);
+      }
+    }, 25_000);
+
     req.on('close', () => {
+      clearInterval(heartbeat);
       clients.delete(res);
     });
   });

packages/web/server/opencode-proxy.test.js

@@ -1,8 +1,9 @@
 import { afterEach, describe, expect, it } from 'vitest';
+import { EventEmitter } from 'node:events';
 import express from 'express';
 import path from 'path';
 
-import { registerOpenCodeProxy } from './lib/opencode/proxy.js';
+import { registerOpenCodeProxy, writeSseChunkWithBackpressure } from './lib/opencode/proxy.js';
 
 const listen = (app, host = '127.0.0.1') => new Promise((resolve, reject) => {
   const server = app.listen(0, host, () => resolve(server));
@@ -81,6 +82,27 @@ describe('OpenCode proxy SSE forwarding', () => {
     expect(seenAuthorization).toBe('Bearer test-token');
   });
 
+  it('waits for drain when writing to a slow SSE response', async () => {
+    const writes = [];
+    const res = new EventEmitter();
+    res.writableEnded = false;
+    res.destroyed = false;
+    res.write = (value) => {
+      writes.push(value);
+      return false;
+    };
+    const controller = new AbortController();
+
+    const write = writeSseChunkWithBackpressure(res, Buffer.from('data: {"ok":true}\n\n'), controller.signal);
+
+    await new Promise((resolve) => setTimeout(resolve, 0));
+    expect(writes).toHaveLength(1);
+
+    res.emit('drain');
+
+    await expect(write).resolves.toBe(true);
+  });
+
   it('routes generic API requests through external OpenCode base URL', async () => {
     const upstream = express();
     upstream.get('/config/providers', (_req, res) => {