diff --git a/src/serve/server-threads.ts b/src/serve/server-threads.ts
index e8c77f9..de07803 100644
--- a/src/serve/server-threads.ts
+++ b/src/serve/server-threads.ts
@@ -1,4 +1,4 @@
-import { openSync } from 'node:fs';
+import { closeSync, openSync } from 'node:fs';
 import type { Server } from 'node:net';
 import { channel } from '../channel.ts';
 import { shared } from '../shared/shared.ts';
@@ -17,7 +17,7 @@ export function serverThreads(
   opts: ServerThreadsOptions = {},
 ): ServerThreads {
   const balance = opts.balance ?? leastConns();
-  const _hwm = opts.highWaterMark ?? DEFAULT_HIGH_WATER_MARK;
+  const hwm = opts.highWaterMark ?? DEFAULT_HIGH_WATER_MARK;
   const listenOpts: Required<ListenOptions> = {
     drainTimeout: opts.listen?.drainTimeout ?? DEFAULT_DRAIN_TIMEOUT,
   };
@@ -29,7 +29,7 @@ export function serverThreads(
   const pushChannels = Array.from({ length: n }, () => new PushChannel<number>());
   // Wrap each PushChannel in moroutine's channel() so it is recognized by
   // moroutine's arg-preparation layer and piped cross-thread via MessagePort.
-  const channels = pushChannels.map((pc) => channel<number>(pc));
+  const channels = pushChannels.map((pc) => channel<number>(pc, { highWaterMark: hwm }));
   // Shared atomic counter slots, one per worker.
   const counters = shared(Array.from({ length: n }, () => int32atomic));
 
@@ -56,8 +56,9 @@ export function serverThreads(
     try {
       pushChannels[idx].send(fd);
     } catch {
-      // send threw because the channel is closed — undo the increment.
+      // send threw because the channel is closed — undo the increment and close the fd.
       counters.elements[idx].sub(1);
+      closeSync(fd);
     }
   };