diff --git a/src/__tests__/fanout.test.ts b/src/__tests__/fanout.test.ts
index bbc2ff02..9a2cde06 100644
--- a/src/__tests__/fanout.test.ts
+++ b/src/__tests__/fanout.test.ts
@@ -141,7 +141,7 @@ describe('FanOutManager', () => {
     )).toHaveLength(1)
   })
 
-  it('destroy cleans up all state', async () => {
+  it('destroy cleans up all state idempotently', async () => {
     const mockSocket = {
       id: 'client-cleanup',
       send: vi.fn(),
@@ -150,7 +150,33 @@ describe('FanOutManager', () => {
 
     fanOutManager.register('XLM/USDC', mockSocket)
     await fanOutManager.destroy()
-    await fanOutManager.destroy()
+    await fanOutManager.destroy() // Idempotent second call
+    expect(mockSocket.close).toHaveBeenCalledWith(1001, 'Server shutting down')
+  })
+
+  it('sends deliberate close code 1001 to all connected clients on shutdown (#347)', async () => {
+    const socketA = { id: 'client-a', send: vi.fn(), close: vi.fn() }
+    const socketB = { id: 'client-b', send: vi.fn(), close: vi.fn() }
+
+    fanOutManager.register('XLM/USDC', socketA)
+    fanOutManager.register('BTC/USDC', socketB)
+
+    await fanOutManager.destroy(1001, 'Server shutting down')
+
+    expect(socketA.close).toHaveBeenCalledWith(1001, 'Server shutting down')
+    expect(socketB.close).toHaveBeenCalledWith(1001, 'Server shutting down')
+  })
+
+  it('respects flush window during shutdown before closing clients (#347)', async () => {
+    const socket = { id: 'flush-client', send: vi.fn(), close: vi.fn() }
+    fanOutManager.register('XLM/USDC', socket)
+
+    const start = Date.now()
+    await fanOutManager.destroy(1001, 'Server shutting down', 50)
+    const elapsed = Date.now() - start
+
+    expect(elapsed).toBeGreaterThanOrEqual(40)
+    expect(socket.close).toHaveBeenCalledWith(1001, 'Server shutting down')
   })
 })
 
diff --git a/src/ws/fanout.ts b/src/ws/fanout.ts
index d8430b49..64a0413d 100644
--- a/src/ws/fanout.ts
+++ b/src/ws/fanout.ts
@@ -117,6 +117,8 @@ interface InternalClient {
 class FanOutManager {
   private clients = new Map<string, InternalClient>() // clientId -> client
   private pairClients = new Map<string, Set<string>>() // pairKey -> Set<clientId>
+  private isDestroying = false
+  private isDestroyed = false
 
   // Redis pub/sub for cross-instance broadcasting
   private subscriber: typeof redis | null = null
@@ -178,6 +180,7 @@ class FanOutManager {
       congestedSince: null,
     }
 
+    this.isDestroyed = false
     this.clients.set(id, client)
     let clients = this.pairClients.get(pairKey)
     if (!clients) {
@@ -445,9 +448,14 @@ class FanOutManager {
   }
 
   /**
-   * Dispose all clients and clean up.
+   * Graceful Shutdown & Cleanup (#347)
+   * Sends close frames with code 1001 ('Server shutting down') and allows a flush window.
+   * Completely idempotent and safe against mid-fanout calls.
    */
-  async destroy(): Promise<void> {
+  async destroy(closeCode = 1001, reason = 'Server shutting down', flushWindowMs = 0): Promise<void> {
+    if (this.isDestroyed || this.isDestroying) return
+    this.isDestroying = true
+
     if (this.bufferSweep) {
       clearInterval(this.bufferSweep)
       this.bufferSweep = null
@@ -459,10 +467,40 @@ class FanOutManager {
         // subscriber may not have connected — ignore
       }
       this.subscriber.disconnect()
+      this.subscriber = null
+    }
+
+    // Allow in-flight sends / queued coalesced messages a brief window to flush (#347)
+    if (flushWindowMs > 0 && this.clients.size > 0) {
+      await new Promise(resolve => setTimeout(resolve, flushWindowMs))
     }
-    for (const [id] of this.clients) {
-      this.unregister(id)
+
+    const clientList = Array.from(this.clients.values())
+    for (const client of clientList) {
+      try {
+        if (client.timer) {
+          clearTimeout(client.timer)
+          client.timer = null
+        }
+        client.destroyed = true
+        client.socket.close(closeCode, reason)
+      } catch {
+        // do not throw
+      }
+      this.unregister(client.id)
     }
+
+    this.clients.clear()
+    this.pairClients.clear()
+    this.isDestroying = false
+    this.isDestroyed = true
+  }
+
+  resetForTesting(): void {
+    this.clients.clear()
+    this.pairClients.clear()
+    this.isDestroyed = false
+    this.isDestroying = false
   }
 }