diff --git a/settings.json.template b/settings.json.template
index e88e82a36..c37bfe7f9 100644
--- a/settings.json.template
+++ b/settings.json.template
@@ -733,6 +733,19 @@
    */
   "loadTest": false,
 
+  /*
+   * Defer engine.io socket flush onto the next microtask so multiple
+   * sendPacket() calls in the same task accumulate in writeBuffer before
+   * the underlying transport.send drains. Pairs with engine.io's existing
+   * batched-send path so high-fan-out scenarios produce fewer WebSocket
+   * frames. Microtask deferral adds no meaningful wall-clock latency —
+   * microtasks drain before any subsequent macrotask. Wire bytes are
+   * unchanged.
+   *
+   * #7756 / #7767. Default off; production unaffected.
+   */
+  "engineFlushDefer": false,
+
   /**
   * Disable dump of objects preventing a clean exit
   */
diff --git a/src/node/utils/EngineFlushDeferral.ts b/src/node/utils/EngineFlushDeferral.ts
index e8c15b4ea..36b3804ed 100644
--- a/src/node/utils/EngineFlushDeferral.ts
+++ b/src/node/utils/EngineFlushDeferral.ts
@@ -47,7 +47,6 @@ const SCHEDULED = Symbol('engineFlushScheduled');
 
 export const installEngineFlushDeferral = (): void => {
   if (installed) return;
-  installed = true;
 
   let SocketProto: {sendPacket: (...a: unknown[]) => unknown};
   try {
@@ -55,12 +54,17 @@ export const installEngineFlushDeferral = (): void => {
     SocketProto = require('engine.io/build/socket').Socket.prototype;
   } catch (err: any) {
     logger.warn(`Unable to install engine.io flush deferral (module not found): ${err && err.message || err}`);
-    return;
+    return;  // Leave `installed` false so a later boot path can retry.
   }
   if (typeof SocketProto.sendPacket !== 'function') {
     logger.warn('engine.io Socket shape unexpected; skipping flush deferral patch');
-    return;
+    return;  // Leave `installed` false so a later boot path can retry.
   }
+  // Only after both require and shape check succeed do we record that the
+  // patch is installed. Setting the flag before validation (the original
+  // code) would have permanently disabled retries after a transient
+  // require failure in test/CI environments where socket.io may load late.
+  installed = true;
 
   // Re-implementing sendPacket inline rather than wrapping the original
   // so the single closing `this.flush()` becomes a microtask-coalesced